Laser-induced breakdown spectroscopy

MISSING IMAGE

Material Information

Title:
Laser-induced breakdown spectroscopy fundamentals, instrumentation and applications
Physical Description:
x, 244 leaves : ill. ; 29 cm.
Language:
English
Creator:
Castle, Bryan C., 1971-
Publication Date:

Subjects

Subjects / Keywords:
Emission spectroscopy   ( lcsh )
Mineral industries   ( lcsh )
Chemistry thesis, Ph. D   ( lcsh )
Dissertations, Academic -- Chemistry -- UF   ( lcsh )
Genre:
bibliography   ( marcgt )
non-fiction   ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1998.
Bibliography:
Includes bibliographical references (leaves 234-243).
Statement of Responsibility:
by Bryan C. Castle.
General Note:
Typescript.
General Note:
Vita.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 029223962
oclc - 41372612
System ID:
AA00012949:00001


This item is only available as the following downloads:


Full Text







LASER-INDUCED BREAKDOWN SPECTROSCOPY: FUNDAMENTALS,
INSTRUMENTATION, AND APPLICATIONS
















By

BRYAN C. CASTLE


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE
UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA




























To my wife, Cherie, and my parents, Leighton and Suzanne.













ACKNOWLEDGMENTS

I would like to express my sincere appreciation to Jim Winefordner for his guidance

and wonderful support extended to me over the last four years. Jim has created a sound

working environment that encourages experimental freedom and, therefore, increases both

productivity and creativity. The Winefordner laboratories, with the wide variety of

instrumentation and materials, are second to none! My gratitude also is extended to Benjamin

Smith for his friendship, assistance, and always calming presence. When things began to look

overwhelming, Benny was always there to straighten the curves in the road. I have been very

fortunate to have two superb research advisors and friends.

I am grateful to all the members of the Winefordner research group for their support

and friendship. I would especially like to acknowledge those who contributed directly to this

work. These people include Andrea Croslyn, Leslie King, Gretchen Potts, David Rusak,

Ricardo Aucelio, Andrew Knight, Hossein Nasajpour, and Omar Hamori. Additionally, I

would like to acknowledge the following visiting scientists for their input to the presented

work: Kobus Visser, Piet Walters, Nico Omenetto, and Karine Talabardon. In particular,

Kobus, is the only person, other than Jim and Ben, who has molded me into the scientist that

I am today. In the eighteen months we spent performing research together he has taught me,

through example, the ways of science.

My family has always provided me with love and support. I am grateful to my parents

for encouraging me to continue my education, and I am very fortunate to have their guidance








and wisdom. I can not express enough how much the love, support, friendship, and the

sacrifice of my wife, Cherie, means to me each and every day. I hope and pray we live a long

and happy life together so I can repay her for these selfless gifts. I would also like to

acknowledge my God who has always been the rock that makes up my foundation, and the

calm in the storm.

Finally, I would like to acknowledge the support of the chemistry department staff

including Jeanne Karably, Steve Miles, Larry Hamly, Joe Shalosky, Gary Harding, and Dailey

Burch. Each of them have made a contribution to the construction of the experimental

apparatus. For financial support I would like to acknowledge the National Science

Foundation Engineering Research Center for Particle Science and Technology and their

industrial partners, the United States Army, and the Florida Institute of Phosphate Research.














TABLE OF CONTENTS


ACKNOWLEDGMENTS...............................................................................iii

ABSTRACT.................. ...........................................................................ix

CHAPTERS

1 INTENT AND SCOPE OF DISSERTATION...................................................1

2 BACKGROUND OF LASER-INDUCED BREAKDOWN SPECTROSCOPY....3

Introduction to Emission Spectroscopy.............................. ...............3
Introduction to Laser-Induced Breakdown Spectroscopy......................5
Review of Laser-Induced Breakdown Spectroscopy Literature................12
Solids.................................... .... .......................... 15
Fundamental studies........................................ .............15
Analytical results......................... ..........................22
L iquids................................................... ...............................3 1
Fundamental studies..................... ..........................31
Analytical results......................... ..............................39
Gases ............... ...................................................46
Fundamental studies....................................... .............46
Analytical results........................ ................. ...........49

3 FUNDAMENTAL INVESTIGATIONS OF LASER
PRODUCED PLASMAS..................................................58

Part I: Spatial and Temporal Dependence of Lead Emission in Laser-
Induced Breakdown Spectroscopy......................... .................................58
Introduction...................... ............ ...........................58
Experimental..................... ............ .........................59
Results and Discussion............................. ......................... 63
Spectra....................................................................................63
Plasma Imaging......................... ... .................................63
Angular Dependence........................ ........... ..............75
Conclusions........................ .......... ...........................77








Part II: Level Populations in a Laser-Induced Plasma on a Lead Target............81
Introduction..................... .......... .......................81
Theory.............................................................................................. 83
Temperature Determinations Using Spectral Line Ratios..............83
Temperature Determination Using Boltzmann Plots...................84
Experimental................................................ ................................. 85
Lead Spectrum, Energy Level Diagram and Line Selection...........85
Data Capture and Graphical Presentation of the Raw Data...........86
Temporal Development of the 360 nm Series of Pb I Line
Intensities........................ .................. ......88
Background Correction........................................... ...............96
Stripping the 367 and 368 nm Line Spectral Overlap....................97
Height Dependence of the 360 Series ofPb I Line Intensities........99
Boltzmann Plot........................... .......................99
Height and Temporal Development of the Boltzmann
Temperature.................................................108
Conclusions........................ ......... ......................... 108

4 LIBS BENCHTOP INSTRUMENT: DEVELOPMENT AND
EVALUATION ................................... ................................. ................110

Introduction................................................110
Developm ent...................................................... 111
Instrument Overview.......................... .......................111
Pierced Mirror.................... ................ 15
Fundamental Evaluations............................ ....... 18
M ass Removal..................................... 118
Plasma Temperature............................................ 120
Optimized Gate Settings................. ......... ....121
Analytical Evaluations.......................... ....... ...126
Sampling Procedure.............................. ..... ...... ....126
A lloys................................. 126
Soils.... ...................... ......... 128
Paints.................................................... 133
Organics...................................................133
Detection Limits...........................................133
Day-to-Day Reproducibility....................... ............. ... 137
Depth Profiling.............................................139
Conclusions.......................... ................. ...............145










5 LIBS PORTABLE INSTRUMENT: DEVELOPMENT AND
EVALUATION ........................................................... .......................146

Introduction............................................ ......... 146
Experim ental........................................... ......... 147
Instrum ent Design................................................................. 147
Spectral Imaging Apparatus........................................ ...........149
Sample Preparation...................... ................................150
Results and Discussion............................ .. ........... ............ 150
Plasm a Imaging.......................... ......................................... 150
Lens-to-Sample Distance.......................................................151
Evaluation of Probe Design..................................................... 154
Lens-to-sample distance........................................154
Spatial filtering......................... ............................... 156
Analytical Applications...................... ................... .............. 156
Analysis of paint samples.......................................156
Analysis of NIST steel samples...................................160
Analysis of NIST iron ore samples................................164
Analysis of NIST organic samples.............................166
Conclusions........................ .... ..................... ........................166

6 VARIABLES INFLUENCING THE PRECISION OF LASER-INDUCED
BREAKDOWN SPECTROSCOPY MEASUREMENTS............................... 169

Introduction..................... ..... ............................... 169
Experimental...................... ........... .........................172
Results and Discussion. ...................................................................173
Choice of Analytical Line......................................................173
Signal Intra-Measurement Development.................................. 173
Dependence of Precision on Sample Movement.......................178
Dependence of Precision on Number of Laser Shots
A ccum ulated....................................................................... 184
Pulse to Pulse Stability of the Laser, and its Effect on Emission
as a Function of Pulse Energy.........................................185
Dependence of Precision on Gate Delay..................................... 188
Dependence of Precision on Surface Roughness.......................189
Dependence of Precision on Background Correction................191
Conclusions........................ .......... .........................193

7 MATRIX EFFECTS IN LASER-INDUCED BREAKDOWN
SPECTROSCOPY................................. ..........................194

Introduction...................................................... 194
Background.................................. ..........................194








Experim ental........................................................ ............................ 197
Results and Discussion...................... .... ........................199
Bulk M atrix Effects............................... .......................199
Internal Standards....................... .............................205
Speciation Effects............................. .. .....................214
Zinc Matrix Effect.............................. .....................215
C onclusions................................................................... ..........219

8 ANALYSIS OF ORES: PROGRESS TOWARDS DEVELOPMENT OF A
PROCESS M ONITOR.............................................................. .......220

Introduction................................................................ ........................220
Experim ental...................................................................... ....... 222
Results and Discussion............................... ................... 222
Phosphate Rock Analysis.........................................................222
Iron Ore Analysis............................ ........................224
Future Directions.......................... ................... ...................... 228

9 CONCLUSIONS AND FUTURE WORK........................................................231

A PPEN D IX ........................................................................ ....................................233

REFER EN C E S............................................. ........................... ............ ..............234

BIOGRAPHICAL SKETCH...................................................244
























viii













Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


LASER-INDUCED BREAKDOWN SPECTROSCOPY: FUNDAMENTALS,
INSTRUMENTATION, AND APPLICATIONS

By

Bryan C. Castle

May, 1998


Chairman: Prof. James D. Winefordner
Major Department: Chemistry


In laser-induced breakdown spectroscopy (LIBS), a laser pulse of sufficiently high

power is focused to produce an irradiance that exceeds the material's breakdown threshold

of 10-100 MW cm2. If the electric field in the focused laser beam is greater than the

dielectric field strength of the material it encounters somewhere in the focal volume,

breakdown occurs at that point and eventually results in the formation of a transient, highly

energetic plasma. For analytical purposes, the emitted electromagnetic radiation is spectrally

resolved, and the emitting species in the laser-induced plasma are identified and quantified by

their unique spectral wavelengths and their line intensities.

Fundamental investigations of laser-induced plasmas have resulted in a better

understanding of their spatial and temporal development. When exploring the lead level








populations in a LIBS plasma, it was determined that over the first 15 ps of the plasma's life

time the energy distribution of the excited atoms does not follow a Boltzmann distribution.

Two different types of LIBS instrumentation have been constructed and evaluated.

A benchtop instrument for laboratory use, and a battery-powered, portable instrument for

field use. Both were evaluated for elemental determinations in alloy, soil, paint, ore and

organic matrices.

Investigations were conducted concerning the variables that influence the precision

of LIBS measurements. These included the choice of analytical line, sample movement,

number of measurements, laser stability, detector settings, surface morphology, and data

treatment.

The effect of a change in matrix on LIBS measurements was explored. Five different

sensitivities were observed for the analysis of zinc and chromium in aluminum, copper,

graphite, sand, and potassium bromide matrices. One possible method for compensation was

through the use of an internal standard. The internal standard can be selected based on the

standard criteria, with the addition of the evaluation of the temporal development of the

analysis and internal standard elements signals.

We also report on the investigation of the usefulness of LIBS as a process monitor in

a mining facility. Satisfactory results have been obtained for both phosphate rock and iron

ore minerals in the laboratory. Current research efforts have focused on the transferring of

the LIBS technology from the laboratory to the processing site.













CHAPTER 1
INTENT AND SCOPE OF DISSERTATION

It is my impression that the forces directing university research have undergone some

drastic changes in recent years. At one time the majority of experiments conducted in the

university setting were driven by the needs of fundamental science. More recently, the focus

has shifted from being fundamentally driven to being industrial driven, that is, research is

accomplished with a set goal in mind. It is not my intension to criticize or praise this change,

it is only mentioned to help place the research presented in this dissertation into the right

context.

From the beginning, my research has been industrially directed. Through our

involvement with the National Science Foundation's Engineering Research Center for Particle

Science and Technology we have had the opportunity to interact with several mining

industries. These include the phosphate mining companies located in Florida, PCS Phosphate,

Inc. (White Springs), Cargill Fertilizers (Fort Meade), and CF Industries (Wauchula), and also

one iron ore company located in Michigan, Cleveland Cliffs Mining (Marquette). Through

these interactions we learned of the industry wide need for an analytical method capable of

providing quantitative, elemental information, on-line, in real time. We believed that the

technique of laser-induced breakdown spectroscopy, as known as LIBS, could be a possible

solution to their problems. It was through this interaction that the research contained in this

dissertation was founded.






2

Chapter 2 provides a brief overview of both emission spectroscopy and laser-induced

breakdown spectroscopy, and concludes with a through review of the recent literature

concerning the fundamentals and applications of LIBS. Chapter 3 presents research on the

fundamental investigations of laser produced plasmas. This was a collaborative project with

Kobus Visser, a visiting professor from University of Stellenbosch in the Republic of South

Africa. This chapter includes all of our initial investigations into LIBS completed over a one

year period, including our research into the spatial and temporal development of lead emission

in LIBS plasma, as well as investigations into the lead level populations in a LIBS plasma.

Chapter 4 presents the research on the construction and evaluation of a LIBS benchtop

instrument. Chapter 5 presents the research on the construction and evaluation of a battery

-powered, portable LIBS instrument. The following three chapters report on further

evaluations conducted using the LIBS benchtop instrument. Chapter 6 reports on the

investigations of the variables influencing the precision of LIBS measurements. Chapter 7

presents studies on matrix effects in LIBS. Finally, chapter 8 reports on the preliminary

results obtained from the analysis of mining minerals, including phosphate and iron ores.

The presentations, publications, and manuscripts that have resulted from this research

are listed in the Appendix.













CHAPTER 2
BACKGROUND OF LASER-INDUCED BREAKDOWN SPECTROSCOPY

Introduction to Emission Spectroscopy

Atomic spectroscopy is based on the concept that atoms, when properly energized,

may re-emit the absorbed energy as electromagnetic radiation having frequencies

characteristic of the radiating species, and with an intensity proportional to the number of

atoms radiating. Therefore, the frequencies at which emission is observed serve to identify

the radiating species, and the intensity of the emitted radiation is a measure of the number of

atoms responsible for the emission. These two properties are the basis of qualitative and

quantitative atomic spectroscopy.

In the late 1600s, Sir Isac Newton was credited with the first investigations of visible

radiation, with his discovery of the seven colors of white light [1]. It wasn't until the mid-

1800s through the work of Kirchhoff and Bunsen, that the qualitative usefulness of analytical

spectroscopy was discovered [2]. Through their work with flame emission, they

demonstrated the identification power of spectroscopy by identifying several elements based

on their flame emissions. The quantitative usefulness of spectroscopy was not totally realized

until the 1920s with the work of Gerlach and Schweitzer [3]. Since this time, the field of

spectroscopy has really blossomed, and currently analytical chemists are realizing the fruits

of these, and many more earlier discoveries.

The three basic components of an emission spectroscopy apparatus include the

excitation source, wavelength selector, and the radiant power detector. The excitation source








is required to produce free atoms from any sample and to transform these atoms into excited

species. The wavelength selector is needed to provide a specific frequency(s) to the

detector. The radiant power detector is capable of quantifying the intensity of

electromagnetic radiation emitted from the excited atoms at a given frequency determined by

the wavelength selector.

Early excitation sources were primarily limited to flames [4]. Since they proved too

inefficient for the excitation of certain materials, in particular refractory materials, arcs and

sparks were developed [4]. Arcs and sparks grew in popularity primarily because of their

ability to analyze most conductive samples with minimal sample preparation. The

disadvantage of these sources is their poor stability. In the 1960's, the development of stable

high-frequency and de plasma devices offered an even more energetic excitation source [5].

These sources offered improved sensitivity which translated into lower detection limits. One

other emission source which has grown in popularity, is the low pressure discharge, one of

which is the glow discharge [6].

There have been several different types of wavelength selectors used in emission

spectroscopy. These devices are primarily based on the principles of absorption, interference,

spatial dispersion, or interferometry. The most commonly used are the spatial dispersion

devices, typically consisting of a ruled grating [7]. The grating is placed in a sealed box

(spectrometer) that allows light to enter and exit through the entrance and exit slits. These

are adjustable to vary the resolution and light collection efficiency of the device. Collimating

optics are used to direct a parallel beam of radiation onto the grating. The grating is a plane

mirror that is ruled with closely spaced groves. The grating operates under the principle that

when different wavelengths of light striking the grating are reflected, they constructively





5

interfere at different angles relative to the normal. Therefore, as the grating is rotated,

different wavelengths of radiation will be directed onto the exit slit of the spectrometer.

The radiation at the exit slit of the spectrometer is monitored by the radiant power

detector. Examples of these are photomultiplier tubes (PMT), photodiodes (PD), photodiode

arrays (PDA), charge-coupled devices (CCD), and charge injection devices (CID) [7]. These

detectors convert the incident photon flux into a measurable electrical signal The PMTs and

PDs are single-channel detectors, while the PDAs and CCDs are multi-channel detectors.

The multi-channel detectors are more complex and expensive, but have the advantage of

simultaneously monitoring several wavelengths and the background between them.

Introduction to Laser-Induced Breakdown Spectroscopy

Soon after the development of the ruby laser, it was realized that when the laser

radiation was focused, the intense light beam was capable of vaporizing and exciting solid

material into a plasma. The possibility of using lasers as excitation sources in atomic emission

spectroscopy was first demonstrated by Breech and Cross in 1962 [8]. This technique,

known as laser-induced breakdown spectroscopy (LIBS), has been reviewed by several

researchers [9-12]. LIBS is based on the concept that when a nanosecond laser pulse of high

energy density strikes the surface of any material, the surface temperature is instantly

increased beyond the vaporization temperature. The coupling of the pulse energy into the

sample occurs through several (often unknown) mechanisms including single and multi-

photon absorption and dielectric breakdown [13-14]. The dissipation of this energy through

vaporization is slow relative to the rate at which energy is deposited. Therefore, before the

surface layer can vaporize, the underlying material reaches critical temperatures and pressures,

causing the surface to explode. The ablated material, in the form of particles, free electrons,





6

atoms, and ionized atoms, expands at a supersonic speed and forms a shock wave in the

surrounding atmosphere. After several microseconds, the plasma plume slows down via

collisions with ambient gas species, and at this point the shock wave separates from the

plasma front and continues propagation at a speed approaching (or exceeding) the speed of

sound. Electron number densities on the order of 10" to 10"9 cm3 and plasma temperatures

in the range of 104 to 105 K have been reported [15-16]. At this stage, the plasma begins to

decay through radiative, quenching, and electron-ion recombination processes that lead to a

formation of a high dense neutral species zone in the post-plasma plume. The decay typically

ends with the formation of clusters, which usually occurs within hundreds of microseconds

after the plasma has been ignited. For analytical purposes, the emitted radiation (integrated

over the first tens of microseconds) is spectrally resolved, and the emitting species in the

laser-induced plasma are identified and quantified by their unique spectral wavelengths and

line intensities.

A simplified experimental apparatus required for LIBS analysis is shown in figure 2-1.

A pulsed laser source is used to vaporize and excite the material present at the focal point of

the focused laser beam. The emitted radiation from the laser-produced plasma is collected

and dispersed by the spectrometer and then quantified by the detector. A digitally enhanced

photograph of a laser-produced plasma is shown in figure 2-2. The plasma consist of three

primary regions, the high temperature core, the lower temperature middle, and the expanding

outer edge known as the shock wave. The total volume occupied by a laser plasma formed

under atmospheric conditions is approximately 3 mm3. The plasma is a transient source with

a typical lifetime ofapproximately 50 ps. Figure 2-3 shows the decay of the emission spectra

observed from a pure lead sample. The front most spectrum was captured at a delay of 0.8











Detector


Spectrometer L




.N d-A Nd-YAG Laser


Figure 2-1. Schematic of basic LIBS experimental apparatus.









HOT, VAPOR PLASMA






SHOCK WAVE




RADIATION


Figure 2-2. Digitally enhanced photograph of a laser-induced breakdown plasma.








t
Pbl
368.35 nm


Pb I
363.96 nm
Pb I
357.27 nm Pb I
373.99 nm
Pb I
367.15 nm












355 360 365 370 375
Wavelength (nm)

Figure 2-3. Temporal development of a series of lead emission lines observed in a LIBS plasma.





10

ps with respect to the laser pulse, and successive spectra at 0.5 js additional increments. It

can be noted from this figure that over a 14 Ps interval (as shown in this case) the emission

signal reaches a maximum and then begins to decay. This indicates that by using gated

detection, the signal-to-noise ratio can be optimized.

There are several attractive features to LIBS. One of the most attractive is that only

optical access to the sample is required. This has allowed LIBS to be used for remote sensing

and process monitoring. Also, this is what distinguishes the LIBS technique from

conventional plasma emission spectroscopy. The sample doesn't need to be transported to

the plasma source, because the plasma is formed within the sample or at the surface of the

sample. Another feature of the technique, from a spectrochemical analysis point of view, is

that it provides simultaneous multi-element capability with minimal, if any, sample

preparation. However, there are some severe problems to overcome with this method

including the variable mass ablation from heterogeneous samples which causes elemental

quantitation to be problematic. This could account for the fact that the LIBS technique has

not yet reached its full potential. An overview of the advantages and disadvantages of LIBS

is listed in table 2-1.

LIBS appears at first glance to be quite simple because the vaporization, atomization,

and excitation processes are carried out in one step by the laser pulse. However, the

mechanism under which these occur is not clearly defined. The reason for the complexity is

based on the large number of variables that influence the ablation process. Several of these

variables and their effects have been studied including: laser pulse temporal duration and

shape,[17] laser wavelength,[18-20] laser energy,[21-23] physical and chemical characteristics

of the target material,[24-25] composition and pressure of the surrounding environment,[26-

28] and effect of magnetic field [29-30].


















Table 2-1. Advantages and disadvantages of laser-induced breakdown spectroscopy.

Advantages Disadvantages

1. Minimal (no) sample preparation 1. Variation in the mass ablated caused by
changes in the bulk matrix.

2. All states of mater can be analyzed, as 2. Difficulty in obtaining matrix matched
well as both conductive and standards.
nonconductive samples

3. Very small amount of material is 3. Detection limits higher (poorer) than
vaporized (10s ofng) standard solution techniques (i.e. ICP-
OES)

4. Easy analysis of refractory materials 4. Poor precision, typically 5 10%
such as ceramics

5. Micro analysis is possible with spatial 5. Standard emission disadvantages, such
resolution of 1-10 pm as spectral interference and self absorption

6. Capability of remote analysis in harsh 6. Possibility of optical component
environments damage from high energy density lasers

7. Atomization and excitation are in one 7. Complexity
step

8. Capable of simultaneous multi-element
analysis








Review of Laser-Induced Breakdown Spectroscopy Literature

There have been several publications in the last decade which have reviewed laser-

induced breakdown spectroscopy either as a unique method of elemental analysis, or as a

member of the family of atomic emission techniques. LIBS has also been addressed in

reviews on laser applications, process monitoring, and materials processing. Many

spectroscopic studies of laser plasmas have also appeared in the physics journals. In order

to construct a meaningful review, the literature must first be reduced to a subset. In this

instance, LIBS in analytical chemistry is chosen.

This review will cover fundamental studies and analytical results and applications of

LIBS related to the field of analytical chemistry. The review is divided according to target

phase. Solids, liquids and gases are treated in sections devoted to each. Articles from the

physics literature are included when they are of interest to the analytical chemist.

Spectroscopic studies of laser-induced plasmas created in pulsed-laser deposition (PLD)

experiments have, for the most part, been omitted under the assumption that these papers are

more pertinent to materials science than to analytical chemistry. Fundamentals and

applications of laser ablation as a sampling technique are also ignored on the grounds that,

in these instances, the laser plasma does not serve as the excitation source.

The time frame to be covered has not been specifically defined because the amount

of literature dealing with each of the three phases of targets differs greatly. The literature on

solids is easily the largest, and so the review of solids is taken almost entirely from

publications within the last 5 years.

Literature on laser-induced breakdown in gases is the oldest. Many of these

publications are in the physics literature and deal with mechanisms of breakdown and plasma





13

diagnostics. There are a smaller number of papers that report upon the determination of trace

metals in gas phase matrices. Most of these are intended to prove the usefulness of LIBS in

hazard monitoring. In the section on gases, the analytical chemistry literature over the past

10 years is reviewed and appropriate additions are made from the physics literature.

The section dealing with liquids includes bulk liquids, isolated droplets, and aerosols

generated from liquids. Among the chemistry journals, this literature is the smallest. The

physics is of interest primarily because of medical applications of laser-induced breakdown,

and publications which are judged to be of interest to the analytical chemist have been

included. All publications related to the elemental analysis of liquids by LIBS during the last

14 years are included.

Previous reviews on this topic begin with a 1984 review by Adrain and Watson titled

"Laser Microspectral Analysis: A Review of Principles and Applications" [31]. This was

followed by Cremers and Radziemski who published "Laser-induced Breakdown

Spectroscopy: Principles, Applications, and Instrumentation" in 1990 [32]. This paper

included a brief review of theory and instrumentation for LIBS followed by a few industrial

applications. In 1992, Thiem et al. reviewed LIBS theory as part of their paper titled "Lasers

in Atomic Spectroscopy: Selected Applications" [33]. The section on laser-induced plasma

covered laser-material interaction, plasma production factors (wavelength, energy, and

properties of target), and emission factors (temperature and electron density).

Majidi and Joseph published "Spectroscopic Applications of Laser-Induced Plasmas"

in 1992 [34]. This publication has perhaps had the most influence in terms of style and

content on the review that follows. Majidi and Joseph reviewed analytical results on solids,

liquids, gases, and mixed phase systems for the years 1987-1992. Emphasis was on





14

applications of LIBS such as determination of hazardous elements in air, toxic elements in

wastewater, and elements of interest in coals and iron ores. The authors stressed the fact that

LIBS required only optical access to samples.

In 1993, Ibrahim and Goddard published "An overview of Laser-Induced Breakdown

Spectroscopy" [35]. They concentrated on the topics of laser-material interaction, local

thermodynamic equilibrium, and plasma diagnostics. The instrumentation was described, the

use of gated detection was explained, and a few applications were presented. It was not a

literature review.

In a 1993 review by Darke and Tyson entitled "Interaction of Laser Radiation with

Solid Materials and its Significance to Analytical Spectrometry," applications of LIBS were

reviewed in the section on laser ablation [11]. In this paper, LIBS was referred to as laser

microprobe optical emission.

The following year application of LIBS to process control was briefly reviewed by

Noll et al [36]. The authors discussed fundamentals of plasma formation and laser-material

interaction with respect to optical and heat penetration, laser energy, density, and absorbance.

The use of fiber optics and on-line sampling was discussed, and a periodic table was presented

showing elements which had been determined and their detection limits in iron ore. This idea

was incorporated into the review that follows. Each section was concluded with a periodic

table or tables with elements that have been determined and shaded according to detection

limit.

Most recently, LIBS was briefly reviewed as a section in the comprehensive review

of atomic emission by Sharp et al. published in 1995 [37].








Solids

Fundamental studies

Many papers in this area stem from the need to more fully understand the LIBS

plasma in order to obtain useful quantitative results. One such paper is the 1988 publication

by Chen and Yeung who used the acoustic signal generated by a laser-induced plasma as an

internal standard [38]. They reported that the magnitude of the acoustic wave was

proportional to the emission signal for major and minor elements within the solid target.

Furthermore, they found that this proportionality was independent of laser power and focus

spot size.

In the same year, Wood et al. studied the effect of laser pulse duration on soft x-ray

emission from a tantalum target [39]. Using a colliding-pulse mode-locked dye laser at 620

nm with pulse durations of 100 and 600 fs and a Nd:YAG laser at 1064 nm with a pulse

duration of 70 ps, the plasma emission in the range from 10-71 nm was observed. It was

found that longer pulses gave relatively shorter wavelength emission and longer emission

lifetimes than short pulses.

In 1989, Coche et al. used laser-enhanced ionization detection in a laser plasma to

study the processes of ionization and recombination [40]. They used a N, laser (337.1 nm,

5 mJ, 10 ns) to ablate a solid target. A dye laser was used to selectively ionize species in the

plasma at different delays relative to the ablation pulse. Optogalvanic detection was used to

give an indication of the number of atoms in the probe volume at the chosen time delay. In

this way, ionization and recombination rates could be inferred.

Also in 1989, lida studied the atomic emission characteristics of a laser plasma in

reduced pressure argon [41]. Using a ruby laser (1.5 J, 20 ns), a plasma about 10 mm in





16

height, and more than 100 (is in duration was formed at pressures between 0 and 50 Torr.

The plasma at 50 Torr had greatly increased line emission and background due to confinement

by the Ar atmosphere. The plasma at lower pressure showed less background and less line

emission.

Another investigation of plasma expansion was done by Balazs et al. in 1991 [20].

These authors investigated ruby laser pulses on a copper target, and developed a two-part

model for the interaction. The first part of the model dealt with the heating and melting of

the solid and included parameters such as thermal diffusivity, pulse duration, and material

density. The second part of the model described the plasma expansion into a vacuum.

Mason and Goldberg have characterized a laser plasma in a pulsed magnetic field [30].

The first part of the paper included spatially resolved emission studies. The authors found

that when a pulsed magnetic field was oriented normal to the laser beam, it caused radial

compression and axial expansion in the plasma. The emission intensities of both atoms and

ions were also increased in this magnetically confined plasma.

In the second part of the paper, the authors discussed time resolved emission and

absorption studies. They concluded that the increase in emission seen in the plasma was due

to Joule heating caused by the induced secondary current in the plasma. This was evident by

the fact that the increase in emission was seen later in time than the maximum of the applied

magnetic field. Also, the intensity increase was attributed to increased atomization efficiency

and longer residence times in the plasma.

The effects of buffer gas type on the plasma produced by a Nd:YAG laser (1064 nm,

100 mJ, 7 ns) on a metal target were studied by Owens and Majidi in 1991 [27]. They

observed an increase in the ratio of Al II / Al I intensity in helium gas relative to argon gas






17

and air. This increase was attributed to the ability of excited helium atoms to transfer energy

to a similar energy level in the aluminum ion.

More metal target studies were evaluated by Lee et al. in 1992 [25]. These authors

used an ArF laser (193 nm, 100 mJ, 10 ns) to produce plasmas on copper and lead. The lead

plasma was much larger than the copper plasma (5 mm vs 2 mm) and had a slightly lower

excitation temperature as determined by Boltzmann plots. The temperature of the copper

plasma was 13200-17200 K, and the lead plasma was 11700-15300 K.

In 1992, Marine et al. studied plasma expansion by optical time-of-flight

measurements [42]. They determined that the velocity distribution of ions produced by a UV

pulse of several nanoseconds duration was broad and not well defined. In contrast, the ions

produced by a picosecond IR pulse traveled with a velocity inversely proportional to the

square root of their mass. Neutral atoms still had a poorly defined velocity distribution.

These authors also noted the appearance of a bi-modal temporal profile for YO' emission and

attributed this to two possible mechanisms for formation of YO'.

Kuzuya et al. studied the effect of laser energy and atmosphere on the emission

characteristics of laser-induced plasmas [22]. They used a Nd:YAG laser at 1064 nm and

pulse energies from 20-95 mJ in atmospheres of He, Ar, and air from 1 Torr to 1 atm. The

authors reported that the maximum emission intensity was observed at 95 mJ in 200 Torr of

Ar. However, the maximum signal to background was obtained in helium at 40 Torr and 20

mJ of power. Images showed the different sizes and shape of these plasmas.

Okana et al. studied mass removal in non-metallic inorganic solids and determined

relationships between laser power and atom yields [43]. Their paper described vacancy

initiated laser ablation as a process by which weakly bound atoms were released from around






18

vacancies and vacancy clusters. The atom yield was determined to be an exponential function

of laser fluence.

Time resolved emission studies from a laser plasma on sodium chloride were reported

by Yago et al. in 1993 [44]. They used a Nd:YAG laser of 150 ns duration focused onto a

NaCI pellet. The emission spectra showed self-reversal in air but not in a vacuum. The

plasma was divided into two zones, a hot core behaving as an emission zone, and a low

temperature periphery behaving as a reabsorption zone. The plasma expansion rate was

shown to be determined by ambient gas pressure.

In 1994, Kagawa et al. used a XeCI laser (308 nm, 15-70 mJ, 20 ns) to produce

plasma on a Zn target in vacuum [45]. Time resolved studies showed a number density jump

which represented the blast wave expansion into the observed volume. This outermost

portion of the plasma was shown to be ideal for analytical measurements because the

background in this area was greatly reduced.

In 1994, Tambay and Thareja studied emission in a laser plasma of Cd metal vapor

formed in a heatpipe [46]. They showed that emission from the vapor was stronger when the

plasma was formed on a tungsten target than it was when the plasma was formed in the gas

itself The authors claimed that this was due to pumping of the vapor by soft x-rays formed

on the tungsten target.

Transition probabilities of 28 Si ion lines were determined using a laser-induced

plasma as a source by Blanco et al. in 1995 [47]. Using a Nd:YAG laser (1064 nm, 280 mJ,

10 ns) to produce a plasma on pure silicon, these authors observed emission in Ar and Kr

atmospheres. The plasma produced was found to have an excitation temperature of 20000

K and an electron density of 10"7 cm'. Absolute transition probabilities for the Si ion lines

were calculated.






19

Jensen et al. published mechanistic studies of laser-induced breakdown on model

environmental samples in 1995 [24]. They used a KrF laser (248 nm, 30 ns) to produce

plasmas on SiO2 containing Eu and Cr, which were added as the solids Eu2O3 and K2Cr2O,

to the sand. Detection limits of 100 ppb for Eu and 2 ppb for Cr were reported. A sample

was also prepared in which the source of Cr was a solution which was added to the sand and

then evaporated. This method of sample preparation gave an order of magnitude less signal

and a different temporal profile for the Cr emission.

In 1995, Tasaka et al. studied the emission of a laser-produced plasma on graphite

[48]. They used a Nd:YAG laser in He and in air to form a plasma in which they observed

a "triple plume" composed of three distinct regions. The authors claimed that these regions

appeared because of three different speeds in the expanding plasma. The fastest region was

composed of carbon ions from the target and N and O ions when the experiment was done

in air. The second plume was composed of the compressed neutrals in the vicinity of the

shockwave. The slowest plume was then the target vapor composed of larger molecules.

Thareja et al. also studied graphite plasmas at low pressure and found similar temporal

profiles [491.

Intense emission from the CN radical has been observed in plasmas produced on

graphite. With the use of a high resolution spectrometer, several authors have resolved the

vibrational and rotational structure in the CN emission bands. The emission from the violet

band of CN around 388 nm has been used to calculate vibrational and rotational temperatures

in the laser plasma [50-531.

In 1996, Al-Wazzan et al. studied three-dimensional number densities of species in

laser produced plumes [541. They used absorption of an expanded dye laser beam to form

shadow images of Ba ions in a plume produced by excimer laser ablation at 248 nm. They






20

also used fluorescence from planar slices of the plasma to obtain sequential cross section

images which could be built into a three-dimensional image.

Al-Wazzan et a. also carried out an experiment in which they observed plasma in

vacuum and in ambient oxygen [551. In oxygen, the expanding plume showed increased

temperature and electron number density at the shock front due to increased collisional

excitation rates. In a vacuum, enhancement at the shock front was not observed.

Bulatov and Liang obtained full spectra at each pixel in the image of a laser-induced

plasma as depicted in figure 2-4 [561. They used this technique to create classification maps

which gave location of any species of interest within the plasma. They studied the effects of

different focal length lenses and different sampling geometries on plasma formation and

location of species within the plasma.

In 1996, Mulatari et al. obtained time resolved images from a laser plasma formed on

a sample at non-normal incidence [571. They varied the angle of the incident laser beam with

respect to the target from perpendicular to nearly parallel. It was found that the plume was

generated in the perpendicular direction regardless of the laser angle. However, the maximum

emission signal was obtained with normal incidence.

Mulatari and Cremers also published a second study that year reporting on the use of

an acousto-optic tunable filter to capture spectrally resolved images [581. They used a series

of different lenses to examine the different distributions of elements within each plasma. By

collecting light from the outermost edge of the plasma, they were able to use ungated

detection to obtain analytically useful spectra with low background.

Nemet and Kozma studied the shape of emission lines produced on gold targets at

different delays relative to the ablation laser [591. The 406.51 and 389.79 nm lines were









Jl1/C


Figure 2-4. Spectroscopic imaging of a laser-induced plasma.


~ -~J~h






22

observed. These lines were asymmetric and shifted relative to their natural wavelength at

times up to 1000 ns after the laser pulse. These lines could be described by asymmetric

Lorentz-type profiles. After 1000 ns, the lines appeared to be very close to their natural

wavelength and were Lorentzian and symmetric.

In 1997, Kuriawan and Kagawa used a long pulse Nd:YAG laser to produce a

plasma on a brass target in vacuum [601. The authors were especially interested in the

secondary plasma formed by compression in the vicinity of the shock wave. The emission in

this secondary plasma was captured, and it was observed that when a wedge of aluminum was

placed very close to the target, the emission signal increased in the vicinity of the wedge. The

authors attributed this effect to increased compression provided by the shock wave interaction

with the immovable aluminum wedge.

In 1997, Martin et al. used a laser-induced plasma to establish spectral calibration of

their detector [611. A Nd:YAG laser (1064 nm, 350 mJ) was focused onto targets of

ceramic, polyethylene, ZnS, and aluminum, all in vacuum. Line pairs in the deep UV were

used with known transition probabilities to determine the relative efficiency of the detection

at each wavelength.

Most recently, Granse et al. modeled a laser-induced plasma and compared their

model to experiments with different lasers and different materials [621. The model they

derived accounted for fluid dynamics of the plasma, absorption of laser energy via inverse

bremsstrahlung, and the dynamics of ionization and recombination.

Analytical results

The number of papers describing analytical results of LIBS studies on solids is easily

larger than the number of papers dealing with either liquids or gases. For this reason, only





23

publications after 1992 are considered in this section. Figures of merit given for specific

determinations differ greatly from one author to the next. In the periodic tables which follow

this section, the lowest published detection limits for the elements in the matrix of interest are

used to determine the shading.

Carbon content in steel was determined by Aguilera et aL in 1992 [631. The authors

observed the emission from the 193.1 nm carbon line because the 247.9 line had a spectral

interference. The experiment was carried out in a CO2 free environment, and a neighboring

iron line was used as an internal standard. A Nd:YAG laser at 1064 nm was used, and it was

found that the slope of the calibration curves decreased with increasing laser power. Also,

there was a slight difference in slope for stainless and non-stainless steels. At a laser power

of 100 mJ, the detection limit for carbon was 65 ppm and the RSD was 1.6 %.

Also in 1992, Hader used LIBS for on-line quality control of rubber mixing [641. The

author called the technique "remote laser microanalysis" and used the acronym RELMA. The

existing technique for quality control of rubber mixing involved a discrete sampling step in

which a small portion of rubber was taken from a batch and analyzed. This led to problems

when it became necessary to distinguish between bulk composition fluctuations and

inhomogenieties; with on-line sampling by LIBS in a number of randomly chosen positions

in the rubber mix, the two problems could be distinguished. Elements were found in the

cross-linkers and plasticizers added to the raw rubber.

Lorenzen evaluated other on-line applications of LIBS in the same year [651. This

time the technique was referred to as "laser-induced emission spectral analysis" and given the

acronym LIESA. The authors described applications such as the determination of minor

elements in liquid steel, and depth profiling of layers on metallic substrates.






24

In 1993, Sabsabi et at. used a KrF laser (248 nm, 100 mJ) to analyze aluminum alloys

[661. They carefully optimized the delay time by using a PMT to look at the temporal

evolution of several different emission lines. An aluminum line was used as an internal

standard and magnesium was determined at a few ppm.

The following year Thiem et al. investigated LIBS of alloy targets [67]. They

determined Al, Cu, Fe, Ni, and Zn using a Nd:YAG laser at 532 nm in a vacuum chamber.

Using non-resonant lines for all the elements, the authors were able to generate linear

calibration curves from a few hundred ppm to a few percent for each element. For the

determination of these elements at the percent level, the authors found many potential

emission lines in the spectral window from 300 to 400 nm.

Thiem and Wolf used LIBS to analyze mining ores and compared the results to those

obtained by a digestion ICP-AES method [681. AI, Ca, Cu, Fe, K, Mg, Mn, Si, and Ti were

determined in aluminum and manganese ore. The comparison showed that both methods

gave roughly the same accuracy, but the ICP technique had better precision. The authors also

pointed out the similarity in cost for the two techniques, about $100,000 for either.

Investigations of the use of resonant wavelengths in laser ablation had been made as

early as 1992 by Borthwick et at [69]. Although most of the work was carried out with time-

of-flight mass spectrometry, the paper is included here because of its relevance to LIBS. The

authors noted that when an ablation laser was scanned through specific wavelengths, an

enhanced ion yield was detected. More specifically, an enhancement in Ga ions in the ablation

of GaAs, and an enhancement in Al ion yields in ablation of steels were noted. The ion yields

were even more pronounced when grazing angles of incidence of the ablation laser were used.





25

In 1995, Allen etal. employed resonant wavelengths in LIBS of thin films [70]. The

authors used resonant laser ablation time-of-flight mass spectrometry to investigate copper

thin films on a silicon substrate. Using a XeCl excimer laser pumped dye laser (5 mJ pulse

energy at 463.51 nm) multi-photon ionization of copper was observed. By using an

unfocussed laser continually and watching the decay of the copper signal, the authors were

able to determine the thickness of the film between 20 and 100 A. It was calculated that

between 10" and 102 A per shot was removed by the laser, although this was at a power

below the breakdown threshold, so emission could not be collected.

Anderson et al used LIBS for depth profiling without the resonance feature of Allen's

experiment [711. A Zn/Ni coating between 2.7 and 7.2 Im thick on a steel substrate was

analyzed and a calibration curve of signal duration with thickness was found to be linear. The

depth resolution was far poorer than in the mass spectrometric experiment of Allen because

the energy density at the target was much greater. Nonetheless, Sn coatings of less than 1 uim,

and Cr coatings on the order of a few nm on steel could be determined by this technique.

Arnold and Cremers used LIBS to determine metal particles on air sampling filters

[72]. TI was collected on filter paper by passing contaminated air through a filter or by

wiping a filter on a TI surface. The laser beam from a Nd:YAG laser at 1064 nm was formed

into a line at the focus by a pair of cylindrical lenses. The detection limit for TI was 40 ng/cm2

of filter paper, and the calibration curve was linear up to 40 pg/cm2. The 535.05 nm TI line

was used for detection.

Sattmann and Sturmalso investigated the use of a multiple Q-switch Nd:YAG laser

for analysis of steel samples [73]. Single, double, and multiple pulses were used to produce

plasmas on a low-alloy steel. Material ablation, emission intensity, electronic temperature,





26

and electron number density were determined for each plasma type. All of these parameters

were greater for the double and multiple pulses than for the single pulses presumably because

of a smaller shielding effect in these cases. Calibration curves were correspondingly steeper.

Davies and Telle used a 100 m fiber optic pair to perform remote LIBS on ferrous

targets [74]. A pair of 550 im (OD) fiber optics were used to transmit laser light to the

target and to return plasma emission to the spectrometer. Detection limits of 200 ppm or less

were found for Cr, Cu, Mn, Mo, Ni, Si, and V. A number of potential emission lines to use

for calibration were given for these elements.

Cremers et al. used a fiber optic probe to determine Ba and Cr in soil [75]. A single

1.5 mm fiber was used both to deliver laser light and collect emission. A glass plate was used

to reflect the emission to the spectrometer. The Ba ion line at 493.41 nm and the Cr atom

line at 425.44 were used for calibration which was linear over 4 orders of magnitude for both

elements. The detection limit for Ba was 26 ppm, and the detection limit for Cr was 50 ppm.

Cremers also determined at what distance LIBS could be performed using

conventional optics rather than fiber optics [76]. By using a beam expander to increase the

diameter of the laser beam and a pair of lenses with adjustable distance to focus this expanded

beam onto a target, he was able to produce a plasma and collect light at a distance of 24 m.

Detection limits in a simulated moon rock were at the level of a few percent due mostly to the

small solid angle of collection at this distance.

Bescos et at. analyzed aluminum samples in 1995 [77]. The authors simultaneously

determined Mg, Mn, Fe, and Pb in aluminum using a spectral window from 380 to 410 nm.

Detection limits were around 100 ppm and calibration curves were linear up to about 1%.

Gonzales et a. also analyzed steel samples [78]. Their 1995 publication dealt with

sulfur determination. They reported a detection limit of 700 ppm using the 180.73 nm atom






27

line. The analysis was done in a N2 atmosphere, and the Fe ion line at 186.47 was used as an

internal standard. Precision was 7%.

Hakkanen and Korppi-Tommola used LIBS to study elemental distributions of paper

coatings [791. A XeCI laser at 308 nm was used to generate a plasma on paper, and Ca or

Si was used as an internal standard. A spatial resolution of about 250 pm could be achieved

with a laser pulse power of 200 mJ. It was estimated that 2 ng of paper coating were

vaporized per shot. Al, Si Mg, Ca, and C were monitored quantitatively in a spectral

window from 220-290 nm.

Sabsabi and Cielo analyzed aluminum alloys by LIBS in 1995 [801. The authors

characterized the laser plasma on aluminum targets using Stark broadening of Al ion lines to

determine electron number densities and using Boltzmann plots a series of Fe atom lines to

determine electronic temperature. Calibration curves for Mg, Mn, Cu, and Si were

constructed and detection limits were as low as 10 ppm, for Mg.

Soil was analyzed by Ciucci et at. in 1996 [81). They used both a Nd:YAG laser at

1064 nm and a XeCI laser at 308 nm. The authors found that the background decay in the

laser plasma was faster for the 308 nm pulse than for the 1064 nm pulse, and used a

correspondingly shorter delay time with the XeCl laser. Cu, Pb, and Cr in soil were

determined, and an entire spectra from 350-700 nm was shown with lines identified for the

geological survey soil sample GXR-2.

Ernst et al. used LIBS to determine Cu in A533b steel [821. The first attempt was

carried out with fiber optic delivery of the laser. This method was significant because in the

hazardous environment of a nuclear reactor pressure vessel, Cu concentration in the steel is

an indicator of radiation embrittlement and of expected material lifetime. Because the fiber






28

optic could not deliver enough power to produce sensitive detection, beam delivery was done

with conventional optics. The Cu line at 324.75 was used for calibration, and by using a

second order calibration curve, Cu could be determined in the range between 100 ppm and

5%.

Geertsen et al. revisited LIBS for aluminum samples in 1996 [831. Using a Nd:YAG

at 1064 nm and a pulse power of 230 mJ the authors formed craters ~5 im deep in aluminum

with a single shot. The detection limits for Mg and Cu were 4 and 40 ppm, respectively, with

a precision of about 8 %. Spatial resolution was assessed by rastering the beam over a sharp

Al-Cu junction. The best spatial resolution obtained was 6 pm.

Marquardt et at. determined Pb in paint using a Nd:YAG laser at 532 nm coupled into

a fiber optic probe [84]. The probe consisted of two fibers, excitation and collection,

terminated in a probe head with an aspheric lens to focus the laser light to a spot and focus

plasma emission into the collection fiber. The common end of this fiber is shown in figure 2-

5. Different combinations of excitation/collection fiber diameters were tried. The detection

limit for Pb was 140 ppm, and precision was 5-10 % even when analysis was carried out

through layers of non-lead containing paint.

Miziolek also reported on a LIBS probe in 1996 [851. This application was for

determination of heavy metals in soils, and the probes were to be used in a cone penetrometer

truck. In one probe, a compact laser was mounted in the probe head. In the other probe,

laser light was carried through a fiber optic to the probe head. The compact laser head

provided the more sensitive probe with detection limits for Pb, Hg, Cr, Cd, and Zn of 1-10

ppm in sand and silt. The fiber optic delivery provided less power but a higher laser repetition

rate; 15 mJ vs 28 mJ, 30 Hz vs 1/3 Hz.














f/2 Lens

Sample
Collection Fiber <.../ ...



Excitation Fiber I- -


Aspheric Lens


Figure 2-5. Common end of fiber optic LIBS probe used by Marquardt et al. for determination of lead in paint.





30

Also in 1996, Cremers et al. introduced a portable LIBS instrument and used it to

determine hazardous elements in soil, in paint, and in samples on filter paper [861. A compact

laser head was used as the probe. Ba, Be, Pb, and Sr were determined in soil with detection

limits of 265, 9.3, 298, and 42 ppm respectively. In paint, the detection limit for lead was

much greater because the 405.8 nm line could not be used due to spectral interference. On

filter paper, Be and Pb were determined with detection limits of 21 ng/cm2 and 5.6 pg/cm2

respectively. In the filter paper studies, some particle size effects were evident.

Palleschi et al. used a 400 mJ Nd:YAG laser at 1064 nm for a variety of

determinations including Hg in air (5 ppm detection limit), and pollutants in power plant

smoke and soil [87]. These authors analyzed the geographical survey GXR-2 sample and

determined concentrations for 18 elements.

Vadillo and Lasema analyzed geological samples of vanadinite, pyrite, garnet, and

quartz [881. These determinations were done in vacuum to increase the lifetime of the ionic

species and of the ion lines observed. Fe, Mn, Mg, and Si were determined in each of the

rocks, and At was also determined in garnet. Differences in composition were as expected.

Lasema et al. used LIBS to analyze the surface of solar cells [891. A N2 laser at 337.1

nm was focused onto the cell and the cell was rastered under the focus providing spatial

resolution of about 30 pm. The C ion line at 588.9, Ag atom line at 546.5, Si atom line at

634.7, and Ti atom line at 625.9 were used in determinations of these four elements. The

concentration of each element was mapped across the surface of the solar cell.

In another application of LIBS to alloys, Kim et al. determined aluminum in a zinc

alloy [901. The authors used a Nd:YAG laser (1064 nm, 105 mJ, 3 ns) in vacuum, air and

argon. The presence of Al in the Zn alloy was important because of its detrimental effect on





31

its welding properties. The delay and the distance from the surface of the target were

optimized in each atmosphere. Because the Al atom line at 308.22 nm was used for

calibration, large delay times were used to insure no contribution from a shorter-lived Zn

interference. In air, a 30 us delay was used and in Ar a 50 us delay was used. The Zn line

at 307.59 was used as an internal standard.

In 1997, Maravelaki et al. used LIBS to monitor the laser cleaning of marble artifacts

[91]. Although lasers had been used previously for the cleaning of such treasures, these

authors were the first to examine the emission from the plasma. Crusts of 20-600 mr

consisting of gypsum, iron oxides, soot, and calcite were ablated from the marble by a

Nd:YAG laser. The plasma formed in this ablation was analyzed spectroscopically to

determine the endpoint of the cleaning.

Figures 2-6 and 2-7 show graphically the detection limits for elements that have been

determined using LIBS in sand, ore, or soil and in steel or alloy.

LiQuids

Fundamental studies

Besides papers in the field of analytical chemistry, there have also been a number of

papers which have dealt with the physics of laser-induced breakdown in aqueous solution.

Most of these deal with the processes of cavitation and the factors which can affect the

breakdown threshold.

In 1984, Armstrong identified three time domains in the heating of an aerosol: the

acoustic regime, the internal conductive regime, and the external conductive regime [921. He

explained the phenomenon of aerosol-enhanced air breakdown as being caused by increased

electron collision frequency in the hot, dense, vapor surrounding the heated aerosol particles

in the beam.










BC N 0 F Ne

Si P S CI Ar


H Cr Fe

Zr MNb Mo Tc Ru


Co G Ge

Rh Pd Ag Cd In Sn


La Hf Ta W Re Os Ir Pt Au Hg TI

Ac


Key: i < 10 ppm
S10 -100 ppm
> 100 ppm


Figure 2-6. Elements determined in ores, rocks, and soils shaded according to detection limit.


Sc

Y


K

Rb

Cs

Fr Ro


As Se Br Kr

Te I Xe

Bi Po At Rn














Ti V Cr Mn

Zr Tc

Hf Ta Re


Os Ir Pt Au Hg


< 50 ppm
50 200 ppm
> 200 ppm


Figure 2-7. Elements determined in metals shaded according to detection limit.


K

Rb

Cs

Fr


Rh Pd


Zn77


Key: D





34

Three years later Armstrong used a CO, laser at 10.6 pm to explode aerosols

generated by a vibrating orifice aerosol generator [931. A UV laser was used to produce

shadow images of the exploding droplet, and a phase Doppler particle analyzer system

determined size and velocity of the expelled particles at a distance of 2 mm from the original

drop. It was found that the CO2 laser power affected the size of particles produced in the

breakdown for original drop sizes between 30 and 50 tm. The speed of the particles

produced was independent of laser power.

Chylek et al. studied the effect of size and material of liquid aerosols on breakdown

thresholds in 1986 [941. The setup that they used is shown in figure 2-8. A Nd:YAG laser

(532 nm, 10 ns) was focused onto droplets generated by a Berglund-Liu vibrating orifice

aerosol generator. Breakdown thresholds were determined on aerosols of different size

generated from liquids of various refractive index, density, surface tension, and chemical

structure. Thresholds were seen to decrease with droplet size. Refractive index had no

apparent effect. Density, surface tension, and chemical structure had effects which could not

be independently examined.

In 1987, Hsieh et al. used a Nd:YAG laser at 532 nm to examine the exact location

of breakdown initiation on water droplets generated by a Berglund-Liu vibrating orifice [951.

They found that as the energy of the laser increased, the breakdown moved from outside the

droplet to the inside. Theoretical calculations showed that the curved liquid gas interface of

a water droplet in air focused laser energy at one point just inside the illuminated face and at

another point just beyond the shadowed face.









Pyroelectric Detector


*
Acoustic Detector Microscope Viewing
A,6 from the top


Beam


Orifice


Frequency Generator

Figure 2-8. Schematic of experimental setup used by Chylek et al. to investigate LIBS of aerosols.





36

The effect of laser wavelength and irradiance on spectra from laser-induced

breakdown of single levitated aerosol droplets was examined by Biswas et al. in 1988 [96].

Droplets of glycerine saturated brine solution approximately 18 pm in diameter were optically

levitated and probed with a Nd:YAG laser at 1064, 532, and 355 nn. Emission lines of Na,

C, and N were observed. It was shown that the energy required for breakdown, and the time

of plasma emission increased with increasing wavelength. The optimum time delay for

determination of these elements increased with laser wavelength and showed more variance

from element to element at longer laser wavelength.

Also in 1988, Zheng et at acquired temporally and spatially resolved spectra of laser-

induced breakdown of a 40 pn 4 M NaCI droplet using a Nd:YAG laser at 532 nm [97].

Using fiber optic ribbons and a streak camera, these authors observed that emission began

first at the shadowed face of the droplet and propagated toward the illuminated face.

Hammer et al. also examined thresholds with ultrashort laser pulses [981. Using

pulses of 2.4 ps, 400 fs, and 100 fs from a pulsed dye amplifier, they determined breakdown

thresholds in saline solution, high purity water, and tap water. The thresholds in these media

were not significantly different. With the 2.4 ps pulse the breakdown threshold was found to

be 5 x 10" W crr, with a400 fs pulse it was 1.3 x 10" W cm2 and with a 100 fs pulse it was

5.65 x 10" W cmO.

Kitamori et at. used the acoustic wave generated during particle-induced breakdown

to detect polystyrene particles in aqueous solution [991. A Nd:YAG laser (532 nm, ~1 mJ,

6 ns, 10 Hz) was focused into a solution. In pure water, the energy density produced by this

beam was not sufficient to cause a breakdown. When a particle entered the probe volume,

however, a breakdown occurred and subsequently an acoustic wave was generated. By





37

detecting these acoustic waves, the probability of breakdown was determined. By analyzing

solutions of known particle concentration, a calibration curve of breakdown probability vs

concentration was generated. This curve was used in the analysis of solutions of unknown

particle concentration.

Vogel et al. used a Nd:YAG laser to produce and image a breakdown in water [1001.

The fundamental wavelength of 1064 nm was focused into the water to produce a breakdown.

A portion of the beam was frequency doubled to produce 532 nm light which passed through

a beam expander and through the underwater breakdown, providing a shadow of the plasma

and the cavitation bubble. The size of the plasma and cavitation bubble were examined for

1064 nm laser pulse durations of 30 ps and 6 ns.

Sacchi discussed the mechanisms of laser breakdown in water and described the

differences in threshold for short pulse and long pulse lasers in 1991 [1011. He claimed that

the avalanche breakdown process or inverse bremstrahlung was responsible for breakdown

when nanosecond laser pulses were used. In this case, the breakdown threshold behaved

probabilistically. The probability of breakdown scaled linearly with the log of laser power for

low powers, and scaled parabolically with the log of laser power at higher power. In contrast,

with laser pulses of less than nanosecond duration, multi-photon ionization appeared to be

the mechanism ofplasma formation, and a definite threshold above which breakdown would

occur could be determined.

Sacchi returned to this topic in 1996 and produced images of laser-induced

breakdown on solids under water. By using a dye laser oriented parallel to the underwater

target and delayed relative to the ablation laser, the authors obtained shadow graphs of the

evolving plume and shock wave. A series of images followed the growth and decay over a

total time of about 7004is. A schematic of the setup used is shown in figure 2-9.































Beam Expander Target


Figure 2-9. Schematic of experimental setup used to obtain shadow graphs of a laser-induced plasma.





39

In 1992, Pinninck et al. studied the effect of resident particles on laser-induced

breakdown thresholds [1021. It was found that even at wavelengths where the particles did

not absorb laser light, there was a decrease in breakdown threshold in particle laden air versus

clean air. This was taken to mean that particles were focusing laser light. Location of plasma

formation (inside, shadowed side, or illuminated side) was found to depend on ionization

potential, gas pressure and laser wavelength. Refractive index had no effect on breakdown

threshold, in agreement with the observations reported by Chylek in 1986 [941.

In 1993, Nyga and Neu created a plasma on calcite submerged in water using two

fiber optics to deliver two pulses from two XeCI lasers [1031. Both pulses were 30 ns pulses

at 308 nm through 600 um fibers which were brought in close proximity to the target with

no focusing lenses. The pulses were separated in time by -300 us, and the second fiber was

equipped with a 308 nm dielectric beam splitter and used to collect emission from the calcite.

No quantitative results were reported.

Feng et al. also examined the effect of laser pulse duration on breakdown thresholds

in water [1041. In their 1997 paper, they determined the mechanism of breakdown with

nanosecond pulses to be due to cascade ionization, and the mechanism of breakdown with

ultra-short pulses to be multi-photon ionization, similar to Sacchi's findings [101]. These

authors, however, went on to model the breakdown phenomenon in water relating the

breakdown threshold to laser power, spot size, and pulse duration. They derived an extended

non-linear Schrodinger equation to describe the relationship.

Analytical results

Quantitative results for LIBS with liquid samples will be presented in two sections

based on the nature of the analysis. The first section will cover determinations done in bulk





40

liquid. This implies that a laser spark is generated either on the surface or beneath the surface

of a relatively large liquid sample. The second section will cover determinations done in

aerosols or droplets generated from liquid samples. Analysis of dry aerosols generated from

solids are included in the last section on gas phase determinations.

Bulk liquid. In 1984, Cremers et at. reported the use of a repetitive single spark

(RSS) and repetitive spark pair (RSP) for the determination of Li, Na, K, Rb, Cs, Be, Mg, Ca,

B, and Al in aqueous and organic solutions [105]. With the single spark, detection limits for

all elements except Li were >1 ppm. The detection limit for B was 1200 ppm. When a

second spark which was generated from a second laser and delayed by 18 ps relative to the

first was used to form the plasma, the detection limit for B was reduced to 80 ppm. This

improvement was attributed to formation of the plasma within a cavitation bubble when the

RSP method was used. The experimental setup is shown in figure 2-10.

In 1987, Wachter and Cremers reported a detection limit of 100 ppm for uranium in

solution by LIBS [106]. The plasma used was formed by a Nd:YAG laser (1064 nm, 260 mi)

on the surface of a 4 M nitric acid solution contained in a small glass vial. Each analysis was

done by averaging 1600 laser shots. This averaging was necessary to overcome poor shot-to-

shot precision due mainly to small variations in lens to sample distance.

In 1987, Cremers conducted an experiment to determine the maximum lens to sample

distance (LTSD) that could be used in a LIBS experiment [761. With a 250 mJ pulse from

a Nd:YAG laser at 1064 nm, it was found that a 2 m focal length lens could produce

breakdown on molten metal The larger problem was collection of the emitted light. A fiber

optic could collect enough light at 0.5 m from the plasma for modest figures of merit to be

obtained. At distances farther than this, the sensitivity of the experiment decreased rapidly.


















5 cm Focal Length
Lenses


1064 nm Pulsed
Laser Beam





Teflon Cell
Spark
Fill Hole


Figure 2-10. Experimental setup used by Cremers et al. to analyze aqueous solutions with LIBS.






42

In 1993, Aragon et al used LIBS to determine carbon content in molten steel [107].

They used a Nd:YAG laser (1064,200 mJ, 8ns) focused onto molten steel in a crucible under

an argon atmosphere. In order to provide a homogeneous sample, a jet of argon gas was

directed downward into the crucible. This served to remove the topmost layer of molten

liquid which would be enhanced in the lighter elements. Carbon was determined by ratioing

of the C 193.09 nm to Fe 201.07 nm intensity and a detection limit of 250 ppm was found

with an RSD of 6%.

In 1995, Stolarski et al. produced sub-surface plasmas in saline solution, triple-

distilled water, and tap water with Nd:YAG laser pulses of <5 mJ and pulse durations of 5

ns and 80 ps [108]. The focussing lens used was 2.54 cm. in diameter and had a focal length

of 17 mm. This lens was chosen to approximate the lens in the human eye. The authors

found that an energy of 1.5 mJ was enough to produce a breakdown at either pulse duration,

and found that the plasma temperatures and electron number densities depended little on pulse

duration for the two studied. These authors observed sodium emission around 590 nm in a

0.9% NaCI solution, but no quantitative analysis was performed.

Also in 1995, Ito et al. used LIBS to determine colloidal iron in water [109]. They

focused aNd:YAG laser (1064 nm, 100 mJ, 10 ns) into a flowing stream of water containing

FeO(OH) as submicron particles. The detection limit for iron was in the ppm range. This

technique differed slightly from previous techniques in that it relied on particle-induced

breakdown for the formation of the plasma. A plasma was formed only when a FeO(OH)

particle was present in the probe volume. Because of the extremely small size of the particles,

however, it was essentially a determination of iron in water. The following year, the same

authors used a second laser delayed by 1 uts relative to the first in a technique similar to






43

Cremer's repetitive spark pair. With this technique, they were able to improve the detection

limit for iron to 16 ppb [110].

In 1996, Knopp and Scherbaum used a dye laser at 500 nm with a pulse energy of 22

mJ to produce a sub-surface breakdown in aqueous solution [111]. They reported detection

limits for Cd, Pb, Ba, Ca, Li, and Na of 500 ppm, 12.5 ppm, 6.8 ppm, 130 ppb, 13 ppb and

7.5 ppb respectively. No signal was obtained for either Hg or Er at 0.1% in solution. When

micron-sized particles of ErBa2Cu30,, rather than a soluble salt, were used as the source of

Er, the detection limits decreased by a factor of 103. The improvement illustrated the

increased sensitivity of particle-induced breakdown as compared to breakdown in particle-free

solution. This experiment also suggested that 500 nm laser light had analytical advantages

over 1064 nm light.

In 1997, Ho and Ng used both a Nd:YAG laser (532 nm, 12 ns) and an ArF laser (193

nm, 15 ns) to determine Na in aqueous solution [112]. In order to increase absorbance of the

laser light, methyl violet was added to the solution to be analyzed. It was found that the 532

nm laser light produced a more visibly intense plasma; because the continuum emission was

higher in the 532 nm produced plasma, the 193 nm laser light actually gave better detection

limits for Na. A detection limit of 230 ppb was reported for Na in the methyl violet solution

with the plasma formed by the ArF laser.

In 1996, a similar experiment was performed by Paksy et al. who used a Nd:YAG

(1064 nm, 15 mi, 4 ns) focused onto the surface of a molten alloy in an argon atmosphere

[113]. Because the laser power used was significantly less than that used by most authors,

the delay of the detection relative to the laser was also less. A 100 ns delay and 1000 ns gate

width were used. Detection limits were 0.001% for Si in Fe, 0.006% for Cr in Fe, 0.06% for

Si in A, and 0.007% for Mn in AL






44

Area et al. determined Cr, Pb, and Cu in aqueous solution with a single laser shot on

the surface of the water [114]. Detection limits were 100, 100, and 50 ppm for the three

elements, respectively. Calibration curves were linear over 1 order of magnitude. The

authors also analyzed aqueous solutions by placing a drop of water to be analyzed on a KBr

pellet. When this technique was used, detection limits were improved by almost an order of

magnitude.

Aerosols and droplets. In 1983, Radziemski et al. used a nebulizer/heat chamber

system to produce an aerosol from a liquid solution and analyzed the aerosol with LIBS

[115]. Detection limits for Na, P, As, and Hg were reported to be 6 ppb, 1.2 ppm, 0.5 ppm,

and 0.5 ppm, respectively. The authors determined that local thermodynamic equilibrium

existed in the aerosol spark for time delays greater than 1 is relative to the laser.

In 1987, Eickmans et al. spatially resolved the emission from laser-induced breakdown

of an aerosol [116]. Approximately 45 gm diameter droplets were formed by a vibrating

orifice aerosol generator which was synchronized with the Nd:YAG laser beam at 532 nm.

Emission of Li, Na and H was observed in 5 M salt solutions of either NaCI or LiC1. Spatial

resolution of the emission revealed that in the plasma plume in the vicinity of the illuminated

face of the droplet, emission lines showed Stark broadening and self-reversal. Near the

shadowed face of the droplet, emission lines were narrower and less intense.

In 1988, Essien et al. used LIBS to determine Cd, Pb, and Zn in an aerosol [117]. A

Nd:YAG laser (1064 nm, 100 mJ, 5 ns) was focused onto an aerosol generated by a

nebulizer/heat chamber. Detection limits were 19 ppb, 210 ppb, and 240 ppb for Cd, Pb, and

Zn, respectively.






45

Also in 1988, Archontaki and Crouch used an isolated droplet generator to produce

equally spaced, uniformly sized droplets [118]. The experimental setup was such that a single

drop of known size was always in the probe volume of the laser for a given repetition rate.

A Nd:YAG laser (1064 nm, 100 mJ) was used to determine detection limits for Li, Na, Mg,

Ca, Mn and Al of 0.3, 2.2, 1.9, 0.4, 7.2, and 5.2 ppm, respectively. Droplet diameter had

little effect on the detection limit for sizes between 58 and 75 nm.

In 1992, Ng etal. used an ArF laser (193 nm, 150 mJ) to analyze an aerosol generated

by a concentric glass nebulizer/spray chamber traditionally used for ICP-AES [119]. The

authors determined the optimum delay for detection to be 6 ps relative to the laser pulse.

Detection limits for Na, Li, In, Al, Ga, Ca, Mg, K, and Sr were determined to be 0.9, 0.3, 10,

3, 3, 8, 3, 2, and 20 ppm, respectively.

The following year Parigger and Lewis used a picosecond XeCI laser (308 nm, 4 mJ)

to produce LIB in 66 pm water droplets [120]. Plasma diagnostics showed a temperature of

-10 000 K and an electron number density of 10"i cm3. The H, line at 656 nm was used as

an internal standard and the detection limit for sodium was ~1 ppm. Poulain and Alexander

performed a very similar experiment in 1995 using a KrF laser (248 nm, 200 mJ). The

detection limit for Na was found to be 165 ppm [121].

Haisch and Paine reported on the characterization of colloidal particles by LIBS in

1996 [122]. Field flow fractionation was used to sort particles by size and deposit them onto

filter paper using partitioned pumping to retain time resolution. A cylindrical lens was used

to focus the laser onto the paper to avoid ablation of the paper which could occur if the

energy density became too high. Detection limits for Si were in the ppb range.






46

Figures 2-11 and 2-12 show graphically the detection limits for elements that have

been determined using LIBS in bulk liquids and in aerosols generated from liquids.



Fundamental studies

In 1988, Kumar and Thareja studied laser-induced gas breakdown in the presence of

an electric field [123]. They used a XeCI laser (308 nm, 60 mJ, 8ns) focused between two

electrodes separated by 6.5 mm. A transverse static electric field was produced by applying

different potentials to the electrodes. In order to study the effect of the field on breakdown

thresholds, the current on the electrodes was adjusted so that no breakdown occurred using

only the electric field or only the laser. The authors were then able to determine that pre-

breakdown electron densities of 10' cm3 created by the field were sufficient to lower the

laser energy density needed to produce breakdown.

Parigger and Lewis used a Nd:YAG laser (1064 nm, 300 mJ, 7.5 ns) to produce a

plasma in CO [124]. They observed the emission of the C2 Swan band around 565 nm to

determine vibrational temperatures in the decaying plasma. The observed spectra was fit with

a model in which temperature was a parameter. At a time delay of 30 ps relative to the laser

pulse, a temperature of 6745 K was obtained. This temperature was confirmed by use of the

CN violet band around 388 nm to calculate the vibrational temperature in a similar way.

Yagi and Huo investigated breakdown in H2 gas at different pressures initiated by a

KrF laser (248 nm, 250 mJ, 20 ns) [125]. This work was related to Raman spectroscopy in

that the breakdown threshold determined the maximum energy with which the Raman

transitions could be pumped. The authors found that below 600 Torr, breakdown thresholds

were pressure independent. Thresholds decreased between 600 and 3000 Torr, and increased

slightly above 3000 Torr.














Ti V Cr Mn Fe Co Ni


Y Zr Nb Mo Tc Ru Rh Pd


Re Os Ir Pt


H

Li

No


Ag In Sn Sb

Au Hg TI Bi


D < 1 ppm
S1 -10 ppm
S> 10 ppm


Figure 2-11. Elements determined in bulk water shaded according to detection limit.


C NO

Si S

Cu ZnGa Ge As Se


Ca I Sc


Rb Sr


Ba La Hf Ta W


Fr Ra Ac


S--








H He

Li B C N Ne

Na Mg Si Ar
K Sc Ti V Cr Mn Fe Co Ni Cu ZnGa Ge Se Br Kr

Rb Y Zr Nb Mo Tc Ru Rh Pd Ag Cd Sn Sb I Xe

Cs Ba La Hf Ta W Re Os Ir Pt Au TI Pb Bi Po At Rn

Fr Ra Ac


Key: l < 10 ppm
S10 -100 ppm
> 100 ppm


Figure 2-12. Elements determined in liquid aerosols shaded according to detection limit.






49

Yalcin et al. investigated the sensitivity of a laser spark produced by a Nd:YAG at 532

nm to ambient conditions such as variation in background gas, presence of particles, and

humidity [126]. This was done in order to assess the potential usefulness of LIBS in toxic

metal monitoring. The authors found that excitation temperature and electron number density

in the plasma remained essentially constant with variations in the ambient conditions.

Analytical results

Cremers and Radziemski again carried out, pioneering work on gasses. In their 1983

publication, they detected chlorine and fluorine in air [127]. Detection limits for these two

gases were 8 and 38 ppm by weight, respectively. These values corresponded to absolute

detection limits of 80 and 2000 ng. In a helium atmosphere, the absolute detection limits

improved to 3 ng for both. Experiments were carried out in which molecular gases containing

both chlorine and fluorine were introduced to the sampling chamber. The relative intensities

of the chlorine and fluorine signal were found to be an indication of the number of each of

these atoms in the molecular gas. All experiments were carried out using a Nd:YAG laser

(1064 nm, 100 mJ, 15 ns) and detecting Cl at 837.6 nm and F at 685.6 nm.

These authors also detected Be in air [128]. The limit of detection in this experiment

was 0.5 ppb by weight. The Be was introduced into a chamber by laser ablation and diluted

by air. A second laser produced a breakdown in the chamber from which the emission was

measured. It was found that the emission from the Be II doublet at 313.1 nm was stronger

than any atomic emission even at delay times of 20 Its. This doublet was used for calibration

which was linear over 4 orders of magnitude.

Cremers et al. used LIBS to determine sodium and potassium in a coal gassifier

stream as early as 1983 [127]. These elements were present as particulates in the stream and






50

so again the question of whether or not the target was a gas or solid arises. Regardless, the

authors were able to determine Na with a detection limit of 4 ppb by weight using the doublet

at 589 nm. Figures of merit for potassium were not given. In conclusion, the authors noted

that sulfur could also be monitored.

Sneddon extended this technique to the determination of P in air and lowered the

absolute detection limit of Cl in air. In a 1988 paper, Sneddon detected phosphorous at 15

ppm and Cl at 60 ppm by weight in air [129]. A calculated plasma volume of 0.010 cm3 was

used to determine an absolute detection limit of 60 pg for Cl.

Ottenson et al. reported on the application of LIBS to analyze particulates in a coal

combustion vent in a 1989 publication [130]. A Nd:YAG laser was triggered on the

scattering from a HeNe laser to explode particles detected in the probe volume. The

experiment used on-line temperature correction to account for different excitation conditions

in plasmas formed on different size particles. Moreover, the size of particles to be sampled

could be adjusted by varying the laser power. With lower laser power, only larger particles

were sampled; the smaller particles were not sufficient to provide plasma formation. The on-

line temperature correction automatically accounted for differences in excitation conditions

with varying laser power.

In 1990, Morris et al. used laser-induced breakdown as a detector for gas

chromatography [131]. The UV laser light was focused into the effluent from a gas

chromatograph at a power below the breakdown threshold of the carrier gas. When eluting

carbon containing molecules were present in the effluent, the threshold for breakdown was

lowered and a plasma formed. The organic analytes were indirectly detected by optically

detecting plasma formation. A schematic is shown in figure 2-13. No emission from the

plasma was measured, and so the potential of this technique was only as a universal detector.









Optogalvanic Probe


Microplasma


Capillary


Gas Sheath


Figure 2-13. Schematic of setup used by Morris et al. for use of laser-induced breakdown as a detector in gas chromatography.


Focused LE
193 nm




Screen






52

In 1991, Cheng et al. studied polyatomic molecular impurities in helium gas at ppm

levels [132]. Using ungated detection, they determined B2z-l, PH3, and AsH, with detection

limits of 1, 3, and 1 ppm, respectively. The ionic phosphorous lines at 602.4, 603.4, 604.3,

and 605.5 nm were used. As lines were observed at 228.8, 235.0, 278.0, and 286.0 nm. B

lines were observed at 336.0 and 434.5 nm. The plasmas were produced by a Nd:YAG laser

at 532 nm.

Also in 1991, He and co-workers focused a pulsed laser onto a metal rod in an

atmosphere of helium or argon seeded with a reaction gas to produce ion-molecule complexes

and observe their emission [133]. Ion lenses were used to preferentially draw ions into the

He or Ar gas for reaction with the seeded gas. Emission from species such as A10, AIO-CO,

Al-Ar and AIH, was observed.

Joseph and Majidi used an electrothermal atomizer to create a gas phase sample from

a liquid for analysis by LIBS [134]. A few 4L sample was deposited in the furnace and was

dried, ashed, and atomized. The setup is shown in figure 2-14. At the onset ofatomization,

a Nd:YAG laser (1064 nm, 100 mJ) was focused into the furnace and fired at a repetition rate

of 10 Hz. Emission from the plasma was collected through the dosing hole of the tube.

Experiments with cobalt and cadmium gave absolute detection limits of 5 and 50 pg.

Casini et al. also determined several elements of interest in air using LIBS [135].

Their experiment used a Nd:YAG laser (1064 nm, 40 mJ, 7 ns) to produce a plasma in an

atmospheric pressure chamber as shown in figure 2-15. Using the saturated concentrations

of air over chosen liquids at the given temperature, and then successively diluting these

concentrations in the chamber, yielded calibration curves for a number of elements. Cl was

detected at 449.0 nm with a detection limit of 60 ppm, S at 415.3 nm with a DL of 200 ppm,






Computer
mI


Tube


\
High Energy
Mirror


Photodiode Array


Figure 2-14. Schematic of setup used by Joseph and Majidi for LIBS in a graphite furnace.









Parabolic Mirror


Laser Controller Computer

Figure 2-15. Schematic of setup used by Cassini et al. for air analysis using LIBS.


Lens


Monochromator






55

P at 442.1 nmwith a DL of200 ppm,Naat 371.1 nm with a DL of 110 ppm, Hg at 404.7 nm

with a DL of50 ppm, Be at 381.4 nm with a DL of 130 ppm, and As at 454.4 nm with a DL

of 130 ppm.

In 1994, Flower et al. used LIBS to monitor metal aerosol emission generated by a

pneumatic nebulizer [136]. A chromium salt solution was passed through the nebulizer and

a heating pipe to produce a gas with known concentration of Cr. The detection limit was 200

ng Cr / standard cubic meter of air (scm). The laser used was a Nd:YAG (1064 nm, 180 mJ)

and detection was at 312 nm.

In 1994, Lazzari et al. detected mercury in air using LIBS with a Nd:glass laser (400

mJ, 8 ns) to produce plasma in a chamber filled with saturated Hg vapor [137]. With

successive dilution of the mercury vapor, they obtained a detection limit of 10 ppb by

observation of the 253.7 line.

Zhang et al. performed LIBS in a particle loaded methane/air flame and in an oil-

fueled combustor vent [138]. In their 1995 publication, they described the addition of a

flange with four movable windows to a pre-existing combustor vent. LIBS was performed

through these windows which were kept clean by a flow ofN, gas and could be switched out

periodically for cleaning.

In 1995, Nordstrom studied the laser-induced plasma emission spectra of N,, 02, and

ambient air from 350 to 950 nm [139]. This range was covered with approximately 0.5 nm

resolution and atomic, ionic, and molecular emission from a plasma created by a CO2 laser

(10.6 prm, 320 mJ, 200 ns) was observed. Spectra were computed for N, 0, and a few

molecular species, and a comparison between experiment and calculation was done.






56

The same year, Parigger et al. published a fundamental paper on laser-induced plasma

in the gas phase [140]. The breakdown was produced by a Nd:YAG (1064 nm, 220 mJ, 6

ns) in hydrogen at 150 and 810 Torr. The H, line at 656 nm was used to calculate electron

number densities in the decaying plasma with temporal resolution as high as 6 ns early in the

plasma and 1 js at later times. Spatial resolution was also achieved by focusing different

portions of the 1:1 plasma image onto the spectrometer entrance slit. Spatial resolution was

approximately 50 mn. Surface plots for electron number densities resolved in time and space

were then created. Electron number densities varied between 10"6 and 10"9 cm'.

In 1996, Haisch et al. used LIBS to detect chlorine in the gas phase [141]. Two

experimental setups were described. The first was a benchtop experiment using a 320 mJ

Nd:YAG laser. The second setup used a miniature Nd:YAG laser built into a sensor head.

This miniature laser provided 18 mJ of pulse energy. Detection of Cl in chlorinated

hydrocarbons was accomplished by focusing the laser onto a copper target in the presence of

the gas to be analyzed. This provided for formation of CuCl in the plasma. The luminescence

of the D-system of CuCl was then detected around 440 nm. Detection limits were a few ppm.

Figure 2-16 shows the elements determined in a gas phase matrix and their relative

detection limits. The unit scm refers to a standard cubic meter of gas.








H

Li Be

Na Mg

K Ca Sc Ti V Cr Mn Fe Co


Rb Sr


Cs I Ba


Fr


Y Zr Nb MoI Tc IRu


La Hf Ta W I Re Os


Ra Ac


Rh Pd Ag In

Ir Pt Au Hg TI


Key: El < 10 lg/scm
S10 100 lg/scm
> 100 gg/scm


Figure 2-16. Elements determined in gas phase matrices shaded according to detection limit.
The unit scm refers to a standard cubic meter of gas.


lil













CHAPTER 3
FUNDAMENTAL INVESTIGATIONS OF LASER PRODUCED PLASMAS

Part I: Spatial and Temporal Dependence of Lead Emission in Laser-Induced Breakdown
Spectroscopy

Introduction

Studies investigating the spatial and temporal development of the LIBS plasma will

provided an insight into the complexity of these pulsed sources. Through the use of time-

resolved spectral imaging, a better fundamental understanding of the plasmas behavior is

attainable. The results of these studies have the potential to considerably improve the

analytical usefulness of LIBS results.

Kuzuya et al. [22] reported on the change in spatial distribution of the LIB plasma

spectral radiation at 400 nm caused by gaseous atmospheres of Ar, air, and He at different

pressures. Their results indicated that breakdown in the gaseous atmosphere above the target

occurred at excessively high laser power densities leading to decreased ablation and emission

from the target material because a major portion of the laser radiation was absorbed by the

opaque gaseous plasma in front of the target material. Sattmann et al. [73] studied the effect

of double laser pulses on the spatial structure of the plasma integrated over time and

wavelength using photographic detection. Temporal and spatial resolution of the LIB plasma

structure was reported by Palau et al.[142] for three different laser wavelengths, without

wavelength resolution. Multari and Cremers[58] reported on the spatial and temporal





59

distribution of the Cr 425 nm line emitted by the LIB plasma when a flat target was tilted at

various angles with respect to the incident laser beam. They concluded that after the plasma

formed on the solid target, it expanded upward along the path of the incident laser pulse, but

once the chromium line emission became dominant over the continuum background, the

emission appeared symmetric about the normal to the surface rather than along the axis of the

incident laser pulse. Vadillo et al. [143] showed non-dispersed, time resolved LIB images

captured by means of a CCD and monochromator in mirror mode (zero order). They

incorrectly inferred the width of the plasma from the digital images, failing to recognize that

the measured image actually represented an image of the entrance slit with non-uniform

irradiance caused by the plasma image projected upon the entrance slit. Bulatov et al. [56]

reported 2D spectral images from the LIB plasma created by means of Fourier transform

visible spectrometry. The wavelength resolution was in 4 nm spectral windows over the range

400-800 nm, which was insufficient for spectral classification of single emission lines. The

results also lacked time resolution.

As part of a research project to develop a portable LIBS instrument for the

determination of toxic metals in the environment and in particulate samples, we have

investigated the temporal and spatial development of lead ionic and atomic emission from a

LIB plasma.

Experimental

A schematic of the LIBS system which was used, is depicted in figure 3-1. It consists

of a laser, motorized X,Y,Z translational stage carrying the sample, spectrometer, ICCD

detector, detector gating and control electronics, and a computer for control and data

acquisition. The Nd:YAG was a Quantel YG580 which delivered 1 J at the fundamental
























X,Y,Z Stage Stage Control Computer


Figure 3-1. Experimental apparatus for capturing time resolved LIBS spectra.






61

wavelength of 1064 nm and produced a 9 ns duration pulse at a maximum of 30 Hz

repetition frequency. A 80/20 beam spliter was used to attenuate the laser output so that only

20% of the laser power was used in the generation of the plasma. The laser beam was

focused on the target using a 2.5 cm diameter, 10 cm focal length lens. The resulting plasma

created a 200 pm diameter crater with an average incident laser irradiance of 18 GW / cm2.

The initial design directed the laser beam perpendicular to the target material. In a later

modification, the laser beam was incident at 450 with respect to the target surface to allow for

the desired movement of the fiber optical system used for radiation collection. Pressed

sample pellets, of 1.4 cm diameter and varying thickness were made with a pellet press at

4500 psi pressure. In pressing the pellets pure lead flakes (Fisher Scientific, Catalog# L-24)

and NIST soil standards (SRM 2711) were used as sample material.

A positioning system consisting of a helium-neon laser, lenses and a position sensing

detector was used to monitor the position of the sample surface with respect to the laser beam

focus and the light collecting system's focal point; control was accomplished with a motorized

X, Y, Z stage (Oriel, Model# 18011). For all measurements, the target was moved

horizontally between shots to allow sampling from a fresh location and to improve the

reproducibility of mass ablation.

A 2.5 cm diameter fused silica lens with a focal length of 5 cm was used as the

collection optic to produce a one-to-one image of the plasma as viewed from the side on the

entrance slit of a 0.5 m focal length spectrometer (Acton Research, Model# SP-500) with a

grating of 1200 grooves / mm. The reciprocal linear dispersion was 1.6 nm / mm at 250 nm

and an entrance slit width of 25 pm was used. The intensified charge coupled device (ICCD),

( Princeton Instruments, Model# ICCD-576S) being used, had pixel dimensions of 22.5 x

22.5 umr and a viewing area of 12.9 mm by 8.6 mm. It was oriented in such a way that the





62

longer dimension corresponded with the wavelength dispersion axis and the shorter dimension

with the entrance slit height. The ICCD was operated by a controller (Princeton Instruments,

Model# ST-130). A programmable pulse delay generator (Princeton Instruments, Model#

PG-200) was used to gate the ICCD. The PG-200 was computer programmable, and was

capable of gating the ICCD with integration times as short as 20 ns. The Q-switch trigger

signal from the laser was connected to the "Ext trig in" input of the programmable pulser

(PG-200) and served as the master control of the delay gate. This trigger signal provided

-250 ns pretriggering with respect to the onset of the laser pulse and enabled the system to

open the ICCD gate before the plasma actually started emitting. The PG-200 high voltage

gate pulse was connected to the ICCD, while its "Aux dly'd trig out" signal was used to

trigger the ICCD Controller ST-130 "Ext sync" input. The PG-200 was programmed to

activate the ST-130 by means of this signal to perform the data read-out from the ICCD and

to transfer data to the control computer 1 us after the completion of the gate pulse. The ST-

130 "not scan" output was connected to the PG-200 "Inhibit in" connector to prevent the PG-

200 from being pretriggered before the most resent data capture, transfer and real time

processing was completed. The measuring system was thus operated in a synchronous mode,

and the detector delay and integration time settings and selection of active pixels were

completely controlled by the software. The entire experimental apparatus was under the

control of a Gateway 2000 Pentium 133 MHz computer running the Winspec V1.4.3.6

(Princeton Instruments) software.

A miniature spectrometer (Ocean Optics, Model# S1000) consisting of collection

fiber, optical components, modular grating disperser, and linear CCD detector with control

electronics was used in the angular dependence measurements.








Results and Discussion



Since existing elemental spectral intensity tables [144-147] are based mainly on

sources such as electric arcs, sparks, and gas discharges, one often finds that they are not

directly applicable to the observed laser-induced plasma intensities. We therefore captured

and analyzed the spectrum of our element of interest, lead. Spectra were accumulated from

10 single laser shots on a solid lead target. Spectra were captured after a delay of 2 ls, using

an integration time of 15 us. Each spectrum covered a wavelength window of~20 nm and

successive sets of measurements were taken with the center of the window shifted by 15 nm

increments to cover a total spectral range from 200-450 nm. The resulting spectra are not

shown due to space limitations. However a classification of the observed lead spectral lines

are given in table 3-1. The relative intensities presented in the table are not corrected for the

detector's varying spectral efficiency due to a lack of a reliable radiance standard for

wavelengths below 250 nm.


Plasma Imaging

Time resolved images of the laser produced plasma were captured by using the

modified experimental setup shown in figure 3-2. The collecting lens was placed at its focal

length (250 mm) from the laser-induced plasma, to provide a nearly parallel beam of radiation

incident on the interference filter. The focusing lens (f=250 mm) focused this transmitted

parallel beam of radiation in its focal plane where the ICCD was placed to capture the

spectrally filtered plasma image. The beam combiner (a neutral density filter of0.3 optical

density) allowed radiation from the plasma as viewed from the front via one of the plane











Table 3-1. Table of lead lines with their relative intensities as observed in the LIB plasma
and their associated states.


Upper Stat Lower State
Wavelength Intensity


(nm) (Relative) Term


205 328

217.000

220.3534*

223.7425

224.689

239.3792

2440 1940

241.1734

244.3829

244.6181

247.6378

257.7260

261.4175

262.8262

266.3157

269.7541

280.1995

282.3189

283.3053

287.3311

357.2729

363.9568

367 1491

368.3462

373.9935

401.9632

405.7807

406.2136


15236 8s'P,

83017 6d'D,

159839 7stS,,

16023 7d'D,

39766 7d'D,

52941 7dVF,

31515 7s'P,

14552 7d'F,

22780 8s'P,

35871 8s3P,

59174 7s'P,

63035 7s'P,

151636 6d'D,

13882 8sP,

115415 7s'P,

9837 14s'P,

161677 6d'F,

90279 6d'D,

140343 7s'P,

109403 6d'F,

62390 7s'P,

64527 7s'P,

13181 8s'P,

72813 7s'Po

55761 7s'P2

10236 6d'F,

116066 7s'P,

23673 6d'D,


Energy Term
(cm ')

48686 87 6p2'p,

46068.57 6p23'P

59448.00 6pP,,

52499.53 6p2'P,

52311.37 6p2'P,

52412.30 6p2'P,

49439.73 6p2'P,

52101.77 6p2'Pi

48726.16 6p2'P,

4868687 6p2'P,

48188.67 6p2'P,

49439.57 9p2'P,

46060.90 6p2'P,

48686.87 6p2'P,

48188.67 6p23'P

58517.67 6p21'D

46328.81 6p2'P2

46060.90 6p2'P,

35287.24 6p2'P,

4544326 6p2'P,

49439.57 6p2'D,

35287.24 6p2'P,

48686.87 6p2'D,

34959.90 6p2'P,

48188.67 6p1D,

46328.81 6p2'D2

35287.24 6p2'P,

46068.57 6p2'D2


Energy
(cm-')

0.00

0.00

14081.00

781937

7819.35

10650.47

7819.35

10650.47

7819.35

7819.35

7819.35

10650.47

7819.35

10650.47

10650.47

21457.90

10650.47

10650.47

0.00

10650.47

21457.90

7819.35

21457.90

7819.35

21457.90

21457.90

10650.47

2145790









Plane mirror for side-on view


Beam combiner


Plane mirror for frontal view

Collecting lens


- Spectral filter


r Focusing lens


ICCD
fs

Figure 3-2. Schematic of the modified optical system for capturing time resolved, 2D spectral images of
the LIB plasma simultaneously from two orthogonal directions.


Plasma





66

mirrors and a side view via the other plane mirror to reach the detector simultaneously. By

slight rotation of the plane mirrors around vertical axes, the two imagesfand s (representing

the frontal and side-on images respectively) could be laterally offset from one another. The

enlargement of the image was given by the ratio of the focal length of the focusing lens to that

of the collecting lens.

This setup allowed simultaneous registering of two perpendicular views of the plasma

during a programmable time interval A solid lead pellet was used as the sample. Interference

filters were used sequentially to produce narrow wavelength band images of the plasma. With

a proper choice of peak wavelength and spectral bandwidth, the interference filter isolated and

transmitted a single transition, providing a means of measuring the temporal and spatial

dependence of the wavelength-associated population distribution. An interference filter with

a spectral FWHM of 8 mn was used to observe the ionic line emission from the transition Pb

II at 220.3534 nm. The filter was tilted at 170 with respect to the optical axis to have the

filter's transmission peak correspond with the ionic line wavelength. This filter could not

completely isolate the ionic line since its full-width at half-maximum permitted a minimal

detection of the Pb I 217.000 nm atomic line transition. An interference filter with a peak

transmission at 280 nm and a FWHM of 8 nm was used to detect the lead atomic line. This

filter permitted radiation from the lead atomic transitions at the wavelengths 280.1995,

282.3189, 283.3053 and 287.3311 nm to be observed simultaneously, with the major

contribution coming from the 280 nm wavelength. It is important to note that during the

initial 500 ns of plasma existence, the observed radiation was mainly due to broadband

continuum radiation at the filtered wavelengths and not due to the elemental line emission.

This was verified by analysis of a sample which did not contain lead (or any spectral emitters






67

at 220 or 280 nm). To obtain a wavelength integrated image, a neutral density filter (optical

density 1) was used to attenuate the radiative flux sufficiently in order to prevent saturation

of the ICCD.

The top two images in figure 3-3 show a mm scale as viewed by the ICCD from the

front (left hand picture) and from the side (right hand picture), when the ruler was placed at

the position where the laser-induced plasma would have formed, using identical imaging. The

physical orientation for the left and right sides of the frontal view and the front and rear of the

side-on view is also shown. The ruler images were obtained from a double exposure blocking

each of the orthogonal paths successively and rotating the ruler through 90* between the

exposures. The clear plastic ruler was back illuminated with a mercury pen lamp. The lower

five frames in figure 3-3 show the wavelength integrated temporal development of the LIB

plasma using a neutral density filter with optical density 1 for radiative flux attenuation. In

each frame, the plasma image on the left is the front view, and the side view is on the right.

The laser beam is entering from the right on the side view, striking the sample at about a 45

angle. During these delay times, only continuum radiation was emitted. These images were

generated by subtracting two successive image files captured with a pulse width of 200 ns,

separated by a 10 ns delay. Even though the PG-200 could gate the ICCD with pulses as

short as 20 ns, it was not practical for full area ICCD imaging, due to the "iris effect"[148]

which caused the central pixels in the ICCD to have almost zero sensitivity at such short pulse

durations. For figures 3-4 thru 3-6, a pulse width of 200 ns was used. Each image at a

specific delay time is from a different laser produced plasma. The data shown here are from

sequentially captured laser shots. No attempt was made to use the "best matching" images.

A normalized intensity scale was used for each figure, 3-4 thru 3-6. This allows comparison







Left RiehrontR


Figure 3-3. Wavelength integrated lead plasma images. Top two images show a back illuminated mm ruler for scale. The
lower five images show the temporal development of the wavelength integrated plasma with increasing delay time.


Left


Right


Front


Rear
























C,, C n


1 1'7M


Figure 3-4. Wavelength integrated lead plasma images on a longer time scale than shown in figure 3-3.























5.ima on a loner time scale than shown in figure 3-4.
Figure 3-5. Wavelength integrated lead plasma images on a

















170 ns 370 ns 570 ns








770 ns 970 ns I 170 ns









1370 ns 1570 ns 1770 ns

Figure 3-6. Images showing temporal and spatial development of the ionic emission from the LIB plasma
transmitted by a 220 nm filter.





72

of images within one figure, but not comparison between figures. Figure 3-3 shows that the

plasma starts from the spot where the laser strikes the target and then continues to grow

along a preferential axis coinciding with the incident laser beam. Figure 3-4 and figure 3-5

shows the further development of the wavelength integrated plasma on a longer time scale

and with a different neutral density filter to show the temporal development of the combined

line emission. Lines have been drawn on the pictures to indicate the surface of the lead target.

Since the top and side of the pellet are highly reflective, anything below this line should be

ignored, since it represents reflected radiation. The reflection from the side of the pellet is

due to back reflection from the neutral density filter. These images show that at ~ 1 gs after

the initiation of the plasma, it has reached a maximum size and brightness, and has begun to

decay. It also shows that the development of the LIB plasma on a homogeneous solid sample

is very reproducible. Finally non-normal incidence of the laser beam onto the target material

leads to non-symmetrical (cylindrical) plasma formation. The protrusion seen in the side-on

view and starting to develop at delay times of > 500 ns occurs long after the termination of

the laser pulse, but along its' incident path.

Figure 3-6 shows the 220 nm ionic line emission. The first two plasma images at

delay times of 170, and 370 ns show primarily the continuum background emission. Note that

the lead ions are confined to a smaller volume, and that they do not extend out into the

plasma tail seen in the images in figure 3-4, thus resulting in a "cut off' shape at the rear of

the side-on view. The ionic emission is also shorter lived. This becomes more obvious

when comparing the ionic emission in figure 3-6 to the atomic emission shown in figure 3-7.

In figure 3-7 and figure 3-8 the atomic emission from the 280 nm atomic line and also

the contribution of emission from a few other minor lines is shown. Initially, during the




















l7O ns


370 ns


1 I'7n ,


I 1 IM I 1 I' 111 ",

Figure 3-7. Images showing temporal and spatial development of the atomic emission from the LIB plasma
transmitted by a 280 nm filter.

































5~7 6T7I.!I / is
n a longer time scale than shown in figure 3-7.

Figure 3-8. Continuation of the 280 mn filtered lead plasma images on a longertim
Figure 3-8. Continuation of the iw0 nm






75

continuum emission, the plasma images, have a "cut off" appearance, similar to the

corresponding images for the lead ionic images. Then, as the plasma decays, it develops a

more cylindrically symmetric shape. These images around 280 nm do not show the protrusion

in the side-on images for the wavelength integrated images of figure 3-4. This would mean

that the emitters responsible for the protrusion in figure 3-4 must be lead atomic or ionic

excitation states other than the ones associated with the 280 or 220 nm lines. Because of a

lack of appropriate narrow band wavelength interference filters, we could not identify the

wavelength(s) responsible for the protrusion.

From these measurements, it is obvious that the elemental spectral line emission in the

LIB plasma is a complicated three dimensional spatial and temporal function, which is

different for ionic and atomic lines and most likely different even for different wavelength lines

from the same element and ionization stage. This indicated that the assumption of local

thermodynamic equilibrium might not be valid [149].

Angular Dependence

Since the plasma images observed in the previous section are not spherically

symmetrical, the spectral brightness will be an anisotropic function. The line of sight along

which the spectral brightness is highest will also be the one providing the optimum sensitivity

in a single parameter optimization. Experiments were carried out to determine if there was

a dependence of the signal intensities on the polar angle of radiation collection when the

whole volume of the LIB plasma was observed. Since it was necessary in these measurements

to align the horizontal axis of rotation of the radiation collecting system with the plasma, the

experimental apparatus shown in figure 3-9 was used. It was necessary to keep the lens-to-

plasma distance constant while the collection optic was rotated along a horizontal axis,










Optical fiber with focusing lens
on polar rotational stage


\ I- -,
&i rT '' __


Laser beam incident at 450

Target on X,Y,Z stage


Figure 3-9. Schematic diagram of the modified radiation collection system used for the angular dependence measurements.





77

otherwise the focus would be changing. For these measurements the Ocean Optics

spectrometer was used. Since the spectrometer we used didn't have the capability of gating,

no temporal resolution was available. Therefore, the time integrated emitted radiation from

the plasma (including background) was used for the measurement. Six successive laser

produced plasmas from a solid lead target were averaged to produce the spectra shown in

figure 3-10(a). The plot shows the 10 background-corrected spectra collected at 100 intervals

as the collection optic was rotated from 90 to 0 with respect to the horizon. Figure 3-10(c)

shows the wavelength integrated net signals over the observed line profile for each lead line

at the corresponding viewing angle. As can be seen in Figure 3-10(c), the signal intensities

are roughly constant from 900 to 40, with a slight increase at 30 before they slope off. The

decrease in signal was due to blocking of the solid angle of collection by the target. We also

observed a greater enhancement at about 400-300 when this experiment was repeated with a

soil sample. This more or less constant behavior with a peak at about 300 with respect to the

horizon is consistent with a cylindrical or mushroom shaped structure.

Conclusions

We found that the spatial and temporal behavior of the transient emission generated

by a laser-induced breakdown plasma is rather complicated. The different spatial and

temporal distributions for the ionic and atomic lines is to be expected during the existence of

the plasma considering the dynamic shift in ionization equilibrium as the plasma initially heats

up and then cools down. What is quite surprising though is that different atomic lead lines

also seem to have vastly different but uniquely reproducible time and space dependencies.

Measured time resolved spectra, to establish the most appropriate time interval producing an

optimum signal to background ratio (or what ever parameter is optimized), will be dependent
















(a) (b)
1400 15000
1200 4 -
1000 10000
goo
600 5000
400
200 0
0 '280 rnm
g00 283 nm

20. 287nm

276 2'8 Z-l !s82 284 2,t 2118 4 20 40 80
Viewing Angle
Wavelength (nm)



Figure 3-10. Angular dependence plots.
(a) Background corrected lead spectra plot; (b) Background plot.














(c) (d)
20000

15000

10000

5000

7 0
1280 nm

287 4m

Viewing Angle Viewing Angle



Figure 3-10 continued.
(c) The wavelength integrated net signal plot; (d) Signal to background ratio plot.






80

on the specific volume of plasma being observed and the line of sight of the detector. These

additional variables affecting the LIB emission should be considered together with the well

documented independent variables such as the laser pulse duration and shape in time, [17]

the laser wavelength [18] and energy, [21] the physical and chemical characteristics of the

target material, [24] and the composition and pressure of the surrounding environment [26].

Multi-variable optimization techniques for increased sensitivity and decreased limits

of detection will consequently also be complex procedures, especially considering that the

physical dimension of the plasma is of the order of millimeters in size and microseconds life

time with steep spatial and temporal gradients in spectral brightness.

With homogeneous material in the solid phase as the target and constant laser and

imaging parameters, the LIB plasma is a very reproducible emission source which is a

promising characteristic for analytical applications. We conclude that time resolved, spectral

imaging has the potential to considerably improve LIBS results and lead to a better

fundamental understanding of the LIB plasma.








Part II: Level Populations In A Laser-Induced Plasma On A Lead Target

Introduction

In laser-induced breakdown spectroscopy the mass ablation rate and characteristic

elemental emission intensity vary non-linearly on a pulse-to-pulse basis (apart from the latter's

temporal and spatial variation during a single shot) because of variations in the laser energy,

the temporal pulse shape and the spatial profile of the laser beam on the irradiated material.

The chemical composition and homogeneity in the targeted volume, the physical properties

(phase, pressure, temperature) and the mechanical properties (smoothness of the surface,

crystalline orientation) of the material and the atmospheric conditions (gas composition,

pressure) play a role, suggesting the likelihood for strong matrix effects if not properly

controlled. Objective interlaboratory comparisons are further obscured by the use of different

(and often unreported) laser focusing conditions (affecting the energy and power density), the

angle of laser beam incidence, the volume or subvolume of plasma actually being observed,

the angle of light collection, and the presence of previously ablated particulates in the

breakdown volume creating a memory effect for subsequent laser shots.

An understanding of the characteristics of the processes involved will enable better

control and lead to improved analytical performance. Temporal and spatial resolved

observations of laser-induced plasmas have been reported [45,150]. In an often cited

publication [115], Radziemski et al. concluded that local thermodynamic equilibrium (LTE)

was established in the LIB plasma at delay times greater than 1 us. Their conclusion was

based on the agreement between calculations from a theoretical one-dimensional

hydrodynamic model and the intensity ratios ofC I / C I; N II / N I; Be II / Be I applying the

Saha equation and a Boltzmann plot for Be I lines in an air plasma at 580 torr. They found






82

time-resolved temperatures ranging from 22000 to 8000 K during the plasma decay. Spatial

resolution using Abel integral inversion was applied only to the Be II / Be I line pair.

Spatially resolved temperatures changed less than 5% compared to the line-of-sight integrated

values, leading Radziemski et al. to conclude that the uncertainties introduced by unfolding

a small source of only approximate cylindrical symmetry would not improve the temperature

or electron density values. Lee et al. [25] measured spatially resolved intensities from lead

lines at 357.27, 363.96, 368.35, 373.99 and 405.78 nm and applied the Boltzmann plot

technique to determine temperatures for the plasma induced in air at atmospheric conditions

by an ArF excimer laser at 193 nm. Their results lacked temporal resolution of the transient

plasma; they reported temperatures in the region 13000 K varying along the plasma height.

Simeonsson et al. [151] measured time resolved temperatures in gaseous LIB plasmas in CO,

CO2, methanol and chloroform applying the Boltzmann equation to two oxygen and two

chlorine lines and the Saha equation to two carbon lines. They found spatially integrated

temporal temperature values ranging from 15000 to 20000 K. They assumed LTE based on

theoretical considerations following the Griem [152] criteria and the experimental

observations ofRadziemski [115]. The 521.82 and 510.55 nm Cu I lines were measured by

Mao and co-workers [21] and applied to a two line Boltzmann technique to determine the

axial variation of the temperature in a LIB plasma on copper targets. No temporal resolution

was measured. They found excitation temperatures ranging from 6000 to 11000 K.

Our impression is that in spite of a vast number of reports citing temperature and

electron number density measurements in LIB plasmas, there is still a need for a complete

spatial and temporal investigation of the population distributions of several different

atomic/ionic species. As part of a research project to develop a portable LIBS instrument for






83

the determination of toxic metals in the environment and in particulate samples, we

investigated the temporal and spatial development of lead atomic emission from a LIB plasma

in air, and inferred from it, the temporal and spatial distribution of the populations of the

upper levels from which the transitions originate.

Theory

Temperature Determination Using Spectral Line Ratios

The following assumptions are required:

* The source volume under observation is in local thermodynamic equilibrium (LTE),

has a homogeneous distribution of emitters, and remains in a steady state during the

time interval of interrogation.

The spectral lines measured are optically thin.

The population distribution amongst the internal energy levels is described by the

Boltzmann equation.

From the radiances, R (spectral energy per unit of time per unit of projected source area per

unit of observational spatial angle and integrated over the line profile), of two spectral lines

(indexed i andj) from different upper excitation energy levels, E, of the same element and

ionization stage, the temperature is given by [5]



T --E-E Rg J5 Ai (3-1)



with the degeneracy of the upper levels denoted by g, the emission transition probability by

A, and the wavelength by X. The two-line method has the advantage compared to a single

line radiance application that one eliminates the need of knowing the value for the effective






84

pathlength through the source, the total particle number density, the partition function, the

need for absolute radiance calibration of the detection system and absolute transition

probabilities. Practical considerations to keep in mind when selecting a line pair are to have

the wavelengths nearly identical and the difference in upper energies of the two transitions as

large as possible. The first criterion is beneficial in the sense that relative radiance calibration

is easier and more accurate and the second assures that the calculated temperature is more

reproducible and not over-sensitive to small fluctuations in the radiance ratio measurement.

Temperature Determination Using Boltzmann Plots

The two-line ratio technique can be extended to a larger number of lines from the

same element and ionization stage. This is similar to applying a least squares linear regression

to a larger number of points to determine the slope and intercept instead of determining the

linear relationship from just two points. Rearranging the spectral line radiance equation for

multiple lines, indexed i, from the same element and ionization stage gives the expression

[153]


( R 1 E, hcln,
In + hIn --0 (3-2)
g,A T k] 4xZ


where 1 is the effective pathlength through the radiation source, n, the particle number density,

Z the partition function and the other symbols have been defined above. If we now treat the

quantity In as the dependent variable and E' as the independent variable for a set of

lines from the same species and otherwise identical conditions, a linear relationship with a

slope of -- and an intercept of In ( results. The Boltzmann plot also needs

calibration of the radiances at each of the analytical wavelengths, just as with the two-line






85

ratio technique. Relative signal values proportional to the radiances and relative transition

probabilities for the measured lines instead of absolute values will not affect the calculated

temperature and will only result in a change of the intercept. As with the two-line ratio

technique, the same practical considerations apply, including the selection of closely matched

wavelengths for accurate relative radiance calibration and large differences in upper energies

from which the transitions originate to provide the best precision in the calculated

temperature.

Experimental

Lead Spectrum. Energy Level Diagram and Line Selection

The experimental apparatus, used for all measurements, was the same as that used in

part I of this chapter. In each wavelength window, a spectrum was accumulated from 10

single laser shots on a solid lead target. The target was moved horizontally between shots to

allow sampling from a fresh location and to improve the reproducibility of mass ablation.

Spectra were captured using an integration time of 15 ps after a delay of 2 ts with respect

to the plasma initiation to avoid the initial intense continuum background. Each spectrum

covered a wavelength window of~20 nm and successive sets of measurements were taken

with the center of the window shifted by 15 nm increments to cover a total spectral range

from 180-705 nm. From the energy level scheme for lead [154], we calculated the

wavelengths in air for all possible electric dipole transitions and associated the observed lines

with their respective upper and lower energy levels. We then used the following criteria in

selecting a set of lines for thermometric purposes:

S The wavelengths of the lines should all fit within the 20 nm wavelength window of

our detection system so that they could be observed simultaneously and radiance

calibration could be considered identical for all the lines.






86

* The lines should be reasonably intense to provide line to background ratios far in

excess of one in order to minimize background noise degrading the precision of the

measured line signals.

* The transition probability values of the lines should be available [145].

* The lower energy level on which the transition ends should preferably exclude the

ground state or other low lying energy levels to minimize self absorption.

* The difference in upper energy levels from which the emission lines originate should

be as large as possible to provide better precision in the temperature measurement.

From these criteria, we selected the lead atomic lines at the wavelengths 357.275, 363.958,

367.151, 368.348 and 373.994 nm as the diagnostic lines. Table 3-2 contains the

wavelengths, transition probabilities, upper and lower state's statistical weight, configuration

and energy level values, from the publication by Wiese and Martin [145]. Figure 3-11 shows

the partial Grotrian diagram for neutral lead with the transitions used in this investigation.

Data Capture and Graphical Presentation of the Raw Data

In each exposure, we simultaneously captured radiational data for the fixed spectral

window around 365 nm containing the selected set of five lead atomic lines emitted from the

LIB plasma and their intensities along the height of the plasma. Each exposure measured the

signal (as a discrete function of wavelength and height) in a time slice with a fixed integration

time of 200 ns and a delay time (with respect to the plasma initiation) that was varied in 500

ns steps for successive files. Each file was the cumulative result from 10 single laser shots on

a solid lead target. The target was moved horizontally between shots. The data files

generated by the Winspec V1.4.3.6 (Princeton Instruments) software were analyzed by means

ofin-house written Matlab 4.0 (The Mathworks Inc.) source programs. Each graph in figure







Energy
(X 103cs np
(x 10 cm')

60 Ionization limit Pb II (P (59821.0 cm')


50 7s P (49439.57 crf') ___ 3
7s P2(48188.67cm- ') 8S P (4868687 crm)

40
7s P3 (35287.24 crr) 357.275 nm
3 1 367.151 nm
30 7s Po (349990 cr1) 373994 nm


20 6p D (21457.90 cm')
363.958 nml .,
368.348 nmJ

10 3
--_ 6p P1 (781935cnr1)

0 ---- 6P3P (ocm')

Figure 3-11. Partial Grotrian diagram for neutral lead atoms and the transitions used for the thermometric diagnostics.






88

Table 3-2. Wavelengths, transition probabilities, and level specifications of the lead lines used
as thermometric species.

Lead I Transitions
Wavelength Transition Upper State Lower State
(nm) Probability
(x l s') Statistical Configuration Energy Level Configuration Energy Level
Weight (cn-1) (cn-1)
357.2748 0.99 3 7s'P, 49439.57 6p2'D, 21457.90
363.9577 0.34 3 7s'P, 35287.24 6p23P, 7819.35
367.1513 0.44 3 8s'P, 48686.87 6p2'D, 21457.90
368.3475 1.50 1 7s'P, 34959.90 6p2'Pi 7819.35
373.9944 0.73 5 7s'P, 48188.67 6p2'D, 21457.90




3-12 shows the variation in each spectrum along the plasma height for sequential delay time

increments of 500 ns. The first spectrum was captured at a delay time of 320 ns and consisted

mainly of an intense continuum with superimposed extremely broadened, barely discernable

lines. As time progressed, the continuum background decreased rapidly while the spectral

lines increased in intensity and reached a peak intensity at around 2 ts delay time after which

they continued with time to show a slower decay in intensity and with decreased line widths.

One can also see in the first frame that the plasma emission extended only about 0.5 mm

above the target surface but then gradually expanded upwards to over 1.5 mm height. From

these figures, it can be seen that the temporal and spatial development of the Pb I 367 nm line

differed significantlyfrom the other four observed lead lines.

Temporal Development of the 360 nm Series ofPb I Line Intensities

A simpler graphical overview of the temporal development of the spectrum was

achieved by accumulating in software the signals over the plasma height for each wavelength

associated column of pixels in each file and then plotting a time sequence of their temporal

development as shown in figure 3-13. The first spectrum measured at a delay time of 320 ns













320 ns
a



3,Il





05 375
Height (mm) 0 36 333 37
Wavelength (nm)



820 ns
4











075 375
e ( 375





1320 ns
5
4^
3
2 a






305 375


Figure 3-12. Time sequence of height dependent spectra for the observed lead
LIBS plasma.







1820 ns

, .


Height (mm)


Wavelength (nm)
2320 ns


2820 ns


Figure 3-12 continued.


((I'




Full Text
xml version 1.0 encoding UTF-8
REPORT xmlns http:www.fcla.edudlsmddaitss xmlns:xsi http:www.w3.org2001XMLSchema-instance xsi:schemaLocation http:www.fcla.edudlsmddaitssdaitssReport.xsd
INGEST IEID E5RYV46LB_M8GLYS INGEST_TIME 2013-02-07T18:24:07Z PACKAGE AA00012949_00001
AGREEMENT_INFO ACCOUNT UF PROJECT UFDC
FILES