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Explosives Detection by Photofragmentation and Nitric Oxide-Ozone Chemiluminescence

Permanent Link: http://ufdc.ufl.edu/UFE0021149/00001

Material Information

Title: Explosives Detection by Photofragmentation and Nitric Oxide-Ozone Chemiluminescence Portability Considerations
Physical Description: 1 online resource (62 p.)
Language: english
Creator: Whiddon, Ronald James Louis
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: 193, chemiluminescence, explosives, nitric, ozone, photofragmentation
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The recent popularity of improvised explosive devices, and the continuing threat presented by unexploded land mines pushes the detection of hidden explosives to the forefront of scientific research. For maximum utility, a detection device should be handheld, be inexpensive, respond quickly, have little interference, and detect explosives without direct contact with the explosive device. Few instruments are available that can meet most of these requirements, primarily because measuring explosives in the vapor phase demands a sensitivity of low parts per billion to parts per trillion of explosive material. The chemiluminescent reaction between nitric oxide and ozone has been used to detect explosives by their decomposition, which produces nitric oxide. As of yet, the instrumentation has not been scaled down to the point that it could be assembled as a handheld detector. This research is the design of a nitric oxide-ozone chemiluminescent reaction chamber that is small enough to be handheld, while still being able to detect explosives in the vapor phase. A 24 mL reaction chamber was designed that was capable of detecting mid parts per billion levels of nitric oxide, and high picogram amounts of TNT. Response times for the instrument were less than 10 s.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Ronald James Louis Whiddon.
Thesis: Thesis (M.S.)--University of Florida, 2007.
Local: Adviser: Winefordner, James D.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021149:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021149/00001

Material Information

Title: Explosives Detection by Photofragmentation and Nitric Oxide-Ozone Chemiluminescence Portability Considerations
Physical Description: 1 online resource (62 p.)
Language: english
Creator: Whiddon, Ronald James Louis
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: 193, chemiluminescence, explosives, nitric, ozone, photofragmentation
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The recent popularity of improvised explosive devices, and the continuing threat presented by unexploded land mines pushes the detection of hidden explosives to the forefront of scientific research. For maximum utility, a detection device should be handheld, be inexpensive, respond quickly, have little interference, and detect explosives without direct contact with the explosive device. Few instruments are available that can meet most of these requirements, primarily because measuring explosives in the vapor phase demands a sensitivity of low parts per billion to parts per trillion of explosive material. The chemiluminescent reaction between nitric oxide and ozone has been used to detect explosives by their decomposition, which produces nitric oxide. As of yet, the instrumentation has not been scaled down to the point that it could be assembled as a handheld detector. This research is the design of a nitric oxide-ozone chemiluminescent reaction chamber that is small enough to be handheld, while still being able to detect explosives in the vapor phase. A 24 mL reaction chamber was designed that was capable of detecting mid parts per billion levels of nitric oxide, and high picogram amounts of TNT. Response times for the instrument were less than 10 s.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Ronald James Louis Whiddon.
Thesis: Thesis (M.S.)--University of Florida, 2007.
Local: Adviser: Winefordner, James D.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021149:00001


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EXPLOSIVES DETECTION BY PHOTOFRAGMENTATION AND
NITRIC OXIDE-OZONE CHEMILUMINESCENCE:
PORTABILITY CONSIDERATIONS



















By

RONALD WHIDDON


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2007



































02007 Ronald Whiddon





























To my father, who is a model of wisdom, patience and charity.












TABLE OF CONTENTS


Page

LI ST OF T ABLE S ............ ..... .__ ...............6....


LI ST OF FIGURE S .............. ...............7.....


LIST OF ABBREVIATIONS AND SYMBOLS .............. ...............8.....


CHAPTER


1 EXPLOSIVES: HISTORY, CLASSIFICATION, AND DETECTION .............. ..............11


Historical Background ................. ...............11.......... ......
Origins ................ ...............11.................
Application .............. ...............12....
Classification .............. ...............13....
Rate ................. ...............13.................

Ignition .............. ...............13....
Functional Group ................. ................. 13..............
Detection of Hidden Explosives ................. ...............14.......... ....
Canine Detection ................. ................. 15..............
Ion M obility Spectrometry .............. ...............15....
Fluorescence ................... ........... ..... .......... .............1
Resonance Enhanced MultiphotonlIonization............... ...............1
Chemiluminescence ................. ...............17.................
Summary ................. ...............18.................


2 MODELING THE NITRIC OXIDE-OZONE CHEMILUMINESCENCE REACTION
FOR A 24 mL REACTION CHAMBER ............ ......_ ...._ ...........1


Introducti on ............. ..... ...............19...
Reaction Kinetics ............ .... __ ...............19..

Model System Parameters .............. ...............21....
Re sults............. ..... ...............22...
Discussion ............. ..... ...............23...


3 CHARACTERIZATION OF A MINIATURE NITRIC OXIDE DETECTOR: NITRIC
OXIDE-OZONE CHEMILUMINESCENCE .............. ...............27....


Introducti on ............. ..... ...............27...

Back ground .............. ..........._ ...............27...
Chemiluminescent Detectors ................. ...............27.................
Ozone Generator............... ...............2

Experimental Methods............... ...............30
D etector .............. ...............30....
Electronics ................. ...............3.. 1..............













Sample Preparation ................. ...............3.. 1..............
Re sults ................ ...............32.................
Discussion ................. ...............34.................


4 EXPLOSIVES DETECTION BY 193 nm PHOTOFRAGMENTATION WITH NOCL
FRAGMENT DETECTION ................. ...............43.......... ......


Back ground ............... ... ................ ...............43.......
Material Phase and Distribution ................ ...............43................

Laser Photofragmentation .............. ...............43....
Catalytic Conversion .............. ...............44....

Experimental Methods............... ...............44
D etector .............. ...............44....

Sample Preparation............... ..............4
Re sults................. ...............46..._..._ ......
Discussion .........._....._ ...............47..._..._ ......


5 FROM THE LAB TO THE FIELD .........._....._ ...............56...._..._ ..


LIST OF REFERENCES ................. ...............58........_.....


BIOGRAPHICAL SKETCH .............. ...............62....










LIST OF TABLES


Table page

1-1 Explosive Classes, Examples, and Bond Structure ................. ............... ......... ...14

1-2 Quick Reference of Explosives Detection Methods. ........... ........... .........._......15












LIST OF FIGURES


Figure page

2-1 Computer Modeled NO-O3 Reaction ................. ...............24........... ...

2-2 Signal Change with Chamber Pressure ................. ...............25...............

2-3 Signal Change with Reactant Flow Rate. ............. ...............26.....

3-1 Image of NO-O3 IHStrument ................. ...............36........... ...

3-2 Reactor-PMT Assembly Schematic............... ...............3

3-3 NO-O3 Detector Schematic............... ...............3

3-4 Signal as a Function of Reactor Pressure ................. ...............38..............

3-5 Signal as a Function of Reactant Flow Rates ................. ...............39........... .

3-6 Instrument Response Function and Pumping Loss as a Function of Reactant Flow
Rate s ................ ...............40.................

3-7 Pumping Loss from Reaction Chamber and Model ................. .............................41

3-8 Signal as a Function of NO Concentration .......... ................ ... ............. ..42

4-1 Energy Levels of NO2 and Dissociation to NO.. ............ ...............48.....

4-2 Image of Explosive Sampling Setup............... ...............49.

4-3 Explosive Sample Stage and Optics.. ............ ...............50.....

4-4 Signal from Explosive as a Function of Distance from Droplet Center.. ................... .......5 1

4-5 Signal Measured from Photofragmented RDX and TNT .......... ............. ..... ........._.52

4-6 Signal Measured for Single Laser Pulses on RDX. ................ .......... ................. .53

4-7 Signal Response as a Function of Photofragmentation Amount. ................ ................. 54

4-8 Signal as a Function of Laser Pulse Energy ................. ...............55........... .










LIST OF ABBREVIATIONS AND SYMBOLS


CAD computer aided drafting

CL chemiluminescence

DNT dinitrotoluene

HMX high molecule weight RDX

IED improvised explosive device

IMS ion mobility spectrometer

LIF laser induced fluorescence

LoD limit of detection

NO nitric oxide

NO2 nitrogen dioxide

NO2* excited state nitrogen dioxide

NOCL nitric oxide ozone chemiluminescence

03 OZOne

PETN pentaerythritol tetranitrate

RDX royal demolition explosive

REMPI resonance enhanced multiphoton ionization

STP standard temperature and pressure

TATP triacetone triperoxide

TEA themal energy analyzer

TNB trinitrobenzene

TNT trinitrotoluene









Roman

M number density of air (molecules cm-3 Torr^)

hv radiation (photons)

fNo mass flow of nitric oxide (molecules of NO/s)

kl excited nitrogen dioxide production rate (cm3mOleCUle-1S-1)

k2 grOund state nitrogen dioxide production rate (cm3mOleCUle-1S-1)

k3 photon emission rate (s l)

k4 excited state quenching rate (cm3mOleCUle-1S-1)



Greek


QCL photon emission (photons/s)

Ot photon emission for time t (photons)

Or photon emission up to time t (photons)

(PCL chemiluminescence quantum efficiency dimensionlesss)

(PL luminescence quantum efficiency dimensionlesss)

cpex excitation quantum efficiency dimensionlesss)

5i instrument transfer function

Dwell dwell time in reaction chamber (s)

Zwo reaction lifetime (s)









Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

EXPLOSIVES DETECTION BY PHOTOFRAGMENTATION AND
NITRIC OXIDE-OZONE CHEMILUMINESCENCE:
PORTABILITY CONSIDERATIONS

By

Ronald Whiddon

August 2007

Chair: James Winefordner
Major: Chemistry

The recent popularity of improvised explosive devices, and the continuing threat presented

by unexploded land mines pushes the detection of hidden explosives to the forefront of scientific

research. For maximum utility, a detection device should be handheld, be inexpensive, respond

quickly, have little interference, and detect explosives without direct contact with the explosive

device. Few instruments are available that can meet most of these requirements, primarily

because measuring explosives in the vapor phase demands a sensitivity of low parts per billion to

parts per trillion of explosive material.

The chemiluminescent reaction between nitric oxide and ozone has been used to detect

explosives by their decomposition, which produces nitric oxide. As of yet, the instrumentation

has not been scaled down to the point that it could be assembled as a handheld detector. This

research is the design of a nitric oxide-ozone chemiluminescent reaction chamber that is small

enough to be handheld, while still being able to detect explosives in the vapor phase. A 24 mL

reaction chamber was designed that was capable of detecting mid parts per billion levels of nitric

oxide, and high picogram amounts of TNT. Response times for the instrument were less than

10 s.









CHAPTER 1
EXPLOSIVES: HISTORY, CLASSIFICATION, AND DETECTION

Historical Background

Origins

Asciano Sobrero, not Alfred Nobel, is the father of high explosives. In 1846, while

employed at the Turin School of Mechanics and Applied Chemistry, Sobrero discovered a

method of nitrifying glycerin in a solution of nitric and sulfuric acids, which produced

nitroglycerin, the first high explosive Laboratory accidents quickly convinced Sobrero that

nitroglycerin was far too unstable a compound to work with. Only then did Immanuel Nobel,

Alfred' s father, invent a method to produce large amounts of nitroglycerin with a modicum of

safety

Nitroglycerin like many high explosives does not explode when exposed to flame, but it is

susceptible to ignition through shock. While the compound would burn in a controlled manner,

attempting to initiate detonation was extremely dangerous. In 1864 Alfred Nobel invented the

blasting cap which would ignite high explosives in a controlled manner, greatly increasing the

safety of explosive use.

While the problem of ignition was solved, the shock sensitivity of nitroglycerin made its

transport dangerous. Alfred Nobel and his employees discovered that mixing three parts

nitroglycerin with one part clay reduced this sensitivity, and in 1867 Guhr Dynamite was born.

Eight years later Nobel replaced the clay with nitrocellulose, an explosive in its own right, and

invented shock stable blasting gelatin, or what is now commonly known as dynamite.

All told, Nobel amassed a fortune through the production of blasting materials, but a large

cost. The accidents in his factories claimed the lives of many employees, as well as his own









brother. That, added to the devastation wrought by the Nobel family product, drove Alfred to

establish the Nobel peace prize3

The discovery of nitroglycerin in 1846 opened the floodgate of invention for high

explosives. 1849 saw the emergence of ammonium nitrate, the most highly produced and

abundantly used explosive material; 1863 the invention of TNT, arguably the most famous high

explosive; PETN in 1894; RDX in 1899. The next maj or addition to the high explosives arsenal

did not occur until 1943 when HMX was invented.

Application

The first use of explosives for combat was recorded by Marcus Graecus, who in 700 A.D.1

described rockets used in battle to disorient and demoralize the enemy. The introduction of the

low explosive black powder to Europe in the 13th century by Friar Roger Bacon, and its

subsequent perfection by Schwartz in 13202 quickly changed the nature of war. Heavily fortified

walls that had offered good defense since the dawn of civilization proved useless against

explosive breaching charges. In reality, modern combat is only an updated application of

explosives.

Perhaps the most terrifying aspect of explosives is the collateral damage they effect when

hidden. Anti personnel land mines, the production of which is now outlawed by a United Nations

agreement, still represent a lingering threat to civilians in war ravaged areas. Improvised

explosive devices (IEDs) can be extremely devastating. These devices are military ordnance such

as explosive mortars, anti-vehicle mines, even bombs; modified with makeshift fuses for use as

booby traps. It is in the interest of humanitarian organizations and the military to develop

methods of detecting these explosive hazards.









Classification

There are several methods of classifying explosives. Each in some way relates to a

physical or chemical property of the material, and each has usefulness depending on the field of

application.

Rate

A first level delineation is by rate of reaction. Low explosives, such as black powder,

deflagrate, meaning they burn at a fast rate. This rapid combustion produces a pressure front

which propagates more slowly than the speed of sound Materials that detonate are termed high

explosives. Their high rate of reaction creates a pressure front which expands faster than the

speed of sound, though many can be burned without inducing detonation.

Ignition

High explosives can be further categorized by the stability of the compound. Primary

explosives are those which are unstable in presence of heat or mechanical shock. They don't

deflagrate. Common examples of primary explosives are mercury fulminate, lead azide, or

potassium permanganate. Secondary explosives are those that are both thermally and

mechanically stable. The maj ority of high explosives fall into this category. These explosives

will deflagrate if ignited, but need the energy of primary explosion to set off a detonation2. Most

functional explosive devices require a primary explosive linked to a secondary explosive.

Functional Group

An explosion is effectively a rapid oxidation reaction, and so explosives need both fuel for

combustion and an oxidizing agent. High explosive mixtures, such as ammonium nitrate-fuel oil,

separate these components, whereas molecular high explosives contain both a fuel and an

oxidizer bonded together. It is common practice to classify molecular explosives by their

oxidizing agent. This approach splits explosives into six categories: nitro compounds, nitrate









esters, nitramines, nitrate and chlorate and perchlorate salts, azides, and everything else2

(Table 1-1).

Table 1-1. Explosive Classes, Examples, and Bond Structure
Nitro Nitrate Bster Nitramine *orates Azides Other



TNT, DNT, Nitrocellu- RDX, HMX KICLO3, Lead Azide TATP
TNB lose, RD-20 KZMnO4
Nitroglycerin

/0 /O R 7 Evident frm N=19N / O- O
R-P~ R-ONN Formula Pb R R


Detection of Hidden Explosives

The detection of hidden explosives is dependant on the type of explosive device being

hunted. So, while there are many methods of analysis for explosive materials, few of them are

suitable for detection of hidden explosives. The focus of this research is on the detection of

physically implanted explosives, such as mines or IEDs. Appropriately, only field portable

methods of explosives detection will be reviewed.

Three things are necessary in a hidden explosives detector: short processing time, low limit

of detection, and utility. A short reaction time, less than ten seconds, minimizes the danger to the

instrument operator. A low limit of detection is necessary to negate the effect of concealment as

most explosives have low vapor pressures. Utility is a general consideration for the instrument

operator. To be useful as a detector, the instrument should be easily carried, simple to operate,

have a long duty cycle, and be accurate. A comparison of current portable detection techniques is

shown in table 1-2









Table 1-2. Quick Reference of Explosives Detection Methods.

Canine IMS LIF REMPI CL

Cost ~$50,000 $40,000 $29,000


Vapor Sampling Yes YesP* Yes* Yes* Yes


Limit of Detection Very Low 100 pg 10 ppm 10 ppm ng ppb
1ppm
Examples German GE-Vapor t t IScirrre
Sheppard Tracar E 3500
*Limit of detection is not low enough for actual vapor phase detection. Samples are collected and vapor-
ized at elevated temperature. CL LoD is higher because of detection at room temperature.
torlable Instrument not available.

Canine Detection

The most successful and widely used method of detecting explosives has always been

bomb sniffing dogs6. Frequently used in ports of entry and military bases, and even in combat,

canine teams are required to have greater than 95% accuracy in detecting hidden explosiveS6,7

From a cost standpoint a bomb dog is comparable to $50,000 instrument; however, unlike an

instrument, the dog requires several years of training prior to use, boarding facilities during

deployment, and care in senescence. Add the fact that dogs perform best with a dedicated

handler to limited availability, and the canine detector shows obvious weakness in terms of mass

deployment.

Ion Mobility Spectrometry

The superstar of field deployed explosives detectors is currently the ion mobility

spectrometer (IMS). The reason for their popularity is primarily their small size and light weight.

Essentially the IMS is a time of flight mass spectrometer that operates at atmospheric pressure.

Because of the slower movement of ions at atmospheric pressure, the drift tubes can be short yet

still allow for separation. Fully functioning IMS instruments have been constructed that displace









about V/2 liter At this point most maj or instrument manufacturers offer some type of portable

explosives detector based on IMS9

Despite their warm reception, IMS detectors have a few characteristics that limit their

effectiveness as field deployable instruments. Ionization is most often caused by radioactive

nickel, a non-selective ionization source. Overabundance of ionizable molecules can flood the

drift tube giving false positives. High humidity and low temperature decrease signal by forming

water-analyte clusters and condensing explosive on the IMS inlet filter Additionally, the limit

of detection for IMS is not very good. In Order to detect explosives, the sample must be

concentrated by collecting on a fabric swipe. The fabric swipe is then heated in a closed

sampling chamber to release vapors into the IMS.

Fluorescence

Fluorescence from explosives and explosive vapors is an area of research that has seen

much interest recently. The truly seductive aspect of a fluorescence based explosives detector is

that it would allow remote sensing of an explosive, a feature that is impossible in competing

detection systems. While the fluorescence signal for TNT is indistinctio, the attached nitro

groups can be can be detected in that manner. Ultraviolet radiation clips the nitro groups off the

parent molecule, yielding nitric oxide (NO) and nitrogen dioxide (NO2). The NO absorbs a

photon of 226 nm and is elevated to an electronically excited state. Relaxation from that state

results in emission in the 226-250 nm range depending on the final vibrational level.

The maj or setback in LIF detection of explosives is the poor limit of detection, 10 ppm

TNT at STP, well above the vapor phase concentration of TNT at room temperatures. To lower

the limit of detection it is necessary to put the sample under vacuum, or raise the sample

temperature, making this far from a standoff detection technique.









Resonance Enhanced Multiphoton Ionization

Resonance enhanced multiphoton ionization (REMPI) is similar to laser induced

fluorescence in how signal is generatedl2. Once the nitro group is clipped off the parent molecule

by the absorbance of a UV photon, two more photons interact with the NO molecule. The first

raises NO from ground to an electronically excited state, and the second from the electronically

excited state to the ionization continuum. Signal is measured with a mass spectrometer, or more

simply by a pair of electrodes near the laser focus; either method makes this a non standoff

technique. While detection with the mass spectrometer can detect vapor phase concentrations, it

demands a non hand portable instrument. At present ion current detectors do not reach a

detection limit necessary for gas phase measurementl3

Chemiluminescence

Two chemiluminescent (CL) reactions are currently used to detect explosiveS14. The older

technique detects relaxation of an excited state of NO2 prOduced from the reaction of NO and

Ozone (OS). The newer CL technique measures emission from oxidation of luminol by NO2.

Chemiluminescence detectors have earned a reputation for having wide linear response, low

background, and low limits of detection"

The nitric oxide-ozone chemiluminescence (NOCL) detector is unable to detect explosives

directly. But, as most explosive groups contain multiple nitro groups attached to an organic

molecule, their detection would be possible after freeing those groups from their organic

backbone. One way of accomplishing this would be vapor phase combustion of the explosive,

which will produce NO: this process is usually done with a heated catalyst. On the other hand,

the nitro groups can be clipped off in the same way REMPI and LIF methods accomplish it, i.e.,

by photofragmenting the explosives with UV light. Photofragmentation can be used directly in

the vapor phase, or from a solid surface.









One limitation of CL systems is their lack of absolute specificity. For an NOCL explosives

detector, interference arises from reactions of 03 with sulfur oxide, alkenes, as well as non

explosive related nitrates; although, non nitrogen emission can be excluded with band pass

filters. Luminol is plagued extensively with interference from a variety of oxidants: chlorates,

permanganate, iodine, peroxide, ozone, sulfur dioxide, etcl6

Summary

The potential for damage that explosives offer encourages extensive development of any

new technique or augmentation that improves their likelihood of detection. The research

embodied in this paper is funded by a Department of Defense grant for vapor phase explosives

detection. It is the goal of this research to investigate the miniaturization of a nitric oxide-ozone

chemiluminescence detector, coupled with photofragmentation for explosives detection.









CHAPTER 2
MODELING THE NITRIC OXIDE-OZONE CHEMILUMINESCENCE REACTION FOR A
24 mL REACTION CHAMBER

Introduction

After three failed attempts at making a nitric oxide-ozone chemiluminescence (NOCL)

detector, computer modeling was performed to assist in understanding the nature of the reaction.

The goal was to gain a solid understanding of how the signal would fluctuate with changes in

reactor pressure, NO flow rate, and 03 flOw rate.

Reaction Kinetics

Kinetics of a reaction between NO and 03 which produced NO2 were known as early as

1954", yet the step responsible for emission was largely missed. The first paper to handle the

chemiluminescent pathway of NO and 03 WAS published in 1964 in the Transactions of the

Faraday Society x. Clyne, Thrush and Wayne winnowed NO-O3 reactions down to four that had

appreciable effect on photon emission.

NO + 03 = NO2* + 02 kl = 4.26*10-"5 cm13 mOleCUle-1 S-1 (2-1)
NO + 03 = NO2 + 02 k2 = 1.6*10-14 CM13 mOleCUle-1 S-1 (2-2)
NO2* = NO2 + hv k3 = 1000 s^l; 20 (2-3)
NO2* + M = NO2 + M k4= 1.49*10 "1 cm3 mOleCUle-1 S-1; 21 (2-4)

Steffenson and Stedman used the reactions and kinetics for the NOCL reaction in

simplified form applicable to reactions with ozonel9. The equation (Equation 2-5) predicts the

emission from an NOCL chamber operating in the continuous flow regime by including the

effect that flow, pressure, and kinetics have on the signal.









The term fwo is the number of molecules of NO entering the reaction chamber per second.

It is found by multiplying the flow rate for the NO sample stream, the number density of that

stream, and the mole fraction of NO in the stream. The equation does not allow for a maximum

signal per second Oct greater than the number of molecules of nitric oxide flowing through the

reaction chamber, since fno is the only term in the equation greater than one. In reality Oct is far

less than the number of NO molecules flowing through the reaction chamber.

First, only a portion of the NO molecules are converted to excited state nitrous oxide

(NO2*), the molecule responsible for photon emission. The term cpex is the splitting ratio for this

excited state. It is a constant that represents the number of molecules reacting through the

equation 2-1 pathway as opposed to the equation 2-2 pathway. The value for cpex is 21%. Next,

there is signal loss from quenching, (PL. If NO2* COllides with any other molecule, it will lose

energy and be unable to emit a photon. The Steffenson Stedman splitting ratio is the number of

NO2" mOleCUleS emitting light divided by the number being quenched. This term is pressure

dependant (Equation 2-4).

While the addition of reactants will result in higher signal flux for a flow system, there is

also the possibility of some signal loss by the transient nature of the reactants. In a static system

the reaction goes to completion before the reaction chamber is evacuated and a new mixture

introduced. In the flow system, reactants are added continuously; and, reactants are removed

continuously. This pumping loss is related to the rate at which molecules flow through the

reaction chamber and the time necessary for those molecules to react completely.

The term (1-exp [-tdwell/two]) brings into the equation the pumping loss of the reaction

chamber. The period tdwell is the amount of time it takes a molecule to travel from the inlet of the

cell to the outlet. It is calculated by dividing the volume of the reaction chamber by the flow rate









of all inlets to the chamber. The period two is the lifetime of NO in the reaction chamber. After

several seconds, the NO has been consumed and effectively the reaction is complete. This period

is mathematically defined as the inverse of the sum of reaction rates kl and k2 multiplied with the

mole fraction of 03 in its stream, the number density of the 03 Stream, and the ratio of the 03

flow to total flow.

The instrumental response function is included as 5i. This term is static and unique for any

individual chemiluminescence instrument. The term 5i is a catch all for the quantum efficiency of

the detector, the solid angle of detection, window losses, reflectance losses, spectral overlap of

the detector and emission source, etc. It is always less than one, but can only be determined

through experimentation.

Model System Parameters

The chemical kinetics program Kinetica 2003, programmed by Dr. Richardson (University

of Florida, Physical Chemistry) was used to model the time dependant reaction concentrations

using reactions and rates given in equations 2-1 through 2-4. Kinetic modeling for both a static

signal and flow modified signal was done with a differential equation program (Polymath 6.10)

used to plot the differential equations 2-6 through 2-10. The Steffenson-Stedman equation (2-5)

was plotted using Excel.


[NO]t = [NO]I + (-kl[NO][O3] k2[NO][O3])dt (2-6)
[O3 t = [O3 I + (-kl[NO][O3] k2[NO][O3])dt (2-7)
[NO2 ]t = [NO2 ]I + (kl[NO][O3] k3[NO2 ]I k4[NO2 ]I[M])dt (2-8)
[NO2 t = [NO2]I + (k2[NO][O3] + k3[NO2 ]I + k4[NO2 ]I[M])dt (2-9)
Qt = GI + (k3[NO2*])dt (2-10)









For best applicability to the existing NOCL reactor all of the modeling was performed with

concentrations, pressures, and flow rates that were possible in the laboratory. NO concentration

was set as 10 ppm, 03 COncentration at 1280 ppm.

Results

The temporal evolution of reactants and products, as predicted by Kinetica 2003, is shown

in Figure 2-1. The photon emission is a running tally of the number of photons

produced, not the instantaneous rate of emission. The result is surprising considering that the

reaction emission is supposed to be a two step process. Consistent with many two step reactions

the NO2" WAS expected to build up, with peak emission rates occurring after peak NO2"

production. However, the model puts the peak photon emission rate right at the time of mixing.

This is supported by the rate constants for the two reactions; the rate of production for NO2" is

much slower than the relaxation rate from that state, thus the intermediate cannot aggregate.

Pressure, which is related to the total count of molecules in a volume, is known in many

luminescences to have to do with the quenching of excited molecules by collision energy

transfer. Equation 2-4 is the quenching step in the NOCL reaction pathway. The extent of signal

loss through quenching was calculated with Polymath for several pressures from 0.01 to 100

Torr (Figure 2-2). Although each successive increase in pressure represents a tenfold increase in

concentration of NO, the losses due to quenching net no increase in signal, only an increase in

reaction rate. Hence, CL efficiency (9cpl,p) decreases with increasing pressure. This is true only

for a static system, as reaction rate takes on greater importance for flow systems. To this point,

only static systems have been simulated, but to properly depict the NOCL detector, we must

model under continuous flow dynamics. Plotting equation 2-5 using a reactor pressure of 1 Torr

and flow rates of 100 to 1000 mL/min yields the signal response shown in figure 2-3. The results

show that highest signal is found for about equal flow rates of NO and 03. The basic difference









between this plot and the static systems results is the inclusion of a pumping loss on total signal.

Excel and Polymath were used to solve the series of differential equations 2-6 through 2-10 to

check the validity of the Steffenson Stedman equation. The surface is similar in shape to figure

2-3, but has a hundredfold higher signal.

Discussion

By modeling the chemiluminescent reaction of NO with 03, we are able to come to a

greater understanding of the temporal dynamics of the reaction. The temporal dynamic shows

that pressure will have a two fold effect on efficiency as increasing it will decrease pumping

losses, but increase quenching loss. Finally we see that slightly higher flow rates of NO

compared to 03 flOw will yield the highest signal.

































6.E+10 -"/


4.E+10 -


2.E+10 -


0.E+00 TT:


-*-NO(calc) -- -NO2(calc) --- NO2*(calc) --x-- hy(calc)


2.E+11


r8.E+07


7.E+07


-r
_


1.E+11 -


3 1.E+11


S8.E+10


o
o


6.E+07


5.E+07 E


3.+0





2.E+07


1.E+07


0O.E+00


* -


7- /
*


"`--~------c~


Time (s)


Figure 2-1: Computer Modeled NO-O3 Reaction. The model is of a static system with no
addition of reactants after time zero. Photon signal (hy) plateaus at 7*10 photons at

10 s.













-*-0.01 Torr -u-0.1 Torr -A-1 Torr -u- 10 Torr -%-100 Torr


1.6E+07

1.4E+07

1.2E+07-l

c 1.0E+07

ii 8.0E+06

c 6.0E+06

4.0E+06

2.0E+06


0 1 2 3 4 5 6 7 8 9

Time(s)


Figure 2-2: Signal Change with Chamber Pressure. Reaction chamber pressures are 0.01 Torr A,
0.1 Torr B, 1 Torr C, 10 Torr D, and 100 Torr E. All pressures reach the same
emission limit of 1.5*107 photons.






















i.E+09

E.E+08

r..E+08

.E+08

13.E+08

5.E+08 signal (countsis)

-1.E+08

3.E+08

2.E+08

i.E+08

0.E+00




Nitric Oxide Flow (mLlmin) LO 8 O 8 Ozn Fo mOmn





Figure 2-3: Signal Change with Reactant Flow Rate. The 3D surface plot shows that highest flow
rates of NO and 03 will give the highest signal. At the higher flow rates a plateau
develops as pumping loss has a greater effect than the addition of new reagents.










CHAPTER 3
CHARACTERIZATION OF A MINIATURE NITRIC OXIDE DETECTOR: NITRIC OXIDE-
OZONE CHEMLUMINESCENCE

Introduction

Nitric oxide has the distinct honor of being one of the first eight identified gases. Joseph

Priestley discovered this "nitrous air" along with carbon dioxide, carbon monoxide, sulfur

dioxide, oxygen, and others in his experiments published in 177620. While the emission from the

reaction of NO and 03 can be seen in the night sky, as a dull reddish/brown glow, the

relationship between the emission and reaction was not fully appreciated until Lord Rayleigh, in

the 1920s, discovered the physical phenomenon of the emission from mixtures of NO and 03

Kinetics of the non emissive reaction were found in the 1950s, but the full understanding of the

reaction was not realized until the mid 1960s.

Background

Chemiluminescent Detectors

The end of World War II saw the proliferation of automobiles and coincidently the buildup

of thick brown layers of smog in American citieS21,22. The brown pollutant, NO2, WAS traced to a

byproduct of high temperature combustion, namely NO. When two molecules of NO2 dissolve in

water, nitric acid and nitrous acid are produced. (Reactions 3-1, 2, 3)These acids of nitrogen,

along with sulfuric acid are responsible for acid rain devastation of the seventies and eightieS23

This environmental danger inspired the first wave of research on NOCL detectors24,25


N2 + 02 + 2NO (3-1)
2NO + Old 2NO2 (3-2)
H20 + 2NO2 & HNO2 + HNO3 (3 -3)

Nitric oxide chemiluminescence detectors have an inherent selectivity for nitrogen compounds

because of their emission spectrum. They are also well suited to real time sample monitoring









since many NOCL detectors operate in the continuous flow regime. The first reported NOCL

detector boasted a low ppb limit of detection; however, in order to achieve that low limit, the

instrument was rather large24. Fontijn's reaction chamber was one liter in volume with pumping

rates of 12.5 L/min. While the sensitivity of NOCL detectors was unmistakable, they were by no

means portable.

A reason for such a large reaction chamber is evident from the relationship between signal,

mass flow, and dwell time of the Steffenson-Stedman equation (Equation 2-5). Essentially the

signal is proportional to the mass flow of NO and the reactor dwell time, so to get a large signal,

use a large reactor with high flow rate. The movement away from this strategy was spurred by

the desire to create instruments that could be carried by weather balloons and high flying

aircraft26-28 This meant two things: the NOCL detector must be small and lightweight. Yet, it is

impossible to reduce the volume of the reaction chamber without reducing the dwell time of the

reactants. Likewise, it is impossible to reduce the pumping speed of a reaction chamber without

reducing the mass flow of reactants. Both of these changes would lead to lower signal. To make

up for the loss in signal brought on by miniaturization, researchers focused on ways to make the

reactor more efficient.

Instrument response function is a factor for a CL detector that has nothing to do with

reaction kinetics. It derives from detector amplification, optical collection efficiency, mixing

dynamics etc. In miniaturizing the reaction cell, efforts were made to increase this factor. For

instance, thorough reactant mixing is important in maximizing chamber efficiencyl9,28

Steffenson and Stedman found that 300 ml reaction chambers with different mixing methods

gave different signalsl9. Also, chamber material was important. Coatings that were highly

reflective at infra red wavelengths would also increase collection efficiency.









Ozone Generator

Ozone as an oxidizer is stronger than peroxide. It is a naturally occurring gas created by

the combination of three molecules of oxygen (Equations 3-4, 3-5). The electric field in the


Ol + energy & 2 O (3-4)
20 + 02 + 203 (3-5)


region of an electric arc, or radiation below 200 nm can supply energy for the dissociation. The

resulting radicals combine with oxygen molecules producing 03.

One method of creating 03 is passing oxygen in front of a mercury lamp (hollow cathode,

pin, or arc) where it is photolyzed by the 185 nm line, creating a small portion of 03. A mercury

pin lamp and pure oxygen will produce 2 ppm of OS, a leVel inSufficient for the NOCL detector.

Low levels of 03 can be concentrated by condensing in liquid nitrogen; however, the danger of

explosion is great since a steady boil off is difficult to maintain. Using high power mercury arc

lamps can create higher levels of ozone, but the heat and brilliant UV light can be a safety

problem.

The most efficient way to produce 03 is with an electrical discharge. In nature, the fresh

smell after a thunder storm arises from an increase in 03 COncentration produced by lightning

strikes. The electric field near an electric discharge is strong enough to break oxygen and

nitrogen bonds. The result is a recombination of these atoms into OS, and excited nitrogen which

gives the blue color to a spark. To reproduce this effect in an instrument, a low current high

voltage is attached to an anode housed in a glass tube, inside a cathode29. The dielectric nature of

glass prevents a true spark, so a multitude of tiny electric discharges set up between the glass and

electrode. Ozone generators with 4% concentration outputs are possible when using the electrical









discharge production method. A detriment to the discharge ozone generator is the emitted RF

noise.

Experimental Methods

Detector

The NOCL detector was designed and assembled in house. The reaction chamber has an

interior volume of 24 mL, two thirds the volume of the next smallest reaction chamber in the

literaturel9. The cavity is machined out of aluminum stock, with a 4 inch length and a 2 inch

outside diameter. The inside of the chamber has a dull finish to promote a Lambertian reflection

profile. The surface is not plated as aluminum already has high reflectivity in the infra red. The

front of the chamber is sealed with a 1 inch low pass filter, 425nm cutoff (Edmund Optics).

Mixing ports are machined in the circumference of the reaction chamber just near the front

window with bores connecting to inlet tubes proj ecting from the back of the chamber. Figure 3-1

shows the instrument and a CAD cutaway of the reaction chamber so that the sample movement

and mixing can be considered. Threads were cut in exterior front of the reaction chamber to

mount in the cooled photomultiplier housing (Products for Research Inc.) creating a light tight

interface between reaction chamber and detector (Figure 3-2).

The detector functions under a light vacuum of 1 Torr. It is necessary for drawing in the

sample and also reduces quenching of the NO2. A roughing pump (BOC Edwards 18) is more

than sufficient for creating vacuum at total flow rates between 200 and 2000 mL/min. The

pressure was adjusted with a needle valve placed between the reaction chamber and the roughing

pump, and a Varian dual range pressure gauge was attached between the chamber and pump. The

reaction chamber connects to the vacuum pump with '/ inch stainless steel tubing and Cajon

fittings. To minimize light leakage all tubing was either stainless steel or copper. The two inlet

tubes are 1/8 inch stainless steel tubing. One tube carries the 03 reagent gas from the ozone










generator to the reaction chamber, whilst the other carries the sample stream. Flow in each tube

is controlled by mass flow controllers (Alicat Scientific) (Figure 3-3). The sample line was kept

as short as possible to minimize transit time to the reaction chamber. In the final configuration, it

took 2.5 s from sample introduction to signal acquisition.

Electronics

Emission is converted to an electrical signal by a 28 mm reflection mode pmt (R955,

Hamamatsu Photonic). Internal gain for the R955 is 1X107 and the dark count rate is on the order

of tens of photons per second. To minimize the dark count rate, the pmt was placed in a cooled

housing. The Peltier cooling device drops the pmt temperature to -40 0 C. At this temperature,

the dark count rate was about 2-3 counts/s. The applied voltage to the photocathode was -950 V.

PMT signal is recorded with a Stanford Research SR-400 fast photon counter. To isolate the

photocathode events from dynode noise, the discriminator was set to trigger on the falling edge

of pulses with at least -6 mV intensity. The photon counter was interfaced to the computer

through a GPIB interface (USB-GPIB, National Instruments)

Sample Preparation

At this point the main goal of research was to optimize the flow and pressure settings in the

newly constructed instrument. All the reactants were used in known concentrations. Ozone

reagent produced by the AC-500 is factory appraised at 500 mg/L which at the onboard flow rate

of 1.5 L/min translates to 2500 ppm. An NO calibration standard (spectra gases 10 ppm NO,

balance N2) WAS used to complete the reagent mix. In some cases, it was necessary to dilute the

NO reagent. In these cases, two flow controllers were attached through a tee to the sample inlet

line, one carrying the calibration standard and the other carrying house nitrogen.










Results

Since this instrument was designed and assembled in the lab, a thorough optimization

regime is needed to appraise its capabilities. The two variables that need to be assessed are

reaction cell pressure and reactant flow rate. Figure 3-4 shows how the instruments response

varies with reaction chamber pressure. Flow rates for NO and 03 were 600 mL/min each. The

emission signal peaks at 10 Torr; however, the efficiency defined as measured signal in

photons/s divided by the number of molecules of NO/s, peaks earlier at 1 Torr. So while higher

signal occurs at a higher pressure, a 1 Torr reaction cell pressure will make better use of the

sample. This should translate to a steeper response curve for the 10 Torr chamber and a

correspondingly higher limit of detection. The drop off in efficiency from above 1 Torr is likely

caused by the quenching of signal, while the drop of below 1 Torr is probably due to a too short

dwell time in the reaction chamber.

A flow dependant signal surface is shown in Figure 3-5. Flow optimization was performed

by measuring the signal generated for an array of flow rates of 03 and NO that ranged from 100

to 1000 mL/min in each reagent. The measured signal (Oct) can be substituted into the

Steffenson Stedman equation in order to determine the instrument response function 5i. The

first step in determining the instrument response function is converting the chemiluminescent

signal, Ocl, to the CL quantum efficiency cp1. The term cpa is equal to the product of cpex, (PL, (-

exp[-tdwell/two]), and 5i.

Dividing the functions cpex, cpL into the CL quantum efficiency reduces the signal to its (1-

exp[-tdwell/two]) and 5i components. The term cpex is constant and can be directly divided out. The

term (PL is Variable with the reaction chamber pressure, but since all measurements were made at

1 Torr, it is universally applied to each measurement, and thus can be removed in the same

manner as cpex.









At this point the signal has been reduced to the value of (1-exp[-tdwell/two]) and 5i

multiplied. When the dwell time is much greater than the reaction lifetime, (1-exp[-tdwell/two]) is

nearly 1. The plateau seen in figure 3-6 satisfies this condition and hence is the measure of 5i.

The instrument response factor this reactor is approximately 2.47*10-6 dimensionlesss), with

error of 6.3E- 5i doesn't vary with pressure, flow, or reagent concentration; so, dividing it out

leaves only pumping loss. Figure 3-7 depicts the cell's pumping loss as a function of NO flow

for 300 mL/min 03 and a function of 03 flOw for 900 mL/min NO. Also included in that figure is

the pumping loss predicted by the Steffenson Stedman equation (2-5). Pumping loss appears to

be proportional to a function of 03 maSs flow and the inverse to the NO mass flow, but it is not a

direct relationship. It seems that the Stefenson-Stedman approximation over accounts for signal

loss due to material transfer through the chamber, but this is not because of the pumping loss.

The inaccuracy is caused by accounting only for emission from the molecules of NO that enter

the chamber during the one second integration, and excluding emission from molecules that have

been in the reaction chamber for the entire dwell time.

The final operation to be performed on this NOCL detector is the evaluation of its

analytical response, and the calculation of the limit of detection. Figure 3-8 depicts the signal for

the NOCL signal measured for 1 Torr of NO at concentrations from 0.50 to 10 ppm. Flow rates

for NO and 03 were each set at 600 mL/min. The experimental limit of detection, obtained under

our present conditions, is approximately 300 ppb. This figure has been calculated from the

conventional definition of the limit of detection, i.e., for a signal being 3 times the standards

deviation of the average background signal









Discussion

A small NOCL detector was designed and constructed by our laboratory. The detector was

among the smallest in existence for ozone-nitric oxide chemiluminescence. The instrument was

optimized for highest effciency by adjusting instrument flow rate and reaction cell pressure.

In the end a fair evaluation of this instrument is that it does not have the sensitivity needed

for atmospheric NO monitoring. It is disappointing that the signal is so low considering that a

240 mL cell with similar mixing designed achieved pptr levels of detection28

Although the experimental limit of detection was found to be insuffcient for the detection

of explosive vapors, one should stress that this limit was obtained with a non-optimized setup.

Indeed, a much lower limit is possible with optimized signal measurement. The spectral overlap

between the PMT and the NOCL emission is from 600 to 900 nm. The average PMT quantum

effciency for this range is 0.041, while the emission signal represents only 6.3 percent of the

total emission30. Multiplying the PMT quantum effciency by the average of percent total

emission from 600 to 900 nm yields a value of 6.5*10-4 dimensionlesss). Dividing the measured

signal by this factor gives the possible signal at 100% PMT quantum effciency, and 100%

spectral overlap, e.g. the signal measured at 0.59 ppm NO would optimally be 3.4*105 photons/s.

The noise associated with the measurement is most likely from PMT dark current, and if there

were no increase in noise in the optimal system, the limit of detection would improve to 15.6

pptr. Hence, this NOCL detector could be vastly improved merely by using a better signal

detection method, such as an infra red sensitive avalanche photodiode.

As previously stated, the instrument response function arises from a variety of non-reaction

components that effect signal. This instrument' s 5i of 2.47*10-6 dimensionlesss) can be

accounted for by estimation of some of the known efficiencies of the detector components.









The efficiency loss from the PMT, as calculated above, is 6.5*1 0-4(dimensionless), leaving

a loss of 3.8*10-3 dimensionlesss) from other sources, such as solid angle, window losses, etc.

As a result of the extensive measurements of signal at various flow rates, it is evident that

the Steffenson-Stedman equation is not wholly accurate. Currently the equation only considers

molecules that enter the cell during the detector integration time. The number of NO molecules

should be expanded to include molecules that are in the cell for the entire dwell time, which

ranges from 7.20 to 0.72 s over 200 ml/min to 2000 ml/min total flow rate.




























Figure 3-1: Image of NO-O3 IHStrument. Inset is cutaway of reaction chamber to highlight the
circumferential mixing.


r-------l /Pumping Port Ir---
NO Inet~'-d~ 03 I let


Reaction
Chamber Long Pass Filter


Sig~rnal Out/ Quartz Lens
Voltage In IL_ __



PMT/

it I Cooled
Housing

Figure 3-2: Reactor-PMT Assembly Schematic. Mixture of NO and 03 enter the chamber near
the long pass filter, and exit through the pumping port. A 1.5" focal length lens
focuses light onto the PMT photocathode. Drawn to scale.









































Figure 3-3: NO-O3 Detector Schematic. Ozone from the generator and NO from a sample source
flow through mass flow controllers, are mixed, and pumped out. Pressure is
controlled by opening or closing the needle valve. Drawn to scale.

















Mass Flow(molecules NOls)
5.2E+13 1.0E+14 1.5E+14 2.0E+14


1.6E+12
4500 +--


4000 -


3500-


3000-


~2500


S2000


1500\


1000-


500


2.5E+14 3.0E+14


8.E-10


7.E-10


6.E-10


5.E-10


4.E-10 .


3.E-10


2.E-10


1.E-10


0.E+00


0.5 10.5 20.5 30.5 40.5 50.5 60.5 70.5 80.5 90.5
Pressure (Torr)


Figure 3-4: Signal as a Function of Reactor Pressure. The diamond plot is of the actual signal,
while the square plot is the reaction efficiency (Photons / Molecules NO).






















-6000


-5000


-4000


-00 Signal
(counts/s)

-2000


-1000


r,


"oo X" ozne vlume Flow
Poo a a (mLlmin)


NO Volume Flow
(mL/min)


Figure 3-5: Signal as a Function of Reactant Flow Rates. 3D surface shows rapid increase in
signal, but no plateau is formed as in the model result.
















4.00E-06

3.50E-06

3.00E-06

2.50E-06
2.0-6 Efficiency

-1- \; 1.50E-06
1.00E-06

5.00E-07

-0.00E+00



NO Mass FlowL a"~, g8~ m
(molecules/mrin) 6O __ mm 0 agg Ozone Volume Flow
OmDWa + -- (mL/min)




Figure 3-6: Instrument Response Function and Pumping Loss as a Function of Reactant Flow
Rates. Dividing the Signal by the NO mass flow and the terms cpex and cpL leaves
only the constant instrument response and the variable pumping loss.




















-*-300mlpm Ozone -E -- 300mlpm Ozone SS -x--900mlpm NO ----900mlpm NO SS


Ozone Mass Flow (moleculests)

4.0E+14 6.0E+14 8.0E+14 1.0E+15


0.0E+00 2.0E+14
140% C


1.2E+15 1.4E+15 1.6E+15
S140%


1209'o




100%


120%


--

I ~.
I
i.

r I


I
80%
rn
c

E
~ 60%
a


-~- --~--"'K


20% /


S0%
600E+12


000E+00


100E+12


200E+12


300E+12

NO Mass Flow (moleculests)


400E+12


500E+12


Figure 3-7: Pumping Loss from Reaction Chamber and Model. Measured pumping loss is shown

in solid traces, model pumping losses in dashed lines. Negative slopes are pumping

loss as a function of NO flow rate, positive slopes are Ozone flow rate dependant



















3500


3000-


2500-



2000-



S1500-


1000-


500-




0 1 2 3 45 67 8 910
[NO] ppm



Figure 3-8: Signal as a Function of NO Concentration. The linear response of the instrument,
with error bars is shown. The limit of detection is about 300 ppb NO.










CHAPTER 4
EXPLOSIVES DETECTION BY 193 nm PHOTOFRAGMENTATION WITH NOCL
FRAGMENT DETECTION

Background

Material Phase and Distribution

The vapor phase concentrations for explosives are very low. Clausius-Clapeyron

expressions for RDX and TNT are shown in equations 4-1 and 4-2. At 298 K, the calculated

concentrations are 6.00 ppb and 9.5 ppb respectively31. Even with complete conversion of nitro

groups to NO, the concentration is below the limit of detection for the NOCL detector studied.

Sampling from a crystallized explosive effectively increases the concentration above normal

vapor phase concentrations.

Log [RDX] (pptr) = -6473/T + 22.50 (4-1)
Log [TNT] (ppb) = -5481/T + 19.37 (4-2)

Deegan and coworkers performed extensive work on the material dispersion in droplets.

Most notable is the presence of a thick ring at the edge of the deposit. As the droplet dries,

evaporation happens uniformly over the surface32. At the edge of the droplet, the curvature

means that there is a higher surface to volume ratio than at the relatively flat center of the

droplet. Evaporation draws liquid to the edge, as well as suspended particles. By the time the

droplet has dried, 90% of the material is contained in the outer ring33

Laser Photofragmentation

There are two possible laser interactions that produce nitric oxide fragments from

explosive molecules. The most important is the cleavage of the nitro-carbon or nitro-amide bond

on the explosive. The bond energies are about 50 kcal/mol. This bond energy equates to a photon

wavelength less than 570 nm, and researches have detected fragmentation at wavelengths than









193, 227 nm, and 454 nm31,34,35. It is unclear whether the photolysis at 454 nm is due to a two

photon process, or direct destabilization of the N-C bond. Products from cleavage are NO, NO2,

and carbon compounds.

A second laser interaction is the photolysis of NO2. When NO2 is impacted by a photon,

this time with an energy equivalent to 250 nm the NO2 will be excited to a semi stable state

where the molecule rapidly dissociates into atomic oxygen and NO (Figure 4-1)36-38, NO2 has

essentially the same absorption cross section as TNT and nitromethane, and is known to

photolyze at the same laser wavelengths39,40. Maximum yield of photolysis product occurs with

nanosecond laser pulses as faster pulses cause multiphoton ionization ofNO2.

Catalytic Conversion

In this instrument, the laser pulse which produces nitric oxide by photofragmentation of

explosive and also photolysis of nitrogen dioxide. The photofragmentation of explosive will

produce a mixture of NO and NO2, however the production ratio is not known. It is necessary to

use a second conversion technique to measure the amount of NO2 prOduced by

photofragmentation. Many transition metals are capable of converting NO2 to NO when heated at

400-600 0 C. Conversion efficiencies for molybdenum, gold, and stainless steel are 100%, while

platinum-gold alloy, and carbon are 57 and 95% respectively25,41-43 These efficiency values

represent optimum values and ultimately depend on converter design

Experimental Methods

Detector

The detector used for this portion of research is the same as that used in Chapter 3. The

reaction chamber is an in house design displacing 24 mL. The chamber is held at vacuum by a

roughing pump. Flow rates and chamber pressure are controlled by a pair of mass flow

controllers on the two inlet tubes and a manual needle valve between the chamber and pump.









The ozone reagent is produced with an EC-500 ozone generator attached through the mass flow

controller to one of the chamber inlet tubes. Signal is captured by a PMT and recorded by a fast

photon counter. To handle solid explosive samples a sample stage was designed and machined

from aluminum (Figure 4-2). The backside of the stage has a machined surface that holds a 1

inch glass slide. The center of the recessed area has a hole covered with a quartz window to

allow the laser beam to impact the sample. A small channel runs through the back of the sample

stage and connects to a 1/8 inch Swagelok adapter. This channel carries air across the sample and

into the reaction chamber via the mass flow controller.

A 193 nm argon fluoride excimer laser (GAM-15, GAM Inc. Orlando FL) is used to

photofragment the explosive sample. The laser' s pulse energy maximum is 15 mJ, the pulse

length 10 ns. The laser shots were triggered externally and ranged from 0.1 to 2000 Hz. The laser

was focused onto a 2 mm spot diameter on the surface of the slide by a quartz 3 inch focal length

lens located between the laser and sample holder. A mechanical shutter placed between the laser

and the quartz lens allowed precise control of shot number (Figure 4-3). This was necessary

because the laser energy was unstable for the first twenty or so shots.

When conversion of NO2 to NO was necessary, it was done using a stainless steel

converter. An 18 inch length of 1/8th inch diameter stainless steel tubing was bent into a coil and

connected between the sample mass flow controller and the reaction chamber. The coil was

heated in a sand bath to 350 o C with a hot plate.

Sample Preparation

Samples used in this experiment were obtained from Chem Services and the office of

Naval Surface Warfare Center. The samples were 2.0% mass/volume TNT in acetone and 2.5%

mass to volume RDX in acetonitrile. For all experiments the volume of solution deposited was

5 uL. Samples were deposited on acetone rinsed 1 inch glass slides. The sample was crystallized










by solvent removal in a gentle dry nitrogen stream. The crystallized droplets were generally

circular and slightly less than 1 cm in diameter. For limit of detection experiments the samples

were diluted in the appropriate solvent (Acetone, 99.9%, Acetonitrile, 99.95% Fisher) prior to

droplet deposition.

Results

To get an estimate of the amount of explosive ablated, it was necessary to find the general

distribution of explosives in the dried droplet. Figure 4-2 inset shows the build up of a visibly

raised ring on the edge of the droplet. A study of material distribution was performed to see if

crystal forming explosive solution might have a different distribution compared to a suspension.

This was done by measuring the NOCL signal wrought from photofragmenting explosive at

positions across the dried droplet. To minimize the error from differences in size and shape of

the droplet, the signal is reported in terms of the ratio of laser spot position to the radius of the

droplet (Figure 4-4). It is difficult to get the amount of explosive in the droplet edge because of

the poor spatial resolution the 2 mm beam diameter affords, but an estimate is shown for the

droplet edge as the outer 10% of the droplet radius.

The amount of material in the droplet edge is found to be roughly 63% of the nonvolatile

material. This was determined by revolving the suggested signal (Figure 4-4), which creates a

disk with a raised edge. Dividing the volume of the edge by the total volume gives the percent of

material in the edge. With this same method, the amount of material covered by the laser spot is

found to be 0.45% of the total nonvolatile material. From the solution concentrations and

volumes deposited, about 6 ng of explosive were in the laser' s sample volume. The average

signals collected by ablating the ~6 ng of RDX and TNT are shown in Figure 4-5. There is a

large variance in the measured signal, which is caused by variations in sample crystallization.










Despite the low limit of detection this NOCL detector exhibited for nitric oxide, extremely

small amounts of explosive could be measured. With the laser power at 8 mJ, a single laser pulse

would produce a measurable quantity of NO from RDX, with all the material in the beam path

removed by the eleventh pulse (Figure 4-6). Figure 4-7 shows the signal at various amounts of

TNT; the smallest signal representing 0.5 nanograms of explosive in the beam path. Experiments

with laser beam energy did not show a definite link of beam energy with signal.

It is possible that the laser beam energy is capable of photofragmenting explosive in one

step of the pulse, and photolyze some of the NO2 prOduced in the same pulse. This was probed

by measuring the signal at three laser pump voltages with and with out the presence of a heated

stainless steel converter (Figure 4-7) the data did not show an increase in the amount of nitric

oxide produced, but rather the higher laser energy had a higher portion of nitrogen dioxide. It is

plausible that the higher laser energies showed a higher nitrogen dioxide content because of the

ablation of explosive rather than photofragmentation of NO2.

Discussion

It was somewhat of a surprise that the detector was able to see any signal at all for

explosive. However, this doesn't mean that there would be enough sensitivity to see vapor phase

explosives. Even at the lowest concentration of ablated explosives there would only be 1012

molecules of explosive in the sample area. Atmospheric pressure would mean a sensitivity of

mid parts per billion; however, the laser probe volume would have to be that entire 1 mL.















NO& O




S2Bi








ON-O Bond Length(~A.U.)


Figure 4-1: Energy Levels of NO2 and Dissociation to NO. Excitation to the 4A2 State induces
dissociation of NO2 to NO. NOCL emission arises from the 2B1 to 2A1 transition.
Adapted from 36.



































Figure 4-2: Image of Explosive Sampling Setup. Inset is of crystallized TNT on a 1 inch glass
slide. The drop diameter is roughly 1 cm and two holes are laser ablation spots.




























Sample\
Holder Window Lens Shutter Iris

Figure 4-3: Explosive Sample Stage and Optics. Beam is trimmed to about 1/4 inch diameter by
a variable iris. The mechanical shutter opens with a remote shutter cable. A 1.5 inch
quartz lens focuses the beam through a quartz window onto a glass slide mounted on
the backside of the sample holder. Ablated material flows into the NOCL detector
(Figure 3-3).













-* Measured Signal e- Suggested Distribution
2.00E 106

1.80E+06 I ~-- -

1.60E+06

1.40E+06

8 1.20E+06

S1.00E+06I

(I5 8.00E+05

6.00E+05 / I

4.00E+05 / I

2.00E+05

D.00E+00
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 110% 120%
r/R

Figure 4-4: Signal from Explosive as a Function of Distance from Droplet Center. The solid line
represents the measured signal from explosive droplets. The dashed line is the
suggested distribution if resolution were better. R is the radius of the droplet, while r
is the distance from the center of the laser spot to the center of the droplet.











O









































: Signal Measured from Photofragmented RDX and TNT. The instrument response to
both explosives is equal. Error in TNT signal is shown and very high for peak. This is
partially due to very slight differences in the time axis between individual
measurements.


O- 4-- Average RDX -m- average TNT


19750




14750-




S9750-




4750-




-2501
1




Figure 4-5


6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96 101 106 111 116
Time (s)












11000-
---Background1
10000 -RX

9000 -RX

8000 -RX

7000-

6000-

S 5000-

c 4000-

3000-

2000-


1000O -icj- IaA i:

10 30 50 70 90 110 130 150 170 190 210 230 250 270 290

Time (s)


Figure 4-6: Signal Measured for Single Laser Pulses on RDX. 0.05 Hz laser rep rate was used to
see if single shot detection was possible. All material in laser spot area is removed by
10 laser pulses.















120000


i I I
Ill l
100000-




80000-




60000-




40000-




20000-






O 2E+14 4E+14 6E+14 8E+14 1E+15 1.2E+15

Molecules TNT


Figure 4-7: Signal Response as a Function of Photofragmentation Amount. TNT sample was
diluted with Acetone to lower the total amount of crystallized explosive in the laser

spot area. Straight line is linear fit with zero intercept












200000


180000-

160000-

140000-

120000-

E 1000000

80000-

60000-

40000-

200000


11kV 11kV-c 12kV 12kV-c 13kV 13kV-c

Laser Pump Voltage

Figure 4-8: Signal as a Function of Laser Pulse Energy. Laser pulse energy was varied to observe
any photolysis of NO2 at higher laser powers. Bars with -c suffix have a stainless steel
catalyst present. Higher laser powers do not indicate photolysis occurs, as the
converted signal is greater in each case, indicating that at higher laser powers more

NO2 is present.









CHAPTER 5
FROM THE LAB TO THE FIELD

Summary

The original aim of research was to design a fully functioning, field portable, explosive

vapor detector. And to that end, this instrument is a total failure. The limit of detection is not low

enough to measure vapor phase concentrations of explosives, and frankly the detector is still too

bulky for deployment

There are some simple changes that could be made to should yield better signal. The

spectrum of nitric oxide lies largely in the infra red beyond the cutoff of the R955. As shown in

Chapter 3, only 0.65% of the available signal can be registered because of the signal detection

method. Some sources report that placing a nickel mesh in the reaction chamber shifts the

emission spectrum to higher energy, moving the peak 1250 nm in the normal NOCL reaction to

800 nm in the catalyzed NOCL reaction. This would put a much larger portion of the signal in

the spectral response range of the PMT44,45. Replacing the PMT with a large surface area

avalanche photodiode, such as those offered by Advanced Photonics, would net a tenfold

increase in detector quantum efficiency, as well as coverage of the entire NOCL emission

spectrum. Additional physical benefits of using a photodiode would be a larger solid angle of

collection by replacing the front window of the reaction chamber with the detector. And a size

reduction since the thermoelectric cooling in the photodiode module is located on chip.

Even though the NOCL reaction chamber is only 24 mL, the roughing pump, cooling

system, ozone generator, pmt, power supply, and especially the laser add up to an instrument that

takes up four feet of bench space and weighs more than 100 lbs. Most of these components can

be replaced with smaller devices without loosing sensitivity. Using a photodiode instead of the

PMT would cut out the cooling apparatus, large power supply, and housing. The roughing pump









is far in excess of what is needed for the system. Tinkering in the lab yielded an ozone generator

that was only a few inches long. But most important is the laser.

Laser photofragmentation is interesting, but unnecessary. Any source of high intensity

ultraviolet light can photolyze NO2 and should also be able to photofragment explosives. An

emerging product that offers the monochromocity of a laser, but with a smaller size and lower

energy use is the excilamp. Once developed, it could be directly substituted for the excimer

laser46. Another photofragmentation device, developed by the NOAA laboratory, uses a 200 W

mercury arc lamp focused into a quartz cell to power photolysis47. Otherwise it is possible to

completely avoid using photofragmentation to produce nitric oxide. Catalyzed pyrolysis, as used

in the TEA detector, would be a tremendous savings in both weight and energy over the excimer

laser, and the mercury arc lamp. Since photofragmentation is prone to the same sources of

interference as the TEA conversion, there is no loss in specificity.









LIST OF REFERENCES


1. Dolan, J., Langer, S. Explosives in the Service of2an (The Royal Society of Chemistry,
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5. Dick, R. A., Fletcher, L. R., D'Andrea, V., A. Explosives and blasting procedures
manual. Department of the Interior, Bureau of Mines IC 8925 Washington D.C. (1983).

6. Furton, K. G., Myers, L. J. The scientific foundation and efficacy of the use of canines as
chemical detectors for explosives. Talanta 54, 487-500 (2001).

7. Nambayah, M., Quickenden, T. I. A quantitative assessment of chemical techniques for
detecting traces of explosives at counter-terrorist portals. Talanta 63, 461-467 (2004).

8. Li, F., Wie, Z., Schmidt, H., Sielemann, S., Baumbach, J. I. Ion mobility spectrometer for
online monitoring of trace compounds. Spectrochim Acta, PartPPPPP~~~~~~~PPPPPP B 57, 1563-1574 (2002).

9. Ewing, R. G., Atkinson, D. A., Eiceman, G. A., Ewing, G. J. A critical review of ion
mobility spectrometry for the detection of explosives and explosive related compounds.
Talanta 54, 515-529 (2001).

10. Arusi-Parpar, T., Heflinger, D., Lavi, R. Photodissociation followed by laser induced
fluorescence at atmospheric pressure and 24 Celsius: a unique scheme for remote
detection of explosives. Appl. Opt. 40, 6677-6681 (2001).

11. Wu, D., Singh, J., Yueh, F., Monts, D. 2,4,6-Trinitrotoluene detection by laser
photofragmentation laser induced fluorescence. Appl. Opt. 35, 3998-4003 (1996).

12. Clark, A., Ledingham, K., Marshall, A., Sander, J., Singhal, R. Attomole Detection of
Nitroaromatic Vapours Using Resonance Enhanced Multiphoton Ionization Mass
Spectrometry. Analyst 118, 601-607 (1993).

13. Cabalo, J., Sausa, R. Trace detection of explosives with low vapor emissions by laser
surface photofragmentation-fragment detection spectroscopy with an improved ionization
probe. Appl. Opt. 44, 1084-1091 (2005).

14. Baeyens, W., Garcia-Campana, A. Chemiluminescence in Analytical Chemistry (Marcel
Dekker, New York, 2007).










15. Jiminez, A. M., Navas, M. J. Chemiluminescence detection systems for the analysis of
explosives. J. Haz. Mat. 106A, 1-8 (2004).

16. Isaacson, U., Wettermark, G. Chemiluminescence in Analytical Chemistry. Anal. China.
Acta 68, 339-362 (1974).

17. Johnston, H., Crosby, H. Kinetics of the fast gas phase reaction between ozone and nitric
oxide. J. Chens. Phys. 22, 689-692 (1954).

18. Clyne, M. A., Thrush, B. A., Wayne, R. P. Kinetics of the chemiluminescent reaction
between nitric oxide and ozone. Trans. Faraday~FFF~~~~FFF~~~FFF Society 60, 359-370 (1964).

19. Steffenson, D., Stedman, D. Optimization of the operating parameters of
chemiluminescent nitric oxide detectors. Anal. Chent. 46, 1704-1709 (1974).

20. Priestley, J. Observations on different kinds of air. Philosophical Trans. 62, 147-264
(1772).

21. Thomas, M. D., et al. Automatic apparatus for determination of nitric oxide and nitrogen
dioxide in the atmosphere. Anal. Chent. 28, 1810-1816 (1956).

22. Friedel, R. A. Spectrometric investigations of atmospheric pollution. Anal. Chent. 28,
1806-1810 (1956).

23. Driscoll, C., et al. Acid rain revisited, advances in scientific understanding since the
passage of the 1970 and 1990 clean air act amendments. Science Links Publications 1,
1-24 (2001).

24. Fontijn, A., Sabadell, A., Ronco, R. Homogenous chemiluminescent measurement of
nitric oxide with ozone. Anal. Chent. 42, 575-579 (1970).

25. Joseph, D., Spicer, C. Chemiluminescence method for atmospheric monitoring of nitric
acid and nitrogen oxides. Anal. Chent. 50, 1400-1403 (1978).

26. Dickerson, R., Delany, A., Wartburg, A. Further modification of a commercial NOx
detector for high sensitivity. Rev. Sci.htst. 55, 1995-1998 (1984).

27. Kondo, Y., Iwata, A., Takagi, M. Balloon-borne chemiluminescent sonde for the
measurement of tropospheric and stratospheric nitric oxide. Rev. Sci.htst. 55, 1328-1332
(1984).

28. Ridley, B. A., Grahek, F. A small, low flow, high sensitivity reaction vessel for NO
chemiluminescence detectors. J. Atnzos. Ocean. Tech. 7, 307-311 (1990).

29. Stone, E., et al. Lightweight ozonizer for field and airborne use. Rev. Sci. hzst. 53, 1903-
1905 (1982).











30. Hamamatsu, R928, R955 Spec. Sheet. (1997).


31. Swayambunathan, V., Singh, G., Sausa, R. Laser photofragmentation-fragment detection
and pyrolysis-laser-induced fluorescence studies on energetic materials. Appl. Opt. 38,
6447-6454 (1999).

32. Deegan, R., et al. Capillary flow as the cause of ring stains from the dried liquid drops.
Nature 389, 827-829 (1997).

33. Deegan, R. Pattern formation in drying drops. Phys. Rev. E 61, 475-485 (2000).

34. Simeonsson, J., Lemire, G., Sausa, R. Trace detection of nitrocompounds by ArF laser
photofragmentation/ionization spectrometry. Appl. Spectrosc. 47, 1907-1912 (1993).

35. Nagakura, S. Ultra-violet absorption spectra and pi-electron structures of nitromethane
and the nitromethyl anion. Mol1. Phys. 3, 152-162 (1960).

36. Gonzalez, A., Larson, C., McMillan, D., Golden, D. Mechanism of decomposition of
nitroaromatics. Laser-powered homogeneous pyrolysis of substituted nitrobenzenes.
J. Phys. Chent. 22, 4809-4814 (1985).

37. Hancock, G., Morrison, M. The 193 nm photolysis of NO2: NO(v) vibrational
distribution, O( D) quantum yield and emission from vibrationally excited NO2.
M~ol. Phys. 103, 1727-1733 (2005).

38. Sun, F., Glass, G. P., Curl, R. F. The photolysis of NO2 at 193 nm. Chens. Phys. Lett. 337,
72-78 (2001).

39. Ledingham, K., Kosmidis, C., Georgiou, S., Couris, S., Singhal, R. A comparison of the
femto-, pico-, and nano-second multiphoton ionization and dissociation processes of NO2
at 248 and 496 nm. Chens. Phys. Lett. 247, 555-563 (1995).

40. Pastel, R., Sausa, R. Spectral differentiation of trace concentrations of NO2 frOm NO by
laser photofragmentation with fragment ionization at 226 and 452 nm: quantitative
analysis of NO-NO2 mixtures. Appl. Opt. 39, 2487-2495 (2000).

41. Winer, A. M., Peters, J. W., Smith, J. P., Pitts, J. N. Response of commercial
chemiluminescent nitric oxide-nitrogen dioxide analyzers to other nitrogen-containing
compounds. Eny. Sci. Tech. 8, 1118-1121 (1974).

42. Bollinger, M. J., Sievers, R. E., Fahey, D. W., Fehsenfeld, F. C. Conversion of nitrogen
dioxide, nitric acid, and n-propyl nitrate to nitric oxide by a gold-catalyzed reduction with
carbon monoxide. Anal. Chent. 55, 1980-1986 (1983).










43. Matthews, R. D., Sawyer, R. F., Schefer, R. W. Interferences in chemiluminescent
measurement of nitric oxide and nitrogen dioxide emissions from combustion systems.
Eny. Sci. Tech. 12, 1092-1096 (1977).

44. Kenner, R., Ogryzlo, E. Orange chemiluminescence from NO2. J. Chem. Phys. 80, 1-6
(1984).

45. Hartek, P., Reeves, R. Formation and reactions of the excited 02(A3 +u) mOleCUleS.
Discuss. Faraday~FFF~~~~FFF~~~FFF Soc. 37, 82-86 (1964).

46. Tarasenko, V. F. Excilamps as efficient UV-VUV light sources. Pure Appl. Chem. 74,
465-469 (2002).

47. Ryerson, T. B., Williams, E. J., Fehsenfeld, F. C. An efficient photolysis system for fast-
response NO2 meaSurement. J. Geophys. Res. 105, 26447-26461 (2000).









BIOGRAPHICAL SKETCH

Ronald James Louis Whiddon was born in Mission Viejo, California. He attended

Bemidji State University in Bemidji, Minnesota, graduating with a Bachelor of Science degree in

biology and chemistry. In fall 2002, he enrolled in the Analytical Division of the Chemistry

Department at the University of Florida. Under the direction of Professor J. D. Winefordner, he

completed his graduate studies with a Master of Science in August 2007.





PAGE 1

1 EXPLOSIVES DETECTION BY PHOTOFRAGMENTATION AND NITRIC OXIDE-OZONE CHEMILUMINESCENCE: PORTABILITY CONSIDERATIONS By RONALD WHIDDON A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2007

PAGE 2

2 Ronald Whiddon

PAGE 3

3 To my father, who is a model of wisdom, patience and charity.

PAGE 4

4 TABLE OF CONTENTS Page LIST OF TABLES................................................................................................................. ..........6 LIST OF FIGURES................................................................................................................ .........7 LIST OF ABBREVIATI ONS AND SYMBOLS............................................................................8 CHAPTER 1 EXPLOSIVES: HISTORY, CLASSIFICATION, AND DETECTION................................11 Historical Background.......................................................................................................... ..11 Origins........................................................................................................................ .....11 Application.................................................................................................................... ..12 Classification................................................................................................................. .........13 Rate........................................................................................................................... .......13 Ignition....................................................................................................................... .....13 Functional Group.............................................................................................................13 Detection of Hidden Explosives.............................................................................................14 Canine Detection.............................................................................................................15 Ion Mobility Spectrometry..............................................................................................15 Fluorescence................................................................................................................... .16 Resonance Enhanced Multiphoton Ionization.................................................................17 Chemiluminescence.........................................................................................................17 Summary........................................................................................................................ ..18 2 MODELING THE NITRIC OXIDE-OZ ONE CHEMILUMINESCENCE REACTION FOR A 24 mL REACTION CHAMBER...............................................................................19 Introduction................................................................................................................... ..........19 Reaction Kinetics.............................................................................................................. ......19 Model System Parameters......................................................................................................21 Results........................................................................................................................ .............22 Discussion..................................................................................................................... ..........23 3 CHARACTERIZATION OF A MINIATURE NITRIC OXIDE DETECTOR: NITRIC OXIDE-OZONE CHEMILUMINESCENCE........................................................................27 Introduction................................................................................................................... ..........27 Background..................................................................................................................... ........27 Chemiluminescent Detectors...........................................................................................27 Ozone Generator..............................................................................................................29 Experimental Methods........................................................................................................... .30 Detector....................................................................................................................... ....30 Electronics.................................................................................................................... ...31

PAGE 5

5 Sample Preparation..........................................................................................................31 Results........................................................................................................................ .............32 Discussion..................................................................................................................... ..........34 4 EXPLOSIVES DETECTION BY 193 nm PHOTOFRAGMENTATION WITH NOCL FRAGMENT DETECTION...................................................................................................43 Background..................................................................................................................... ........43 Material Phase and Distribution......................................................................................43 Laser Photofragmentation...............................................................................................43 Catalytic Conversion.......................................................................................................44 Experimental Methods........................................................................................................... .44 Detector....................................................................................................................... ....44 Sample Preparation..........................................................................................................45 Results........................................................................................................................ .............46 Discussion..................................................................................................................... ..........47 5 FROM THE LAB TO THE FIELD........................................................................................56 LIST OF REFERENCES............................................................................................................. ..58 BIOGRAPHICAL SKETCH.........................................................................................................62

PAGE 6

6 LIST OF TABLES Table page 1-1 Explosive Classes, Examples, and Bond Structure............................................................14 1-2 Quick Reference of Expl osives Detection Methods..........................................................15

PAGE 7

7 LIST OF FIGURES Figure page 2-1 Computer Modeled NO-O3 Reaction.................................................................................24 2-2 Signal Change with Chamber Pressure..............................................................................25 2-3 Signal Change with Reactant Flow Rate...........................................................................26 3-1 Image of NO-O3 Instrument...............................................................................................36 3-2 Reactor-PMT Assembly Schematic...................................................................................36 3-3 NO-O3 Detector Schematic................................................................................................37 3-4 Signal as a Function of Reactor Pressure...........................................................................38 3-5 Signal as a Function of Reactant Flow Rates.....................................................................39 3-6 Instrument Response Function and Pump ing Loss as a Function of Reactant Flow Rates.......................................................................................................................... .........40 3-7 Pumping Loss from Reac tion Chamber and Model...........................................................41 3-8 Signal as a Function of NO Concentration........................................................................42 4-1 Energy Levels of NO2 and Dissociation to NO.................................................................48 4-2 Image of Explosive Sampling Setup..................................................................................49 4-3 Explosive Sample Stage and Optics..................................................................................50 4-4 Signal from Explosive as a Functi on of Distance from Droplet Center............................51 4-5 Signal Measured from Phot ofragmented RDX and TNT..................................................52 4-6 Signal Measured for Single Laser Pulses on RDX............................................................53 4-7 Signal Response as a Function of Photofragmentation Amount.......................................54 4-8 Signal as a Function of Laser Pulse Energy.......................................................................55

PAGE 8

8 LIST OF ABBREVIAT IONS AND SYMBOLS CAD computer aided drafting CL chemiluminescence DNT dinitrotoluene HMX high molecule weight RDX IED improvised explosive device IMS ion mobility spectrometer LIF laser induced fluorescence LoD limit of detection NO nitric oxide NO2 nitrogen dioxide NO2* excited state nitrogen dioxide NOCL nitric oxide ozone chemiluminescence O3 ozone PETN pentaerythritol tetranitrate RDX royal demolition explosive REMPI resonance enhanced multiphoton ionization STP standard temperature and pressure TATP triacetone triperoxide TEA themal energy analyzer TNB trinitrobenzene TNT trinitrotoluene

PAGE 9

9 Roman M number density of air (molecules cm-3 Torr-1) h v radiation (photons) fNO mass flow of nitric oxide (molecules of NO/s) k1 excited nitrogen di oxide production rate (cm3molecule-1s-1) k2 ground state nitrogen dioxide production rate (cm3molecule-1s-1) k3 photon emission rate (s-1) k4 excited state quenching rate (cm3molecule-1s-1) Greek CL photon emission (photons/s) t photon emission for time t (photons) I photon emission up to time t (photons) CL chemiluminescence quantum efficiency (dimensionless) L luminescence quantum efficiency (dimensionless) ex excitation quantum efficiency (dimensionless) i instrument transfer function dwell dwell time in reaction chamber (s) NO reaction lifetime (s)

PAGE 10

10 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science EXPLOSIVES DETECTION BY PHOTOFRAGMENTATION AND NITRIC OXIDE-OZONE CHEMILUMINESCENCE: PORTABILITY CONSIDERATIONS By Ronald Whiddon August 2007 Chair: James Winefordner Major: Chemistry The recent popularity of improvised explosive de vices, and the continuing threat presented by unexploded land mines pushes the detection of hidde n explosives to the forefront of scientific research. For maximum utility, a detection devi ce should be handheld, be inexpensive, respond quickly, have little interference, and detect explosives without di rect contact with the explosive device. Few instruments are available that can meet most of these requirements, primarily because measuring explosives in the vapor phase de mands a sensitivity of low parts per billion to parts per trillion of explosive material. The chemiluminescent reaction between nitric oxide and ozone has been used to detect explosives by their decomposition, which produces nitric oxide. As of yet, the instrumentation has not been scaled down to the point that it could be assembled as a handheld detector. This research is the design of a nitric oxide-ozone chemiluminescent reaction chamber that is small enough to be handheld, while still being able to detect explosives in the vapor phase. A 24 mL reaction chamber was designed that was capable of detecting mid part s per billion levels of nitric oxide, and high picogram amounts of TNT. Response times for the instrument were less than 10 s.

PAGE 11

11 CHAPTER 1 EXPLOSIVES: HISTORY, CLAS SIFICATION, AND DETECTION Historical Background Origins Asciano Sobrero, not Alfred Nobel, is the father of high explosives. In 1846, while employed at the Turin School of Mechanics a nd Applied Chemistry, Sobrero discovered a method of nitrifying glycerin in a solution of nitric and sulfuric acids, which produced nitroglycerin, the fi rst high explosive1. Laboratory accidents quickly convinced Sobrero that nitroglycerin was far too unsta ble a compound to work with. Only then did Immanuel Nobel, Alfreds father, invent a met hod to produce large amounts of nitr oglycerin with a modicum of safety2. Nitroglycerin like many high explosives does not explode when exposed to flame, but it is susceptible to ignition through shock. While the compound would burn in a controlled manner, attempting to initiate detonation was extremel y dangerous. In 1864 Alfred Nobel invented the blasting cap which would ignite high explosives in a controlle d manner, greatly increasing the safety of explosive use. While the problem of ignition was solved, the shock sensitivity of nitroglycerin made its transport dangerous. Alfred Nobel and his em ployees discovered that mixing three parts nitroglycerin with one part clay reduced this sensitivity, and in 1867 Guhr Dynamite was born. Eight years later Nobel replaced the clay with ni trocellulose, an explosive in its own right, and invented shock stable blasti ng gelatin, or what is now commonly known as dynamite. All told, Nobel amassed a fortune through the pr oduction of blasting ma terials, but a large cost. The accidents in his factories claimed th e lives of many employees, as well as his own

PAGE 12

12 brother. That, added to the de vastation wrought by the Nobel fam ily product, drove Alfred to establish the Nobel peace prize3. The discovery of nitroglycer in in 1846 opened the floodgate of invention for high explosives. 1849 saw the emergence of ammoni um nitrate, the most highly produced and abundantly used explosive material4; 1863 the invention of TNT, arguably the most famous high explosive; PETN in 1894; RDX in 1899. The next major addition to the high explosives arsenal did not occur until 1943 wh en HMX was invented. Application The first use of explosives for combat was recorded by Marcus Graecus, who in 700 A.D.1 described rockets used in battle to disorien t and demoralize the enemy. The introduction of the low explosive black powder to Europe in the 13th century by Friar Roger Bacon, and its subsequent perfection by Schwartz in 13202 quickly changed the nature of war. Heavily fortified walls that had offered good defense since the dawn of civilization proved useless against explosive breaching charges. In reality, modern combat is only an updated application of explosives. Perhaps the most terrifying aspect of explosives is the collateral damage they effect when hidden. Anti personnel land mines, the production of which is now outlawed by a United Nations agreement, still represent a lingering threat to civilians in war ravaged areas. Improvised explosive devices (IEDs) can be extremely devast ating. These devices are military ordnance such as explosive mortars, anti-vehicle mines, even bombs; modified with makeshift fuses for use as booby traps. It is in the interest of humanitari an organizations and the military to develop methods of detecting these explosive hazards.

PAGE 13

13 Classification There are several methods of classifying e xplosives. Each in some way relates to a physical or chemical property of the material, and each has usefulness depending on the field of application. Rate A first level delineation is by rate of react ion. Low explosives, such as black powder, deflagrate, meaning they burn at a fast rate. Th is rapid combustion produces a pressure front which propagates more slowly than the speed of sound5. Materials that det onate are termed high explosives. Their high rate of r eaction creates a pressure front which expands faster than the speed of sound, though many can be burned without inducing detonation. Ignition High explosives can be further categorized by the stability of the compound. Primary explosives are those which are unstable in pres ence of heat or mechanical shock. They dont deflagrate. Common examples of primary explosiv es are mercury fulminate, lead azide, or potassium permanganate. Secondary explosiv es are those that are both thermally and mechanically stable. The majority of high explosiv es fall into this category. These explosives will deflagrate if ignited, but need the energy of primary explosion to set off a detonation2. Most functional explosive devices require a primary explosive linked to a secondary explosive. Functional Group An explosion is effectively a rapid oxidation reaction, and so explosiv es need both fuel for combustion and an oxidizing agent. High explosive mi xtures, such as ammonium nitrate-fuel oil, separate these components, wher eas molecular high explosives contain both a fuel and an oxidizer bonded together. It is common practice to classify molecular explosives by their oxidizing agent. This approach splits explosiv es into six categories: nitro compounds, nitrate

PAGE 14

14 esters, nitramines, nitrate and chlorate and perchlorate salts, azide s, and everything else2 (Table 1-1). Table 1-1. Explosive Classes, Examples, and Bond Structure Detection of Hidden Explosives The detection of hidden explosives is dependa nt on the type of e xplosive device being hunted. So, while there are many methods of analys is for explosive materials, few of them are suitable for detection of hidden explosives. The fo cus of this research is on the detection of physically implanted explosives, such as mine s or IEDs. Appropriately, only field portable methods of explosives dete ction will be reviewed. Three things are necessary in a hidden explos ives detector: short processing time, low limit of detection, and utility. A shor t reaction time, less than ten seconds, minimizes the danger to the instrument operator. A low limit of detection is n ecessary to negate the e ffect of concealment as most explosives have low vapor pressures. Utility is a general considerat ion for the instrument operator. To be useful as a de tector, the instrument should be easily carried, simple to operate, have a long duty cycle, and be accurate. A compar ison of current portable detection techniques is shown in table 1-2

PAGE 15

15 Table 1-2. Quick Reference of Explosives Detection Methods. Canine Detection The most successful and widely used method of detecting explosives has always been bomb sniffing dogs6. Frequently used in ports of entry a nd military bases, and even in combat, canine teams are required to have greater than 95% accuracy in detecting hidden explosives6,7. From a cost standpoint a bomb dog is compar able to $50,000 instrument; however, unlike an instrument, the dog requires severa l years of training prior to use, boarding facilities during deployment, and care in senescence. Add the f act that dogs perform best with a dedicated handler to limited availability, and the canine de tector shows obvious weakness in terms of mass deployment. Ion Mobility Spectrometry The superstar of field deployed explosives detectors is currently the ion mobility spectrometer (IMS). The reason for their popularity is primarily their sma ll size and light weight. Essentially the IMS is a time of flight mass sp ectrometer that operates at atmospheric pressure. Because of the slower movement of ions at atmo spheric pressure, the drift tubes can be short yet still allow for separation. Fully functioning IMS in struments have been constructed that displace

PAGE 16

16 about liter8. At this point most major instrument ma nufacturers offer some type of portable explosives detector based on IMS9. Despite their warm reception, IMS detectors ha ve a few characteristics that limit their effectiveness as field deployable instruments. Ionization is most often caused by radioactive nickel, a non-selective ionization source. Overabundance of ionizable molecules can flood the drift tube giving false positives. High humidity and low temperature decrease signal by forming water-analyte clusters and condensin g explosive on the IMS inlet filter1. Additionally, the limit of detection for IMS is not very good9. In order to detect explosives, the sample must be concentrated by collecting on a fabric swipe. Th e fabric swipe is then heated in a closed sampling chamber to release vapors into the IMS. Fluorescence Fluorescence from explosives and explosive vapo rs is an area of research that has seen much interest recently. The trul y seductive aspect of a fluorescence based expl osives detector is that it would allow remote sensi ng of an explosive, a feature th at is impossible in competing detection systems. While the fluor escence signal for TNT is indistinct10, the attached nitro groups can be can be detected in that manner. Ul traviolet radiation clips the nitro groups off the parent molecule, yielding nitric oxi de (NO) and nitrogen dioxide (NO2). The NO absorbs a photon of 226 nm and is elevated to an electronica lly excited state. Rela xation from that state results in emission in the 226-250 nm range depending on the final vibrational level. The major setback in LIF detection of expl osives is the poor limit of detection, 10 ppm TNT at STP, well above the vapor phase c oncentration of TNT at room temperature11. To lower the limit of detection it is necessary to put the sample under vacuum, or raise the sample temperature, making this far from a standoff detection technique.

PAGE 17

17 Resonance Enhanced Multiphoton Ionization Resonance enhanced multiphoton ionization (REMPI) is similar to laser induced fluorescence in how signal is generated12. Once the nitro group is clipped off the parent molecule by the absorbance of a UV photon, two more photons interact with the NO molecule. The first raises NO from ground to an electronically excite d state, and the second from the electronically excited state to the ionization co ntinuum. Signal is measured with a mass spectrometer, or more simply by a pair of electrodes near the laser focus; either method makes this a non standoff technique. While detection with the mass spectrometer can detect vapor phase concentrations, it demands a non hand portable instrument. At pr esent ion current dete ctors do not reach a detection limit necessary for gas phase measurement13. Chemiluminescence Two chemiluminescent (CL) reactions are currently used to detect explosives14. The older technique detects relaxation of an excited state of NO2 produced from the reaction of NO and Ozone (O3). The newer CL technique measures emission from oxidation of luminol by NO2. Chemiluminescence detectors have earned a re putation for having wide linear response, low background, and low limits of detection15. The nitric oxide-ozone chemiluminescence (NOCL) detector is unable to detect explosives directly. But, as most explosive groups contai n multiple nitro groups attached to an organic molecule, their detection would be possible after freeing those groups from their organic backbone. One way of accomplishing this would be vapor phase combustion of the explosive, which will produce NO: this process is usually do ne with a heated catalyst. On the other hand, the nitro groups can be clipped off in the same way REMPI and LIF methods accomplish it, i.e., by photofragmenting the explosives with UV light. Photofragmentati on can be used directly in the vapor phase, or from a solid surface.

PAGE 18

18 One limitation of CL systems is their lack of absolute specificity. For an NOCL explosives detector, interference arises from reactions of O3 with sulfur oxide, alkenes, as well as non explosive related nitrates; although, non nitrog en emission can be excluded with band pass filters. Luminol is plagued extensively with interference from a variety of oxidants: chlorates, permanganate, iodine, peroxide ozone, sulfur dioxide, etc16. Summary The potential for damage that explosives o ffer encourages extensive development of any new technique or augmentation that improves th eir likelihood of detection. The research embodied in this paper is funded by a Department of Defense grant for vapor phase explosives detection. It is the goal of this research to investigate the miniat urization of a nitric oxide-ozone chemiluminescence detector, coupled with phot ofragmentation for explosives detection.

PAGE 19

19 CHAPTER 2 MODELING THE NITRIC OXIDE-OZONE CHEMILUMINESCENCE REACTION FOR A 24 mL REACTION CHAMBER Introduction After three failed attempts at making a nitr ic oxide-ozone chemiluminescence (NOCL) detector, computer modeling was performed to assi st in understanding the nature of the reaction. The goal was to gain a solid understanding of how the signal would fluctu ate with changes in reactor pressure, NO flow rate, and O3 flow rate. Reaction Kinetics Kinetics of a reaction between NO and O3 which produced NO2 were known as early as 195417, yet the step responsible for emission was la rgely missed. The first paper to handle the chemiluminescent pathway of NO and O3 was published in 1964 in th e Transactions of the Faraday Society18. Clyne, Thrush and Wayne winnowed NO-O3 reactions down to four that had appreciable effect on photon emission. Steffenson and Stedman used the reactions and kinetics for th e NOCL reaction in simplified form applicable to reactions with ozone19. The equation (Equation 2-5) predicts the emission from an NOCL chamber operating in the continuous flow re gime by including the effect that flow, pressure, and kinetics have on the signal. NO + O3 = NO2 + O2 NO + O3 = NO2 + O2 NO2 = NO2 + h v NO2 + M = NO2 + M k1 = 4.26*10-15 cm3 molecule-1 s-1 k2 = 1.6*10-14 cm3 molecule-1 s-1 k3 = 1000 s-1; 20 k4= 1.49*10-11 cm3 molecule-1 s-1; 21 (2-1) (2-2) (2-3) (2-4) i NO dwell L ex NO clf exp 1(2-5)

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20 The term fNO is the number of molecules of NO en tering the reaction chamber per second. It is found by multiplying the flow rate for the NO sample stream, the number density of that stream, and the mole fraction of NO in the stream. The equation does not allow for a maximum signal per second cl greater than the number of molecule s of nitric oxide flowing through the reaction chamber, since fNO is the only term in the equati on greater than one. In reality cl is far less than the number of NO molecules flowing through the reaction chamber. First, only a portion of the NO molecules ar e converted to excited state nitrous oxide (NO2*), the molecule responsible for photon emission. The term ex is the splitting ratio for this excited state. It is a constant that represents the number of molecules reacting through the equation 2-1 pathway as opposed to the equation 2-2 pathway. The value for ex is 21%. Next, there is signal loss from quenching, L. If NO2* collides with any other molecule, it will lose energy and be unable to emit a photon. The Steffens on Stedman splitting ratio is the number of NO2* molecules emitting light divided by the number being quenched. This term is pressure dependant (Equation 2-4). While the addition of reactants will result in hi gher signal flux for a flow system, there is also the possibility of some signal loss by the transi ent nature of the reactant s. In a static system the reaction goes to completion before the reac tion chamber is evacuated and a new mixture introduced. In the flow system, reactants are added continuously; and, reactants are removed continuously. This pumping loss is related to th e rate at which molecules flow through the reaction chamber and the time necessary for those molecules to react completely. The term (1-exp [tdwell/tNO]) brings into the equation the pumping loss of the reaction chamber. The period tdwell is the amount of time it takes a molecu le to travel from the inlet of the cell to the outlet. It is calcul ated by dividing the volume of the reaction chamber by the flow rate

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21 of all inlets to the chamber. The period tNO is the lifetime of NO in the reaction chamber. After several seconds, the NO has been consumed and e ffectively the reaction is complete. This period is mathematically defined as the i nverse of the sum of reaction rates k1 and k2 multiplied with the mole fraction of O3 in its stream, the number density of the O3 stream, and the ratio of the O3 flow to total flow. The instrumental response function is included as i. This term is static and unique for any individual chemiluminescen ce instrument. The term i is a catch all for the quantum efficiency of the detector, the solid angle of detection, window losses, reflectance losses, spectral overlap of the detector and emission source, etc. It is always less than on e, but can only be determined through experimentation. Model System Parameters The chemical kinetics program Kinetica 2003, programmed by Dr. Richardson (University of Florida, Physical Chemistry) was used to model the time dependant reaction concentrations using reactions and rates given in equations 2-1 through 2-4. Kine tic modeling for both a static signal and flow modified signal was done with a differential equation program (Polymath 6.10) used to plot the differential equations 2-6 th rough 2-10. The Steffenson-Stedman equation (2-5) was plotted using Excel. [NO]t = [NO]I + (-k1[NO][O3] k2[NO][O3]) dt [O3]t = [O3]I + (-k1[NO][O3] k2[NO][O3]) dt [NO2*]t = [NO2*]I + (k1[NO][O3] k3[NO2*]I k4[NO2*]I[M]) dt [NO2]t = [NO2]I + (k2[NO][O3] + k3[NO2*]I + k4[NO2*]I[M]) dt t = I + (k3[NO2*]) dt (2-6) (2-7) (2-8) (2-9) (2-10)

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22 For best applicability to the existing NOCL r eactor all of the modeli ng was performed with concentrations, pressures, and fl ow rates that were possible in the laboratory. NO concentration was set as 10 ppm, O3 concentration at 1280 ppm. Results The temporal evolution of reactants and products, as predicted by Kinetica 2003, is shown in Figure 2-1. The photon emission is a r unning tally of the number of photons produced, not the instantaneous rate of emission. The result is surprising considering that the reaction emission is supposed to be a two step process. Consiste nt with many two step reactions the NO2* was expected to build up, with peak emission rates occurring after peak NO2* production. However, the model puts the peak phot on emission rate right at the time of mixing. This is supported by the rate constants for the two reactions; the rate of production for NO2* is much slower than the relaxation rate from that state, thus th e intermediate cannot aggregate. Pressure, which is related to the total count of molecules in a volume, is known in many luminescences to have to do with the quenching of excited molecules by collision energy transfer. Equation 2-4 is the quenching step in the NOCL reaction pathway. The extent of signal loss through quenching was calcula ted with Polymath for several pressures from 0.01 to 100 Torr (Figure 2-2). Although each successive increase in pressure represents a tenfold increase in concentration of NO, the losses due to quenching net no increase in signal, only an increase in reaction rate. Hence, CL efficiency ( cl,p) decreases with increasing pr essure. This is true only for a static system, as reaction rate takes on great er importance for flow systems. To this point, only static systems have been simulated, but to properly depict the NOC L detector, we must model under continuous flow dynami cs. Plotting equation 2-5 using a reactor pressure of 1 Torr and flow rates of 100 to 1000 mL/min yields the si gnal response shown in figure 2-3. The results show that highest signal is found fo r about equal flow rates of NO and O3. The basic difference

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23 between this plot and the static systems results is the inclusion of a pu mping loss on total signal. Excel and Polymath were used to solve the seri es of differential equa tions 2-6 through 2-10 to check the validity of the Steffenson Stedman eq uation. The surface is similar in shape to figure 2-3, but has a hundredfold higher signal. Discussion By modeling the chemiluminescent reaction of NO with O3, we are able to come to a greater understanding of the temporal dynamics of the reaction. The temporal dynamic shows that pressure will have a two fold effect on e fficiency as increasing it will decrease pumping losses, but increase quenching loss. Finally we see that slightly higher flow rates of NO compared to O3 flow will yield the highest signal.

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24 0.E+00 2.E+10 4.E+10 6.E+10 8.E+10 1.E+11 1.E+11 1.E+11 2.E+110246810Time (s)Concentration (molecules/mL)0.E+00 1.E+07 2.E+07 3.E+07 4.E+07 5.E+07 6.E+07 7.E+07 8.E+07Photon Count NO(calc) NO2(calc) NO2*(calc) hv(calc) Figure 2-1: Computer Modeled NO-O3 Reaction. The model is of a static system with no addition of reactants after time zero. Photon signal (hv) plateaus at 7*107photons at 10 s.

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25 Figure 2-2: Signal Change with Chamber Pressu re. Reaction chamber pressures are 0.01 Torr A, 0.1 Torr B, 1 Torr C, 10 Torr D, and 100 Torr E. All pressures reach the same emission limit of 1.5*107 photons. 0.0E+00 2.0E+06 4.0E+06 6.0E+06 8.0E+06 1.0E+07 1.2E+07 1.4E+07 1.6E+07 0123456789 Time(s)Signal (photons) 0.01 Torr 0.1 Torr 1 Torr 10 Torr 100 Torr A B C D E

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26 100 200 300 400 500 600 700 800 900 1000 100 200 300 400 500 600 700 800 900 1000 0.E+00 1.E+08 2.E+08 3.E+08 4.E+08 5.E+08 6.E+08 7.E+08 8.E+08 9.E+08 1.E+09Signal (counts/s) Ozone Flow (mL/min) Nitric Oxide Flow (mL/min) Figure 2-3: Signal Change with Re actant Flow Rate. The 3D surface plot shows that highest flow rates of NO and O3 will give the highest signal. At the higher flow rates a plateau develops as pumping loss has a greater e ffect than the addition of new reagents.

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27 CHAPTER 3 CHARACTERIZATION OF A MINIATURE NI TRIC OXIDE DETECTOR: NITRIC OXIDEOZONE CHEMILUMINESCENCE Introduction Nitric oxide has the distinct honor of being one of the first eight identified gases. Joseph Priestley discovered this nitrous air al ong with carbon dioxide, carbon monoxide, sulfur dioxide, oxygen, and others in his experiments published in 177620. While the emission from the reaction of NO and O3 can be seen in the night sky, as a dull reddish/b rown glow, the relationship between the emission and reaction was not fully appr eciated until Lord Rayleigh, in the 1920s, discovered the physical phenomenon of the emission from mixtures of NO and O3. Kinetics of the non emissive reaction were found in the 1950s, but the full understanding of the reaction was not realized until the mid 1960s. Background Chemiluminescent Detectors The end of World War II saw the proliferation of automobiles and coincidently the buildup of thick brown layers of smog in American cities21,22. The brown pollutant, NO2, was traced to a byproduct of high temperature combustion, na mely NO. When two molecules of NO2 dissolve in water, nitric acid and nitrous acid are produced. (Reactions 31, 2, 3)These acids of nitrogen, along with sulfuric acid are responsible for acid rain devastation of th e seventies and eighties23. This environmental danger inspired the fi rst wave of research on NOCL detectors24,25. Nitric oxide chemiluminescence de tectors have an inherent sel ectivity for nitrogen compounds because of their emission spectrum. They are al so well suited to real time sample monitoring N2 + O2 2NO 2NO + O2 2NO2 H2O + 2NO2 HNO2 + HNO3 (3-1) (3-2) (3-3)

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28 since many NOCL detectors operate in the con tinuous flow regime. The first reported NOCL detector boasted a low ppb limit of detection; however, in order to achieve that low limit, the instrument was rather large24. Fontijns reaction chamber was one liter in volume with pumping rates of 12.5 L/min. While the sensitivity of NOC L detectors was unmistak able, they were by no means portable. A reason for such a large reaction chamber is evident from the relationship between signal, mass flow, and dwell time of the Steffenson-St edman equation (Equation 2-5). Essentially the signal is proportional to the mass fl ow of NO and the reactor dwell time, so to get a large signal, use a large reactor with high flow rate. The mo vement away from this strategy was spurred by the desire to create instrume nts that could be carried by weather balloons and high flying aircraft26-28. This meant two things: the NOCL detector must be small and lightweight. Yet, it is impossible to reduce the volume of the reaction ch amber without reducing th e dwell time of the reactants. Likewise, it is impossible to reduce the pumping speed of a reaction chamber without reducing the mass flow of reactants. Both of thes e changes would lead to lower signal. To make up for the loss in signal brought on by miniaturization, researcher s focused on ways to make the reactor more efficient. Instrument response function is a factor for a CL detector that has nothing to do with reaction kinetics. It derives fr om detector amplification, opti cal collection efficiency, mixing dynamics etc. In miniaturizing th e reaction cell, efforts were made to increase this factor. For instance, thorough reactant mixing is impor tant in maximizing chamber efficiency19,28. Steffenson and Stedman found that 300 ml reac tion chambers with different mixing methods gave different signals19. Also, chamber material was impor tant. Coatings that were highly reflective at infra red wavelengths woul d also increase collection efficiency.

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29 Ozone Generator Ozone as an oxidizer is stronge r than peroxide. It is a natu rally occurring gas created by the combination of three molecules of oxygen (E quations 3-4, 3-5). The electric field in the region of an electric arc, or radiation below 200 nm can suppl y energy for the dissociation. The resulting radicals combine with oxygen molecules producing O3. One method of creating O3 is passing oxygen in front of a mercury lamp (hollow cathode, pin, or arc) where it is photolyzed by th e 185 nm line, creating a small portion of O3. A mercury pin lamp and pure oxygen will produce 2 ppm of O3, a level insufficient for the NOCL detector. Low levels of O3 can be concentrated by condensing in liquid nitrogen; however, the danger of explosion is great since a steady boil off is difficult to maintai n. Using high power mercury arc lamps can create higher levels of ozone, but th e heat and brilliant UV light can be a safety problem. The most efficient way to produce O3 is with an electrical disc harge. In nature, the fresh smell after a thunder storm arises from an increase in O3 concentration produced by lightning strikes. The electric field n ear an electric discharge is strong enough to break oxygen and nitrogen bonds. The resu lt is a recombination of these atoms into O3, and excited nitrogen which gives the blue color to a spark. To reproduce this effect in an instrument, a low current high voltage is attached to an anode hous ed in a glass tube, inside a cathode29. The dielectric nature of glass prevents a true spark, so a multitude of tiny electric discharges set up between the glass and electrode. Ozone generators with 4% concentratio n outputs are possible when using the electrical O2 + energy 2 O 2O + O2 2O3 (3-4) (3-5)

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30 discharge production method. A detriment to the discharge ozone generator is the emitted RF noise. Experimental Methods Detector The NOCL detector was designed and assemble d in house. The reaction chamber has an interior volume of 24 mL two thirds the volume of the next smallest reaction chamber in the literature19. The cavity is machined out of aluminum stock, with a 4 inch length and a 2 inch outside diameter. The inside of the chamber has a dull finish to promote a Lambertian reflection profile. The surface is not plated as aluminum al ready has high reflectivity in the infra red. The front of the chamber is sealed with a 1 in ch low pass filter, 425nm cutoff (Edmund Optics). Mixing ports are machined in the circumference of the reaction chamber just near the front window with bores connecting to in let tubes projecting from the back of the chamber. Figure 3-1 shows the instrument and a CAD cutaway of the reaction chamber so that the sample movement and mixing can be considered. Threads were cut in exterior front of the reaction chamber to mount in the cooled photomultiplier housing (Produc ts for Research Inc.) creating a light tight interface between reaction chamber and detector (Figure 3-2). The detector functions under a light vacuum of 1 Torr. It is necessary for drawing in the sample and also reduces quenching of the NO2. A roughing pump (BOC Edwards 18) is more than sufficient for creating vacuum at tota l flow rates between 200 and 2000 mL/min. The pressure was adjusted with a needle valve pl aced between the reaction chamber and the roughing pump, and a Varian dual range pressure gauge wa s attached between the chamber and pump. The reaction chamber connects to the vacuum pump with inch st ainless steel tubing and Cajon fittings. To minimize light leakage all tubing was e ither stainless steel or copper. The two inlet tubes are 1/8 inch stainless steel tubi ng. One tube carries the O3 reagent gas from the ozone

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31 generator to the reaction chamber, whilst the other carries the sample stream. Flow in each tube is controlled by mass flow contro llers (Alicat Scientific) (Figure 3-3). The sample line was kept as short as possible to minimize transit time to the reaction chamber. In the final configuration, it took 2.5 s from sample introduc tion to signal acquisition. Electronics Emission is converted to an electrical signal by a 28 mm reflection mode pmt (R955, Hamamatsu Photonic). Internal gain for the R955 is 1X107 and the dark count rate is on the order of tens of photons per second. To minimize the dark count rate, the pmt was placed in a cooled housing. The Peltier cooling device drops the pmt te mperature to -40 C. At this temperature, the dark count rate was about 2-3 counts/s. Th e applied voltage to th e photocathode was -950 V. PMT signal is recorded with a Stanford Res earch SR-400 fast photon counter. To isolate the photocathode events from dynode noise, the discrimi nator was set to trigger on the falling edge of pulses with at least -6 mV intensity. The photon counter was interfaced to the computer through a GPIB interface (USB-G PIB, National Instruments) Sample Preparation At this point the main goal of research was to optimize the flow and pressure settings in the newly constructed instrument. All the reactants were used in known concentrations. Ozone reagent produced by the AC-500 is factory appraised at 500 mg/L which at the onboard flow rate of 1.5 L/min translates to 2500 ppm. An NO ca libration standard (spectra gases 10 ppm NO, balance N2) was used to complete the reagent mix. In so me cases, it was necessary to dilute the NO reagent. In these cases, two fl ow controllers were attached th rough a tee to th e sample inlet line, one carrying the calib ration standard and the ot her carrying house nitrogen.

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32 Results Since this instrument was designed and a ssembled in the lab, a thorough optimization regime is needed to appraise its capabilities. The two variables that need to be assessed are reaction cell pressure and reac tant flow rate. Figure 3-4 shows how the instruments response varies with reaction chamber pressure. Flow rates for NO and O3 were 600 mL/min each. The emission signal peaks at 10 Torr; however, th e efficiency defined as measured signal in photons/s divided by the number of molecules of NO/s, peaks earli er at 1 Torr. So while higher signal occurs at a higher pressure, a 1 Torr react ion cell pressure will make better use of the sample. This should translate to a steeper response curve for th e 10 Torr chamber and a correspondingly higher limit of detection. The drop o ff in efficiency from above 1 Torr is likely caused by the quenching of signal, wh ile the drop of below 1 Torr is probably due to a too short dwell time in the reaction chamber. A flow dependant signal surface is shown in Figure 3-5. Flow optimization was performed by measuring the signal generated fo r an array of flow rates of O3 and NO that ranged from 100 to 1000 mL/min in each reagent. The measured signal ( Cl) can be substituted into the Steffenson Stedman equation in order to determine the instrument response function i. The first step in determining the instrument res ponse function is converti ng the chemiluminescent signal, Cl, to the CL quantum efficiency Cl. The term Cl is equal to the product of ex, L, (1exp[tdwell/ tNO]), and i. Dividing the functions ex, L into the CL quantum efficiency reduces the signal to its (1exp[tdwell/ tNO]) and i components. The term ex is constant and can be directly divided out. The term L is variable with the reacti on chamber pressure, but since all measurements were made at 1 Torr, it is universally applied to each meas urement, and thus can be removed in the same manner as ex.

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33 At this point the signal has been reduced to the value of (1-exp[tdwell/ tNO]) and i multiplied. When the dwell time is much gr eater than the reaction lifetime, (1-exp[tdwell/ tNO]) is nearly 1. The plateau seen in figure 3-6 satisf ies this condition and he nce is the measure of i. The instrument response factor this reactor is approximately 2.47*10-6 (dimensionless), with error of 6.3E-8. i doesnt vary with pressure, flow, or reagent concentration; so, dividing it out leaves only pumping loss. Figur e 3-7 depicts the cells pumping loss as a function of NO flow for 300 mL/min O3 and a function of O3 flow for 900 mL/min NO. Also included in that figure is the pumping loss predicted by the Steffenson Sted man equation (2-5). Pumping loss appears to be proportional to a function of O3 mass flow and the inverse to the NO mass flow, but it is not a direct relationship. It seems that the Stefen son-Stedman approximation over accounts for signal loss due to material transfer through the chamber, but this is not because of the pumping loss. The inaccuracy is caused by accounting only for em ission from the molecules of NO that enter the chamber during the one second integration, an d excluding emission from molecules that have been in the reaction chamber for the entire dwell time. The final operation to be pe rformed on this NOCL detector is the evaluation of its analytical response, and the calc ulation of the limit of detecti on. Figure 3-8 depicts the signal for the NOCL signal measured for 1 Torr of NO at concentrations from 0.50 to 10 ppm. Flow rates for NO and O3 were each set at 600 mL/min. The experi mental limit of detection, obtained under our present conditions, is approximately 300 ppb. This figure has been calculated from the conventional definition of the limit of detection, i.e., for a signal being 3 times the standards deviation of the average background signal

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34 Discussion A small NOCL detector was de signed and constructed by our laboratory. The detector was among the smallest in existence for ozone-nitri c oxide chemiluminescence. The instrument was optimized for highest efficiency by adjusting inst rument flow rate and reaction cell pressure. In the end a fair evaluation of th is instrument is that it does not have the sensitivity needed for atmospheric NO monitoring. It is disappointing that the signal is so low considering that a 240 mL cell with similar mixing designe d achieved pptr levels of detection28. Although the experimental limit of detection wa s found to be insufficient for the detection of explosive vapors, one should stress that this limit was obt ained with a non-optimized setup. Indeed, a much lower limit is possible with opti mized signal measurement. The spectral overlap between the PMT and the NOCL emission is fr om 600 to 900 nm. The average PMT quantum efficiency for this range is 0.041, while the em ission signal represents only 6.3 percent of the total emission30. Multiplying the PMT quantum efficien cy by the average of percent total emission from 600 to 900 nm yields a value of 6.5*10-4 (dimensionless). Dividing the measured signal by this factor gives the possible signal at 100% PMT quantum efficiency, and 100% spectral overlap, e.g. the signal measured at 0.59 ppm NO woul d optimally be 3.4*105 photons/s. The noise associated with the measurement is most likely from PMT dark current, and if there were no increase in noise in the optimal syst em, the limit of detection would improve to 15.6 pptr. Hence, this NOCL detector could be va stly improved merely by using a better signal detection method, such as an infra red sensitive avalanche photodiode. As previously stated, the instrument response function arises from a variety of non-reaction components that effect signal. This instruments i of 2.47*10-6 (dimensionless) can be accounted for by estimation of some of the known efficiencies of the detector components.

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35 The efficiency loss from the PM T, as calculate d above, is 6.5*10-4(dimensionless), leaving a loss of 3.8*10-3 (dimensionless) from other sources, such as solid angle, window losses, etc. As a result of the extensive measurements of si gnal at various flow rates, it is evident that the Steffenson-Stedman equation is not wholly ac curate. Currently the eq uation only considers molecules that enter the cell during the detector integration time. The number of NO molecules should be expanded to include molecules that ar e in the cell for the en tire dwell time, which ranges from 7.20 to 0.72 s over 200 ml/min to 2000 ml/min total flow rate.

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36 Figure 3-1: Image of NO-O3 Instrument. Inset is cutaway of reaction chamber to highlight the circumferential mixing. Figure 3-2: Reactor-PMT Assembly Schematic. Mixture of NO and O3 enter the chamber near the long pass filter, and exit through the pumping port. A 1.5 focal length lens focuses light onto the PMT photocathode. Drawn to scale. NO Inlet O3Inlet Quartz Lens PMT Reaction Chamber Long Pass Filter Pumping Port Cooled Housing Signal Out/ Voltage In To Needle Valve & Pump Ozone PMT Flow Pressure Reaction

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37 Figure 3-3: NO-O3 Detector Schematic. Ozone from the ge nerator and NO from a sample source flow through mass flow cont rollers, are mixed, and pumped out. Pressure is controlled by opening or closing the needle valve. Drawn to scale. AC-500 Ozone Generator Alicat MP series Mass Flow Controller Voltage In/ Signal Out Cooled PMT Housing Reaction Chamber Pressure Sensor O 3 Strea m N O Strea m N eedle Valve To Roughing Pump

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38 0 500 1000 1500 2000 2500 3000 3500 4000 4500 0.510.520.530.540.550.560.570.580.590.5 Pressure (Torr)Signal (counts/s)0.E+00 1.E-10 2.E-10 3.E-10 4.E-10 5.E-10 6.E-10 7.E-10 8.E-10 1.6E+125.2E+131.0E+141.5E+142.0E+142.5E+143.0E+14 Mass Flow (molecules NO/s)Efficiency Avg signal efficiency Figure 3-4: Signal as a Function of Reactor Pressure. The diamond plot is of the actual signal, while the square plot is the reacti on efficiency (Photons / Molecules NO).

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39 100 200 300 400 500 600 700 800 900 1000 100 200 300 400 500 600 700 800 900 1000 0 1000 2000 3000 4000 5000 6000Signal (counts/s)Ozone Volume Flow (mL/min)NO Volume Flow (mL/min) Figure 3-5: Signal as a Function of Reactant Fl ow Rates. 3D surface shows rapid increase in signal, but no plateau is formed as in the model result.

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40 100 200 300 400 500 600 700 800 900 1000 5.37E+11 1.07E+12 1.61E+12 2.15E+12 2.68E+12 3.22E+12 3.76E+12 4.29E+12 4.83E+12 5.37E+12 0.00E+00 5.00E-07 1.00E-06 1.50E-06 2.00E-06 2.50E-06 3.00E-06 3.50E-06 4.00E-06 Efficiency Ozone Volume Flow (mL/min) NO Mass Flow (molecules/min) Figure 3-6: Instrument Response Function and Pu mping Loss as a Function of Reactant Flow Rates. Dividing the Signal by th e NO mass flow and the terms ex and L leaves only the constant instrument res ponse and the variable pumping loss.

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41 0% 20% 40% 60% 80% 100% 120% 140% 0.00E+001.00E+122.00E+123.00E+124.00E+125.00E+126.00E+12 NO Mass Flow (molecules/s)Pumping Loss0% 20% 40% 60% 80% 100% 120% 140%0.0E+002.0E+144.0E+146.0E+148.0E+141.0E+151.2E+151.4E+151.6E+15Ozone Mass Flow (molecules/s) 300mlpm Ozone 300mlpm Ozone SS 900mlpm NO 900mlpm NO SS Figure 3-7: Pumping Loss from Reaction Chamber and Model. Measured pumping loss is shown in solid traces, model pumping losses in dashed lines. Negative slopes are pumping loss as a function of NO flow rate, positive slopes are Ozone flow rate dependant

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42 y = 324.19x R2 = 0.9896 0 500 1000 1500 2000 2500 3000 3500 012345678910 [NO] ppmCounts/s (b.s. ) Figure 3-8: Signal as a Function of NO Concentration. The linear response of the instrument, with error bars is shown. The lim it of detection is about 300 ppb NO.

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43 CHAPTER 4 EXPLOSIVES DETECTION BY 193 nm PHOTOFRAGMENTATION WITH NOCL FRAGMENT DETECTION Background Material Phase and Distribution The vapor phase concentrations for explos ives are very low. Clausius-Clapeyron expressions for RDX and TNT are shown in e quations 4-1 and 4-2. At 298 K, the calculated concentrations are 6.00 p pb and 9.5 ppb respectively31. Even with complete conversion of nitro groups to NO, the concentration is below the li mit of detection for th e NOCL detector studied. Sampling from a crystallized e xplosive effectively increases th e concentration above normal vapor phase concentrations. Deegan and coworkers performed extensive wo rk on the material dispersion in droplets. Most notable is the presence of a thick ring at the edge of the deposit. As the droplet dries, evaporation happens uniformly over the surface32. At the edge of the droplet, the curvature means that there is a higher surface to volume ra tio than at the relatively flat center of the droplet. Evaporation draws liquid to the edge, as well as suspende d particles. By the time the droplet has dried, 90% of the materi al is contained in the outer ring33. Laser Photofragmentation There are two possible laser in teractions that produce nitric oxide fragments from explosive molecules. The most important is the cleavage of th e nitro-carbon or nitro-amide bond on the explosive. The bond energies are about 50 kcal/mol. This bond energy equates to a photon wavelength less than 570 nm, and researches have detected fragmentation at wavelengths than Log [RDX] (pptr) = -6473/T + 22.50 Log [TNT] (ppb) = -5481/T + 19.37 (4-1) (4-2)

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44 193, 227 nm, and 454 nm31,34,35. It is unclear whether the photolysis at 454 nm is due to a two photon process, or direct dest abilization of the N-C bond. Produc ts from cleavage are NO, NO2, and carbon compounds. A second laser interaction is the photolysis of NO2. When NO2 is impacted by a photon, this time with an energy equivalent to 250 nm the NO2 will be excited to a semi stable state where the molecule rapidly dissociates into atomic oxygen and NO (Figure 4-1)36-38, NO2 has essentially the same absorption cross secti on as TNT and nitromethane, and is known to photolyze at the same laser wavelengths39,40. Maximum yield of photolysis product occurs with nanosecond laser pulses as faster puls es cause multiphoton ionization of NO2. Catalytic Conversion In this instrument, the laser pulse which produces nitric oxide by photofragmentation of explosive and also photolysis of nitrogen di oxide. The photofragmentation of explosive will produce a mixture of NO and NO2, however the production ratio is not known. It is necessary to use a second conversion techniqu e to measure the amount of NO2 produced by photofragmentation. Many transition me tals are capable of converting NO2 to NO when heated at 400-600 C. Conversion efficienci es for molybdenum, gold, and st ainless steel are 100%, while platinum-gold alloy, and carbon are 57 and 95% respectively25,41-43. These efficiency values represent optimum values and ulti mately depend on converter design Experimental Methods Detector The detector used for this portion of research is the same as that used in Chapter 3. The reaction chamber is an in house design displacing 24 mL. The chamber is held at vacuum by a roughing pump. Flow rates and chamber pressu re are controlled by a pair of mass flow controllers on the two inlet t ubes and a manual needle valve between the chamber and pump.

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45 The ozone reagent is produced with an EC-500 ozone generator attached through the mass flow controller to one of the chamber inlet tubes. Si gnal is captured by a PMT and recorded by a fast photon counter. To handle solid explosive samp les a sample stage was designed and machined from aluminum (Figure 4-2). The backside of the stage has a machined surface that holds a 1 inch glass slide. The center of the recessed ar ea has a hole covered with a quartz window to allow the laser beam to impact the sample. A sm all channel runs through the back of the sample stage and connects to a 1/8 inch Swagelok adapter. This channel carries air across the sample and into the reaction chamber via the mass flow controller. A 193 nm argon fluoride excimer laser (GAM-15, GAM Inc. Orlando FL) is used to photofragment the explosive sample. The lasers pulse energy maximum is 15 mJ, the pulse length 10 ns. The laser shots were triggered externally and range d from 0.1 to 2000 Hz. The laser was focused onto a 2 mm spot diameter on the surf ace of the slide by a quartz 3 inch focal length lens located between the laser and sample holder. A mechanical shutter placed between the laser and the quartz lens allowed precise control of shot number (Figure 4-3). This was necessary because the laser energy was unstable fo r the first twenty or so shots. When conversion of NO2 to NO was necessary, it was done using a stainless steel converter. An 18 inch length of 1/8th inch diameter stainless steel tubing was bent into a coil and connected between the sample mass flow contro ller and the reaction chamber. The coil was heated in a sand bath to 350 C with a hot plate. Sample Preparation Samples used in this experiment were obtai ned from Chem Services and the office of Naval Surface Warfare Center. The samples we re 2.0% mass/volume TNT in acetone and 2.5% mass to volume RDX in acetonitrile. For all expe riments the volume of solution deposited was 5 u L. Samples were deposited on acetone rinsed 1 in ch glass slides. The sample was crystallized

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46 by solvent removal in a gentle dry nitrogen str eam. The crystallized droplets were generally circular and slightly less than 1 cm in diamet er. For limit of detection experiments the samples were diluted in the appropriate solvent (Aceton e, 99.9%, Acetonitrile, 99. 95% Fisher) prior to droplet deposition. Results To get an estimate of the amount of explosive ablated, it was necessary to find the general distribution of explosives in th e dried droplet. Figure 4-2 inset shows the build up of a visibly raised ring on the edge of the droplet. A study of material distribution was performed to see if crystal forming explosive soluti on might have a differe nt distribution compared to a suspension. This was done by measuring the NOCL signal wrought from photofragmenting explosive at positions across the dried droplet. To minimize the error from differences in size and shape of the droplet, the signal is reported in terms of the ratio of laser spot positi on to the radius of the droplet (Figure 4-4). It is difficu lt to get the amount of explosive in the droplet edge because of the poor spatial resolution the 2 mm beam diamet er affords, but an estimate is shown for the droplet edge as the outer 10% of the droplet radius. The amount of material in the droplet edge is found to be roughly 63% of the nonvolatile material. This was determined by revolving the suggested signal (Figure 4-4), which creates a disk with a raised edge. Dividi ng the volume of the edge by the to tal volume gives the percent of material in the edge. With this same method, the amount of material covere d by the laser spot is found to be 0.45% of the total nonvolatile mate rial. From the solution concentrations and volumes deposited, about 6 ng of explosive were in the lasers sample volume. The average signals collected by ablating the ~6 ng of RD X and TNT are shown in Figure 4-5. There is a large variance in the measured signal, which is caused by variations in sample crystallization.

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47 Despite the low limit of detection this NOCL de tector exhibited for n itric oxide, extremely small amounts of explosive could be measured. W ith the laser power at 8 mJ, a single laser pulse would produce a measurable quantit y of NO from RDX, with all the material in the beam path removed by the eleventh pulse (Figure 4-6). Figu re 4-7 shows the signal at various amounts of TNT; the smallest signal repres enting 0.5 nanograms of explosive in the beam path. Experiments with laser beam energy did not show a defi nite link of beam energy with signal. It is possible that the laser beam energy is capable of photofragmenting explosive in one step of the pulse, and photolyze some of the NO2 produced in the same pulse. This was probed by measuring the signal at three laser pump voltages with and with out the presence of a heated stainless steel converter (Figure 4-7) the data di d not show an increase in the amount of nitric oxide produced, but rather the highe r laser energy had a higher portion of nitrogen dioxide. It is plausible that the higher laser en ergies showed a higher nitrogen dioxide content because of the ablation of explosive rather than photofragmentation of NO2. Discussion It was somewhat of a surprise that the dete ctor was able to see any signal at all for explosive. However, this doesnt mean that th ere would be enough sensitivity to see vapor phase explosives. Even at the lowest concentration of ablated explosives there would only be 1012 molecules of explosive in the sample area. Atmo spheric pressure would mean a sensitivity of mid parts per billion; however, the laser probe volume would have to be that entire 1 mL.

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48 Figure 4-1: Energy Levels of NO2 and Dissociation to NO. Excitation to the 4A2 state induces dissociation of NO2 to NO. NOCL emission arises from the 2B1 to 2A1 transition. Adapted from 36. 4A 2 2B 1 2A 1 NO & O

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49 Figure 4-2: Image of Explosive Sampling Setup. In set is of crystallized TNT on a 1 inch glass slide. The drop diameter is roughly 1 cm and two holes are laser ablation spots. Sample Quartz Shutter Iris

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50 Figure 4-3: Explosive Sample Stag e and Optics. Beam is trimmed to about 1/4 inch diameter by a variable iris. The mechanical shutter ope ns with a remote shutter cable. A 1.5 inch quartz lens focuses the beam through a qua rtz window onto a gla ss slide mounted on the backside of the sample holder. Ablate d material flows into the NOCL detector (Figure 3-3). Iris Shutter Lens Window Sample Holder Sample to Instrument Beam

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51 0.00E+00 2.00E+05 4.00E+05 6.00E+05 8.00E+05 1.00E+06 1.20E+06 1.40E+06 1.60E+06 1.80E+06 2.00E+06 0%10%20%30%40%50%60%70%80%90%100%110%120% r/RSignal (counts) Measured Signal Suggested Distribution Figure 4-4: Signal from Explosiv e as a Function of Distance from Droplet Center. The solid line represents the measured signal from expl osive droplets. The dashed line is the suggested distribution if resolu tion were better. R is the ra dius of the droplet, while r is the distance from the center of the laser spot to the center of the droplet.

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52 -250 4750 9750 14750 19750 16111621263136414651566166717681869196101106111116 Time (s)Signal (counts/s) Average RDX Average TNT Figure 4-5: Signal Measured from Photofragmented RDX and TNT. The instrument response to both explosives is equal. Error in TNT signa l is shown and very high for peak. This is partially due to very slight differences in the time axis between individual measurements.

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53 1030507090110130150170190210230250270290 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 Signal (counts/s)Time (s) Background1 RDX1 RDX2 RDX3 RDX4 RDX5 Figure 4-6: Signal Measured for Si ngle Laser Pulses on RDX. 0.05 Hz laser rep rate was used to see if single shot de tection was possible. All material in laser spot area is removed by 10 laser pulses.

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54 y = 8.23E 11x R2 = 9.11E 01 0 20000 40000 60000 80000 100000 120000 02E+144E+146E+148E+141E+151.2E+15 Molecules TNTSignal (counts) Figure 4-7: Signal Response as a Function of Photofragmentation Amount. TNT sample was diluted with Acetone to lower the total amount of crystallized e xplosive in the laser spot area. Straight line is linear fit with zero intercept

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55 0 20000 40000 60000 80000 100000 120000 140000 160000 180000 200000 11kV11kV-c12kV12kV-c13kV13kV-c Laser Pump VoltageSignal (counts) Figure 4-8: Signal as a Function of Laser Pulse Energy. Laser pulse energy was varied to observe any photolysis of NO2 at higher laser powers. Bars with -c suffix have a stainless steel catalyst present. Higher laser powers do not indicate photolysis occurs, as the converted signal is greater in each case, i ndicating that at higher laser powers more NO2 is present.

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56 CHAPTER 5 FROM THE LAB TO THE FIELD Summary The original aim of research was to design a fully functioning, field portable, explosive vapor detector. And to that end, th is instrument is a total failure. The limit of detection is not low enough to measure vapor phase concentrations of e xplosives, and frankly the detector is still too bulky for deployment There are some simple changes that could be made to should yield better signal. The spectrum of nitric oxide lies largely in the in fra red beyond the cutoff of the R955. As shown in Chapter 3, only 0.65% of the available signal can be registered because of the signal detection method. Some sources report that placing a nickel mesh in the reaction chamber shifts the emission spectrum to higher energy, moving the peak 1250 nm in the normal NOCL reaction to 800 nm in the catalyzed NOCL reaction. This woul d put a much larger portion of the signal in the spectral response range of the PMT44,45. Replacing the PMT with a large surface area avalanche photodiode, such as those offered by Advanced Photonics, would net a tenfold increase in detector quantum efficiency, as well as coverage of the entire NOCL emission spectrum. Additional physical benefits of usi ng a photodiode would be a larger solid angle of collection by replacing the front window of the reaction chamber with the detector. And a size reduction since the thermoelectric cooling in the photodiode module is located on chip. Even though the NOCL reaction chamber is only 24 mL, the roughing pump, cooling system, ozone generator, pmt, power supply, and esp ecially the laser add up to an instrument that takes up four feet of bench space and weighs more than 100 lbs. Most of these components can be replaced with smaller devices without loosin g sensitivity. Using a photodiode instead of the PMT would cut out the cooling apparatus, la rge power supply, and housing. The roughing pump

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57 is far in excess of what is needed for the syst em. Tinkering in the lab yi elded an ozone generator that was only a few inches long. Bu t most important is the laser. Laser photofragmentation is interesting, but unnecessary. Any source of high intensity ultraviolet light can photolyze NO2 and should also be able to photofragment explosives. An emerging product that offers the monochromocity of a laser, but with a smaller size and lower energy use is the excilamp. Once developed, it c ould be directly substi tuted for the excimer laser46. Another photofragmentation device, de veloped by the NOAA laboratory, uses a 200 W mercury arc lamp focused into a quartz cell to power photolysis47. Otherwise it is possible to completely avoid using photofragmentation to produ ce nitric oxide. Catalyzed pyrolysis, as used in the TEA detector, would be a tremendous savings in both weight and energy over the excimer laser, and the mercury arc lamp. Since photofra gmentation is prone to the same sources of interference as the TEA conversion, there is no loss in specificity.

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58 LIST OF REFERENCES 1. Dolan, J., Langer, S. Explosives in the Service of Man (The Royal Society of Chemistry, Cambridge, UK, 1997). 2. Akhavan, J. The Chemistry of Explosives (The Royal Society of Chemistry, Cambridge, UK, 1998). 3. Pauli, H. E. Alfred Nobel: Dynamite King-Architect of Peace (L.B. Fischer, New York, 1942). 4. Fordham, S. High Explosives and Propellants, 2nd ed. (Pergamon Press, Oxford, 1980). 5. Dick, R. A., Fletcher, L. R., D'Andrea, V., A. Explosives an d blasting procedures manual. Department of the Interior, Bureau of Mines IC 8925 Washington D.C. (1983). 6. Furton, K. G., Myers, L. J. The scientific foundation and efficacy of the use of canines as chemical detectors for explosives. Talanta 54 487-500 (2001). 7. Nambayah, M., Quickenden, T. I. A quantitat ive assessment of chemical techniques for detecting traces of explosives at counter-terrorist portals. Talanta 63, 461-467 (2004). 8. Li, F., Wie, Z., Schmidt, H., Sielemann, S., Baumbach, J. I. Ion mobility spectrometer for online monitoring of trace compounds. Spectrochim Acta, Part B 57 1563-1574 (2002). 9. Ewing, R. G., Atkinson, D. A., Eiceman, G. A ., Ewing, G. J. A critical review of ion mobility spectrometry for the detection of explosives and explosive related compounds. Talanta 54 515-529 (2001). 10. Arusi-Parpar, T., Heflinger, D., Lavi, R. Photodissociation followed by laser induced fluorescence at atmospheric pressure and 24 Celsius: a unique scheme for remote detection of explosives. Appl. Opt. 40 6677-6681 (2001). 11. Wu, D., Singh, J., Yueh, F., Monts, D. 2,4,6-Trinitrotoluene detection by laser photofragmentation laser induced fluorescence. Appl. Opt. 35 3998-4003 (1996). 12. Clark, A., Ledingham, K., Marshall, A., Sande r, J., Singhal, R. Attomole Detection of Nitroaromatic Vapours Using Resonance Enhanced Multiphoton Ionization Mass Spectrometry. Analyst 118, 601-607 (1993). 13. Cabalo, J., Sausa, R. Trace detection of explosives with low vapor emissions by laser surface photofragmentation-fragme nt detection spectroscopy with an improved ionization probe. Appl. Opt. 44 1084-1091 (2005). 14. Baeyens, W., Garcia-Campana, A. Chemiluminescence in Analytical Chemistry (Marcel Dekker, New York, 2007).

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59 15. Jiminez, A. M., Navas, M. J. Chemilumin escence detection systems for the analysis of explosives. J. Haz. Mat. 106A 1-8 (2004). 16. Isaacson, U., Wettermark, G. Chemiluminescence in Analytical Chemistry. Anal. Chim. Acta 68 339-362 (1974). 17. Johnston, H., Crosby, H. Kinetics of the fast gas phase reaction between ozone and nitric oxide. J. Chem. Phys. 22 689-692 (1954). 18. Clyne, M. A., Thrush, B. A., Wayne, R. P. Kinetics of the chemiluminescent reaction between nitric oxide and ozone. Trans. Faraday Society 60 359-370 (1964). 19. Steffenson, D., Stedman, D. Optimiza tion of the operating parameters of chemiluminescent nitric oxide detectors. Anal. Chem. 46 1704-1709 (1974). 20. Priestley, J. Observations on different kinds of air. Philosophical Trans. 62 147-264 (1772). 21. Thomas, M. D., et al. Automatic apparatus for determination of nitric oxide and nitrogen dioxide in the atmosphere. Anal. Chem. 28 1810-1816 (1956). 22. Friedel, R. A. Spectrometric inve stigations of atmospheric pollution. Anal. Chem. 28 1806-1810 (1956). 23. Driscoll, C., et al. Acid rain revisited, advances in scientific understanding since the passage of the 1970 and 1990 clean air act ammendments. Science Links Publications 1 1-24 (2001). 24. Fontijn, A., Sabadell, A., Ronco, R. Ho mogenous chemiluminescent measurement of nitric oxide with ozone. Anal. Chem. 42 575-579 (1970). 25. Joseph, D., Spicer, C. Chemiluminescence me thod for atmospheric monitoring of nitric acid and nitrogen oxides. Anal. Chem. 50 1400-1403 (1978). 26. Dickerson, R., Delany, A., Wartburg, A. Fu rther modification of a commercial NOx detector for high sensitivity. Rev. Sci.Inst. 55 1995-1998 (1984). 27. Kondo, Y., Iwata, A., Takagi, M. Ball oon-borne chemiluminescent sonde for the measurement of tropospheric and stratospheric nitric oxide. Rev. Sci.Inst. 55 1328-1332 (1984). 28. Ridley, B. A., Grahek, F. A small, low fl ow, high sensitivity reaction vessel for NO chemiluminescence detectors. J. Atmos. Ocean. Tech. 7 307-311 (1990). 29. Stone, E., et al. Lightweight ozonizer for fi eld and airborne use. Rev. Sci. Inst. 53 19031905 (1982).

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60 30. Hamamatsu, R928, R955 Spec. Sheet. (1997). 31. Swayambunathan, V., Singh, G., Sausa, R. La ser photofragmentation-fragment detection and pyrolysis-laser-induced fluorescen ce studies on energe tic materials. Appl. Opt. 38 6447-6454 (1999). 32. Deegan, R., et al Capillary flow as the cause of ri ng stains from the dried liquid drops. Nature 389 827-829 (1997). 33. Deegan, R. Pattern formation in drying drops. Phys. Rev. E 61 475-485 (2000). 34. Simeonsson, J., Lemire, G., Sausa, R. Trace detection of nitrocompounds by ArF laser photofragmentation/ionization spectrometry. Appl. Spectrosc. 47 1907-1912 (1993). 35. Nagakura, S. Ultra-violet absorption spectra and pi-electron structur es of nitromethane and the nitromethyl anion. Mol. Phys. 3 152-162 (1960). 36. Gonzalez, A., Larson, C., McMillan, D., Go lden, D. Mechanism of decomposition of nitroaromatics. Laser-powered homogeneous pyrolysis of substituted nitrobenzenes. J. Phys. Chem. 22 4809-4814 (1985). 37. Hancock, G., Morrison, M. The 193 nm photolysis of NO2: NO(v) vibrational distribution, O(1D) quantum yield and emission from vibrationally excited NO2. Mol. Phys. 103 1727-1733 (2005). 38. Sun, F., Glass, G. P., Curl, R. F. The photolysis of NO2 at 193 nm. Chem. Phys. Lett. 337 72-78 (2001). 39. Ledingham, K., Kosmidis, C., Georgiou, S., C ouris, S., Singhal, R. A comparison of the femto-, pico-, and nano-second multiphoton io nization and dissociation processes of NO2 at 248 and 496 nm. Chem. Phys. Lett. 247 555-563 (1995). 40. Pastel, R., Sausa, R. Spectral differe ntiation of trace concentrations of NO2 from NO by laser photofragmentation with fragment ionization at 226 and 452 nm: quantitative analysis of NO-NO2 mixtures. Appl. Opt. 39 2487-2495 (2000). 41. Winer, A. M., Peters, J. W., Smith, J. P., Pitts, J. N. Response of commercial chemiluminescent nitric oxide-nitrogen dioxide analyzers to other nitrogen-containing compounds. Env. Sci. Tech. 8 1118-1121 (1974). 42. Bollinger, M. J., Sievers, R. E., Fahey, D. W., Fehsenfeld, F. C. Conversion of nitrogen dioxide, nitric acid, and n-propyl nitrate to nitric oxide by a gold-catalyzed reduction with carbon monoxide. Anal. Chem. 55, 1980-1986 (1983).

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61 43. Matthews, R. D., Sawyer, R. F., Schefer, R. W. Interferences in chemiluminescent measurement of nitric oxide and nitrogen dioxide emissions from combustion systems. Env. Sci. Tech. 12 1092-1096 (1977). 44. Kenner, R., Ogryzlo, E. Orange chemiluminescence from NO2. J. Chem. Phys. 80 1-6 (1984). 45. Hartek, P., Reeves, R. Formati on and reactions of the excited O2( A3+ u) molecules. Discuss. Faraday Soc. 37 82-86 (1964). 46. Tarasenko, V. F. Excilamps as efficient UVVUV light sources. Pure Appl. Chem. 74 465-469 (2002). 47. Ryerson, T. B., Williams, E. J., Fehsenfeld, F. C. An efficient photol ysis system for fastresponse NO2 measurement. J. Geophys. Res. 105 26447-26461 (2000).

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62 BIOGRAPHICAL SKETCH Ronald James Louis Whiddon was born in Mission Viejo, Californi a. He attended Bemidji State University in Bemidji, Minnesota, gr aduating with a Bachelor of Science degree in biology and chemistry. In fall 2002, he enrolled in the Analytical Division of the Chemistry Department at the University of Florida. Under th e direction of Professor J. D. Winefordner, he completed his graduate studies with a Master of Science in August 2007.