Hadamard transform and programmed scan multi-element analysis

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Title:
Hadamard transform and programmed scan multi-element analysis multiplex versus single channel atomic fluorescence spectrometry
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vii, 103 leaves. : illus. ; 28 cm.
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Plankey, Francis William, 1945-
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Chemistry thesis Ph. D
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Thesis -- University of Florida.
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Bibliography: leaves 100-102.
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Typescript.
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Vita.

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HADAMARD TRANSFORM AND PROGRAMMED SCAN
MULTIELEMENT ANALYSIS MULTIPLEX VERSUS
SINGLE CHANNEL ATOMIC FLUORESCENCE SPECTROMETRY






By





FRANCIS WILLIAM PLANKEY, JR.


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




UNIVERSITY OF FLORIDA
1974


I














This work is dedicated to my wife, Bonnie, who encouraged and

supported me in many, many ways and who spent lonely, patient hours in

order to make this work and my graduate program a success.














ACKNOWLEDGMENTS



The author wishes to acknowledge the outstanding assistance of

his teachers and associates, especially the help and guidance of Professor

Sidney Siggia during the early years and Professor James D. Winefordner

during Graduate School. The enthusiasm and encouragement of these

latter two persons was the driving force of his academic work. Further-

more, he would like to thank Dr. Dave Johnson, Lucas Hart and Gail

Pokrant for .their help in many instances.














TABLE OF CONTENTS



Page

ACKNOWLEDGMENTS . . .. .

ABSTRACT ........... .. vi

Chapter

I INTRODUCTION TO FLAME SPECTROMETRY AND MULTIELEMENT
ANALYSIS . . 1

Atomic Methods of Analysis .. 1
Single Element Analysis . ... 2
Multielement Analysis . .. 3

II MULTIPLEX SPECTROMETRY SYSTEMS .. . 5

Types of Multiplex Spectrometry. . .. 5
Previous Spectroscopic Studies with Multiplex Methods. 6
Signal-to-Noise Relationships for UV-Visible HTS 8

III ATOMIC FLUORESCENCE COMPONENTS COMMON TO THE SYSTEMS
STUDIED .. . . 15

The Atomic Fluorescence Excitation Source 15
The Atomization Cell . . 16
The Optical System . . 20

IV THE HADAMARD TRANSFORM SPECTROMETER (HTS) SYSTEM 24

Hadamard Transform Spectrometry ..... 24
Implementation of the HTS System 25
The Fast Hadamard Transform (FTS) .. 30
Analytical Procedure with the HTS System ... 34
Results and Discussion .. . 36





iv








Chapter Page

V SINGLE CHANNEL SCANNING SPECTROMETER (SCSS) SYSTEM 49

Preliminary Discussion . 49
Description of SCSS Experimental Setup .. 50
Operation of the SCSS System . .... 56
Analytical Procedure with the SCSS System 57
Results and Discussion .. 59

VI COMPARISON OF MULTIPLEX AND SCANNING TECHNIQUES 77

VII PROGRAMMED SLEW SPECTROMETER (PSS) SYSTEM 82

Preliminary Discussion ............. 82
Description of Software . .. 83
Analytical Procedure with the PSS System 84
Results and Discussion . 86

VIII COMPARISON OF THE THREE SYSTEMS . 90

APPENDICES

I AVERAGE SIGNAL-TO-NOISE RATIOS FOR THE UV-VISIBLE
SPECTRAL REGION .. . 93

II THE COMPUTER SYSTEM .......... 95

LITERATURE CITED .. . 100

BIOGRAPHICAL SKETCH . .. .. 103














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



HADAMARD TRANSFORM AND PROGRAMMED SCAN MULTIELEMENT
ANALYSIS MULTIPLEX VERSUS SINGLE CHANNEL
ATOMIC FLUORESCENCE SPECTROMETRY

By

Francis William Plankey, Jr.

August, 1974

Chairman: James D. Winefordner
Major Department: Chemistry

The practice of sequential analysis for many trace elements by

flame spectrometry is explained and techniques for rapid sequential and

simultaneous multielement analysis are reviewed. Consideration of the

three flame spectrometric techniques, atomic emission, atomic absorption

and atomic fluorescence, leads one to the conclusion that the latter is

most suitable for multielement work. A description of, and justification

for using, a xenon arc continuum source and an argon separated

air/acetylene flame for multielement analysisare discussed.

Multiplex spectrometry is explained and types of multiplex

methods for simultaneously coding many spectral intervals at a detector

are covered. Results of the application of multiplex methods with the

advantage derived from multiplexing are presented. Of the three

multiplexing techniques, Fourier transform, frequency correlation and


a








Hadamard transform, only the latter has been chosen for implementation

In this work. Signal-to-noise relationships are investigated for the

multiplex and the single channel cases, and example calculations

are shown.

A description of the 255 spectral interval Hadamard transform

spectrometer (HTS) covering about 25 nm in about 25 s, and the software

for performing the transform is presented followed by a description of

the analytical procedure and the results obtained for Ni, Co, Fe, Mn, Mg,

Cu, Ag and Cr analysis. Spectra and estimates of the limits of detection

are given for these elements.

The construction and operation of a computer controlled single

channel scanning spectrometer (SCSS) is described, and the results for the

analysis of those elements done with the HTS plus Zn, Cd, Au, Pb, Sn and

Tl are presented. Next, a comparison of the results of analyses of the

two methods is made, and the cause of the serious disadvantage of the

multiplex method, based on proportional noise in the system, is proposed.

The results indicate that in the case of atomic fluorescence flame

spectrometry, the large background signal results in a factor of about

100 times poorer signal-to-noise ratio for the HTS system as compared

to the SCSS when the scanning times are equivalent.

Using the same equipment as in the SCSS, it is possible to slew

rapidly between intervals of interest and thereby spend more time

measuring analytically useful wavelengths. The S/N increase, proportional

to the measuring time, is demonstrated, and the improved limits of detection

for 13 elements are compared with the SCSS limits.


~Lj














CHAPTER I

INTRODUCTION TO FLAME SPECTROMETRY AND MULTIELEMENT ANALYSIS



Atomic Methods of Analysis

Flame spectrometric techniques have become the most widely used

methods for the determination of trace quantities of metals in solution.

Atomic emission spectrometry (AES) has been reviewed by Pickett and

Koirtyohann (1), atomic absorption spectrometry (AAS) has been described

by Lewis (2) and atomic fluorescence spectrometry (AFS) has been

explained by Winefordner and Elser (3). The three techniques have been

critically compared by Winefordner, Svoboda and Cline (4).

Each of the flame techniques measures the interaction or emission

of electromagnetic radiation by ground state or excited state atoms.

In AES, the excitation of ground state atoms is by thermal means in a

flame. The excited atoms emit light of a characteristic (for each

element) frequency, and the intensity of the light is directly proportional

to the concentration of atoms in the flame. In AAS, the ground state

atoms in the flame absorb radiation (from a suitable source) of a certain

frequency, characteristic of the element. The amount of light absorbed

is proportional to the concentration of the specified element in the

flame cell. In AFS, a light source is also used but in this case, the

radiation is used to excite ground state atoms in a flame. These







radiationally excited atoms emit characteristic radiation and the

Intensity of fluorescence is proportional to the concentration of atoms

In the flame cell.

Conventionally, the three techniques differ substantially only

in the mechanism of excitation of the atomic species; otherwise, they

share much of the same instrumentation. Usually, a dispersive device

monochromatorr) is used to isolate the characteristic radiation of

Interest-and a photomultiplier/amplifier/readout detection system is

used to produce a signal proportional to the radiant flux striking the

photocathode and thus proportional to the intensity of the emission,

fluorescence or absorption. More detailed descriptions of flame

techniques are available in books by Mavrodineanu (5) and Hermann

and Alkemade (6).



Single Element Analysis

The usual procedure in the analysis of several metals in several

samples is as follows. The monochromator dispersing element, usually a

grating, is positioned so that a spectral line of the first element of

interest is Isolated at the detector. A blank solution, and a series

of standard solutions containing known concentrations of the first

element, are then aspirated into the flame, and the appropriate intensity

readings are recorded. The concentration range should include or bracket

the concentration of the unknowns. If such a concentration range cannot

be determined immediately then dilution of the sample and subsequent

analyses are generally necessary. Each,of the unknown samples is then

aspirated and the corresponding intensity readings are recorded. After

the analytical curve of intensity (corrected for blank) vs. concentration

is constructed, the concentration of the first element in each sample is







determined from its intensity value on this curve. The next element is

selected, the monochromator is scanned to the appropriate wavelength

and the next characteristic frequency spectral line is isolated. The

procedure is repeated for each element.



Multielement Analysis

Recently, there has been much effort expended to develop

multielement techniques with which many elements can be determined

simultaneously or in rapid sequence. The longest established multielement

technique has involved the use of the photographic emulsion detector.

Because this method is very slow and usually requires a subsequent

scanning of the plate, photographic detection has been bypassed as a

viable system by most flame spectroscopists. Recently, Busch and

Morrison (7) reviewed multielement systems. Types of detection systems

which have been investigated include scanning monochromators (8-12),

movable detector (13), non-dispersive rotating filter detector (14),

sequentially pulsed source AFS (15), direct reading spectrographs (16-19)

and imaging devices (20-26).

For detector-noise limited situations, as in the infrared (IR)

spectral region, multiplex methods have been used to increase the

signal-to-noise ratio and to decrease the analysis time. Speculation

about the use of a multiplex method (7,24,27), such as Fourier transform

or Hadamard transform spectrometry, in the ultraviolet (UV) region has

led to the present study of the comparison of multiplex and single

channel methods in multielement atomic fluorescence spectrometry.

Each of the three atomic techniques, AES, AAS and AFS,has been

used by various researchers for multielement analysis. Each technique

has certain advantages and certain disadvantages. The instrumental






4
system for AES requires no light source and is quite simple. A flame

atomizer supplies sufficient energy to excite most elements whose

resonant lines lie above about 350.0 nm (4). However, the spectrum of

many elements is very complex in this spectral region and a good high

resolution monochromator is necessary. Also, some elements do not have

resonance lines which can be excited by the thermal energy present in

analytical flames. Recent developments in induction coupled plasmas (28)

may improve this excitation energy problem but AES is presently limited

in multielement analytical flame spectrometry.

In AAS, almost every metallic element can be measured at trace

quantities. The source need not be extremely intense since the measured

quantity in AAS is the attenuation of the source intensity by the absorbing

atoms. However, the radiation must be directed from the source through

the flame and into the dispersive system. In order to avoid very complex

optical arrangements, one source must then be used for multiple elements.

This is a severe limitation because commercially available AAS sources

are limited to about six elements.

In AFS, an intense source of radiation is required for the

radiational excitation of the atoms in the flame. If such a source is

available, AFS becomes the technique of choice because fluorescence

spectraare relatively uncomplicated compared with those in AES and no

optical geometry problems are present because fluorescence is emitted in

all directions and a non-1800 source-flame-monochromator optical

alignment is suitable. Because AFS has these advantages, it alone was

studied in this project.














CHAPTER II

MULTIPLEX SPECTROMETRY SYSTEMS



Types of Multiplex Spectrometry

Normal single channel spectrometer systems measure the intensity

in one spectral interval at a time. Three multiplexedd methods" have

been devised to measure the intensities of more than one spectral

interval at the same time with one detector. Each of these multiplex

methods uses a coding device to enable the transformation of the total

Intensity into the intensities at the individual spectral intervals.

In Fourier transform multiplex spectroscopy, a Michelson

interferometer is used to encode the high frequency electromagnetic

radiation used in spectroscopy into an audio frequency interferogram (29).

In this system, all of the spectral intervals, over a wavelength region

determined only by optics and detector, are multiplexed on the detector.

A-computer is used to perform a Fourier transform of the coded interferogram

to decode the intensity versus mirror displacement signal to an intensity

versus wavelength spectrum. This technique has found wide application in

the infrared (IR) spectral region. Low (30-33) has written an excellent

review of Fourier transform spectrometry (FTS).

A second type of multiplex method can be based on the modulation

of line sources at different frequencies. If signals due to the sources







are allowed to fall on a single detector, each of the wavelengths in the

sources will be frequency coded and multiplexed (34). The individual

wavelength intensities can be recovered either by locking-in on their

modulation frequency or, alternatively, by performing a Fourier

transformation. This type of correlation spectrometry can only be used

in analyses where line excitation sources are used such as AAS or AFS.

No published results of this type of system are available at the present

time.

In the third multiplex method, a normal grating system is used to

disperse a wavelength region onto a specially constructed mask which

allows some spectral interval intensities to pass onto the detector

while blocking some other intensities. In this way, an equation relating

the total intensity (or the flux reaching the detector) to a linear

combination of the separate spectral interval intensities (fluxes reaching

the detector) is formed. A series of linearly independent equations is

formed for example when the mask is constructed in a manner related to

mathematical Hadamard matrices, and so this method is called Hadamard

transform spectrometry (HTS). HTS thus codes a specific wavelength

region in a binary,-on (intensity passed) or off (intensity blocked),

fashion. A computer program is used to solve the set of simultaneous

equations formed in the HTS process. A review of HTS principles and

application in the IR spectral region can be found in articles by Ibbett,

Aspinall and Grainger (35), Decker and Harwit (36) and Nelson and

Fredman (37).



Previous Spectroscopic Studies with Multiplex Methods

Of these three multiplex methods discussed in the previous

section, only the first and third have been previously used in







spectroscopic studies. Commercial instruments are available for FTS and

tITS in the IR region. The utility of these multiplex methods in the IR

is a result of the signal-to-noise (S/N) advantage. Fellgett (38) has

shown that in a detector-noise limited case (such as IR spectrometry),

a signal-to-noise ratio increase can be expected. The so-called Fellgett,

or multiplex, advantage is the ratio of the S/N of the multiplex method

to the S/N of a single channel (slit) system and is given by



(S/N)
F m = N

SC



*here N is the number of spectral intervals multiplexed on the detector.




N = 2 X1





I and X) are the extremes of the wavelength range reaching the detector

and S) is the bandpass of the spectral interval. In FTS, 6X depends on

the displacement increment of the mirror while in HTS, 6X depends on the

dimensions (size of a unit slit) of the Hadamard mask. In either case,

N usually ranges from a few hundred to a few thousand.

The Fellgett advantage is calculated using equal analysis time for

the single channel (slit) and the multiplex system. If, on the other hand,

it is desired to maintain the same signal-to-noise ratio in both systems,

the increase in analysis speed is directly proportional to N for the

multiplex methods. It is this factor which led to the investigation of

the use of a multiplex method for atomic analysis in the ultraviolet (UV)


j








spectral region, in a desire to be able to rapidly scan a fairly wide

wavelength region with good resolution.

FTS systems utilize an interferometer to transform radiation,
14
e.g., frequencies of 010 Hz, to an interferogram, e.g., frequencies

of '102 Hz. Interferometric methods are adaptable without tremendous

problems to the IR and near IR regions, where wavelengths are of the order

of fractions of a mm to a few pm. Because alignment procedures in

Interferometry require tolerances of a fraction of the wavelength of the

light being observed, it is quite difficult to achieve stable interfer-

ometry in the 200-700 nm region of the spectrum. For this reason, FTS has

not been evaluated for multielement atomic analysis.

Correlation spectroscopy has recently been discussed, but since

it is limited to use with line sources and requires individual modulation

for each element to be analyzed, it has not yet been studied.

The HTS method uses well-developed dispersion technology and

requires only machine shop tolerances. It is capable of retrofit into

existing monochromator systems, and only a Hadamard mask is needed to

implement the system. The HTS system was therefore chosen for investi-

gation of the use of a multiplexing technique to decrease analysis time

in multielement atomic analysis.



Signal-to-Noise Relationships for UV-Visible HTS

The Fellgett advantage is only applicable to multiplex systems

in which the major source of noise is in the detector. In other words,

the advantage is only realized in systems where the noise does not

increase with an increase in the signal reaching the detector. This Is

the case in the IR region where only noisy detectors (high noise

equivalent sources) are available. In the UV-visible region, photomultiplier







detectors of very low noise are used and the major source of noise is

the quantum noise or shot noise of the source. This type of noise bears

a square root dependence on the total signal; it is due to the random

arrival of photons at the detector. No gain in S/N can be expected from

the Fellgett advantage for multiplex methods in the UV-visible region if

the S/N is averaged over the entire wavelength region measured (39).

(See Appendix I for a proof of this statement.)

A single channel scanning spectrometer (SCSS) system which covers

N spectral intervals in T seconds will measure each interval for T/N

seconds. The signal in that time will be proportional to the number of

photons incident on the photomultiplier tube, I, and the time, T, and so

will be IT/N. Assuming that shot noise is the dominant noise, the noise

will be the square root of the signal. (The signal-noise relationship

follows a Poisson distribution, where the standard deviation of the

population is equal to the square root of the mean of the population.)

Now, the signal-to-noise for one spectral interval will be
1 _
(IT/N)/(IT/N) = (IT/N)2. Therefore, the S/N ratio depends only on

the signal which is involved with the particular spectral interval of

Interest and is independent of the signals in other spectral intervals.

There are three limiting cases which are useful for comparing

HTS in the UV-visible region with the SCSS:

First, the case of one line falling in one spectral interval

will be considered. This single line, of flux I, will be allowed to

pass to the detector for (N+1)/2 separate measurements and will be

summed each time. With the assumption of no background, the other

(N-1)/2 measurements will have no intensity reaching the detector.

The total signal from the line, of.intensity I, will be IT(N+1)/2N and

the signal-to-noise is thus (IT(N+1)/2N)2 (still assuming only shot noise).


'A







The gain in signal-to-noise at the spectral interval of interest, due to

the longer observation time, is then given by



(S/N)
G = HTS [IT(N+1)/2N] N+
(S/N) 1 [.1 2
SCSS [ IT/N]


This advantage is essentially the same as the Fellgett advantage for HTS

in the IR (37). It should be pointed out that the noise of all the N

spectral intervals is exactly the same as the noise associated with the

spectral Interval which contains the single spectral line and, again

assuming no background, this noise would now cause the baseline to

fluctuate around a level of zero in the other N-1 spectral intervals.

Therefore, .the average (S/N) is exactly equal to the average (S/N)scsS
HTS CS
and there is no average advantage. However, in the SCSS case almost all

of the noise is associated with the line peak, and the baseline thus has

little noise. In the HTS, the noise is distributed evenly over the

entire spectrum, and therefore the peak signal has less noise in the

HTS than in the SCSS, and the baseline signals have less noise in the

SCSS. This is true in any case where the HTS has an advantage and

the advantage is only associated with the peak signals.

The second case of interest involves the S/N relationship of a

weak line in the presence of a strong line. In this case, the noise in

each spectral interval will essentially be due to the noise from the

strong line. Thus, the S/N of the weak line will be equal to the S/N

of the strong line divided by the ratio of intensity of strong line to

intensity of weak line. In the case of two lines of intensity ratio

(N+1)/2 to 1 or greater, the S/N of the weak line would be the same or


_








better in the SCSS system as in the HTS system. This degradation of the

S/N is called the multiplex disadvantage and results when two or more

dissimilar signals are measured simultaneously with one detector. If

the noise is related to the total signal at the detector, the multiplex

disadvantage will degrade the S/N of very small signals in the presence

of very large signals.
B
The third case is somewhat more realistic since a background, I ,

is now considered at all of the spectral intervals observed by HTS. At

all N mask positions approximately N/2 slots allow the background to

pass and at (N+1)/2 positions the line of interest with intensity Is Is

also passed. The total signal, ITHT,which will be summed will be
HTS



N+1
2
s B
I = N+I I + N I
THTS 2
I=1



The shot noise figure associated with this signal will be




2
s B
N .= N I + N I
HTS 2 j i
i=1


Since the signal of interest is still N+1 Is the S/N ratio in the HTS
2
system is given by









(S/N)
HTS


N+1
2


N+1 I
2 J
N+1
2
+-N

+ N

1=1


In the SCSS, the signal of interest is given by

signal is


ITcss
SCSS


B

I


Is while
J


the total


= I + I
j


The noise is then (I.
J


+ 1.8 ) and the S/N ratio is
J


(S/N)
SCSS


j
s B
(I. + I. )
J J


The gain in S/N at the line peak is then


(S/N)
G = HTS
(S/N)
SCSS


-8
where I1


is the average background intensity,


.8
1


(N+1)/2
Z B
= i=1
N+1
2


N+1 5s
2 [


+ I


N+1 I +
2


- B A
NI II
1


I






R 13

Evaluating the limiting cases shows that if the average background is

small compared to the intensity of the line of interest (I B s)
1 J
then G reduces to [(N+1)/2] as in the first case considered.

Otherwise, if Is << B then G becomes 1//2. In this circumstance the

HTS system is worse by a factor of 0.7 compared with the SCSS.

In the previous exercise, only shot noise (or quantum noise) was

considered and noise which is proportional to the background (flicker

noise from a flame for example) or to the source (i.e., ripple noise in

an AFS source) was neglected. Instrumentation has been developed to

minimize the effects of these types of noises but even the best

instrumental system may allow some small part of this so-called

"proportional" noise. This noise is characterized by being directly

proportional to the total signal which is associated with the noise.

For low level signals the few percent proportional noise is usually

negligible compared to the square root relationship shot noise. However,

in the HTS system, with a significant background at each spectral
-B
interval the total influence of an average background of 11 at each
2 B -B
interval is N2 + N 7B compared with II contribution in the SCSS.
2
This factor of (N2 + N)/2 would almost certainly cause the majority of

noise in the Hadamard system to be due to proportional noise if the

background is significant.

To illustrate the concepts of this chapter it is instructive to

examine a possible example in multielement AFS by the two methods.

Consider the use of photon counting. Also, consider that the background

consists of 1000 counts accumulated in each spectral interval for the

counting period and that two lines of 100 counts and 5000 counts per

measurement interval are to be determined in the same analysis time,

by the HTS and SCSS system (255 spectral intervals are to be measured in

25.5 seconds).







The SCSS will measure each spectral interval for 0.1 s and will

accumulate 1000 counts for each spectral interval while measuring the back-

ground alone and 1100 and 6000 counts while measuring the line peak spectral

intervals. The shot noise on 1000 counts is about 32 and the shot noises at

the lines plus background are about 33 and 77 respectively. If proportional

noise is all due to the background and is 1.0% then proportional noise is

10 counts at each channel. Shot noise is thus dominant, and the (S/N)scsS

of the weak and strong lines are 3.0 and 64.5 respectively.

Now, for the HTS, the background is measured at 128 spectral intervals

(on the average) and is summed for each channel 255 times. The lines are

summed 128 times. The shot noise associated with each spectral interval

is thus
( (255) (128)(1000) + (128)(100) + (128)(5000) )

or 5770. The signal for the first line is measured 128 times and so is 12800;

the second line signal is 640000. From shot noise considerations, the (S/N)
HTS
of the first line is then 2.2 and the second line is 111. However, if

proportional noise is 1% of the background level then for 128 spectral

intervals the proportional noise is 1280 counts. This noise is independently

measured 255 times, and therefore the porportional noise is

( (255)(1280)2)1 = 20440.
Noise values add quadratically and so the total noise in the HTS system is

( (5770)2 + (20440)2)i = 21239
Now the (S/N)HTS for the two lines is reduced to 0.6 and 30, a factor of 5,
HTS
and 2.2 times worse for the two lines by HTS as compared with the SCSS.

It can be seen that in a real system, the scanning technique might

be shot noise limited while the multiplex technique may suffer tremendous

losses in S/N because of proportional noise.













CHAPTER III

ATOMIC FLUORESCENCE COMPONENTS
COMMON TO THE SYSTEMS STUDIED



The Atomic Fluorescence Excitation Source

In recent years, most AFS workers have been using intense line

sources of excitation. Metal vapor discharge lamps (MVL's)(40,41),

hollow cathode lamps (HCL's)(42,43), electrodeless discharge lamps

(EDL's)(44,45) and, more recently, tunable dye lasers (TDL's)(46) have

been shown to be excellent single element AFS sources. In most cases,

these sources have to be tuned, adjusted or thermostated for optimum

performance for one specific element. The results obtained thereby are

quite good for most elements limits of detection range from a few

hundred ng/mi down to fractions of a ng/ml for more than 30 elements (3).

Some attempts have been made to use multielement HCL's (47) and

-especially multielement EDL's (48) as AFS sources. Combination problems

and optimization trade-offs have severely limited use of multielement

sources in multielement AFS (49). If single element sources are used in

multielement AFS each must be individually powered and adjusted for

optimum conditions. Furthermore, the spatial arrangement of such a

situation becomes difficult if more than 4 to 6 sources are to be used.

Continuum sources have been used in the past (50-53) with some

-success. Workers note two difficulties with continuum sources: the spectral








radiance of xenon arcs falls off rapidly below about 250 nm (where many

useful resonance lines occur), and scattering by the incompletely

vaporized particles in the flame leads to high backgrounds and thus to

poorer detection limits when compared to intense line sources. Despite

these shortcomings, the xenon arc lamp appears to be the source of choice

in multielement AFS at the present time for three reasons: first,

although the lamp's output drops off below 250 nm, some fraction of the

output is still available down to about 200 nm (see Figure 1) allowing

excitation of just about every element of interest; secondly, only one

source is necessary, which simplifies the front-end geometrical

arrangement, e.g., even if six-element combination sources (as EDL's)

could be made, still five source/power supplies, lens/chopper systems

would be needed to allow excitation of 30 elements, compared with one

continuum source with one power supply; finally, the continuum source

is convenient to operate, requires only minimal adjustment and Is

stable after a few minutes warm up time, as shown in Figure 2. The

particular continuum lamp used in this study is a high pressure, short

arc xenon lamp with aTr internal aluminum parabolic reflector. The

manufacturer claims that this lamp collimates 85% of the total arc

output and uses a sapphire window for output down to at least 200 nm.



The Atomization Cell

Flames are the most common atomizers for present-day atomic

spectroscopy. A sample is aspirated into a flame, usually with a

premixed, laminar flow burner, where the solvent is evaporated, and the

solute vaporized and atomized by the thermal action of the flame. Much

of the previous work in AFS has made use of the low quenching combustible

gases such as oxyhydrogen or hydrogen-air diffusion flames; molecular








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species such as CO and CO2 enhance the non-radiational deactivation

(quenching) of excited atoms in the flame. The hydrogen flames are also

much cooler than hydrocarbon flames and have higher burning velocities

and therefore the desolvation, vaporization and atomization processes,

which are very necessary to avoid scattering of the continuum source

radiation,.are not as efficient. The use of the cooler hydrogen flame

and real samples necessitates a scatter correction even when used with

line sources (54).

In order that the multielement systems investigated would have

maximum versatility, the higher temperature air/acetylene flame was used

for all of the AFS work in this study. Although the quenching species in

this flame, together with a higher background, could mean a factor of

5 to 100 lower (poorer) fluorescence quantum yields, the flame proved

to be an excellent atomization source and no noticeable scatter was

observed even when aspirating solutions with more than 500 jg/ml of solids.

The flame was produced by a premixed nebulizer/burner assembly

together with a capillary type burner head (55). The flame was separated

by an argon sheath. The fuel/air flow rates were established so that the

flame was slightly fuel-rich; the cones above the capillaries were

somewhat "fuzzy" and had some noticeableyellow color. This fuel/air

ratio led to the lowest background and allowed good atomization of all of

the-elements studied. Some improvement might be possible if the flame

were changed to maximize the S/N for each individual element, but this

would have been too complicated for the present study and so was not
-*1
attempted. The flow rate for acetylene was 1.5 1 min for air 9.7 1 min"

and for argon 15.5 1 min" as measured with a wet test meter.








The Optical System

The front-end optics and source/atomizer were the same in all

of the work described later in this study. The source was focused

through a chopper operated at about 50 Hz. The chopped radiation was

collected by a lens and focused into the flame. On the other side of

the flame a 140 mm diameter, UV enhanced mirror with a 62 mm focal

length focused the source radiation back into the flame. At right

angles to the source light path, a lens collected the fluorescence and

formed a 1:1 image of the flame at the entrance slit of the monochromator.

All of the lenses used were of fused quartz. Figure 3 shows a block

diagram of the front-end arrangement used throughout this study, and

Table 1 lists the components used and their sources.

Few problems were encountered with the present set-up. However,

It should be stressed that the mirror behind the flame and in line with

the source should not focus the exciting beam directly back into the

continuum lamp because of the danger of overheating the source and

destroying the lamp. A procedure involving slight defocusing of the

mirror-source optics was adopted after the explosive destruction of a

lamp. This procedure reduced the fluorescence signal by only 10-20%.

The major emphasis in this study was to develop and evaluate

systems for multielement analysis. While doing this, it was also

possible to compare multiplex and single channel methods in the UV-visible

spectral region. Much of the equipment used throughout this study was

exactly the same for both the Hadamard and programmed scan systems;

however, in the multiplex system an analog detection system was used

and in the single channel set-up a digital detection (photon counting)

system was employed. This led to problems in a direct comparison of the

Hadamard and scanning methods. The primary advantage of the digital













Xenon Arc


Fused Qu
Lenses,



Light Trap


Head


Flame Cell
O Monochro -
.Entrance



Collecting
Lens


motor
Slit
iF,


/
/ Doubling Mirror






Figure 3.-- Atomic Fluorescence Source and Flame Cell Arrangement used
throughout this study.





22


TABLE 1

COMPONENTS OF THE ATOMIC FLUORESCENCE
SOURCE AND FLAME CELL.


DESCRIPTION
(MODEL NUMBER)


SOURCE


Xenon Arc Continuum
and Power Supply

Nebulizer/Burner


Lenses


Doubling Mirror


Chopper


VIX-150-7UV


303-0110


2" diameter,
2*' focal length

140 mm diameter,
62 mm focal length

46 Hz, 50% duty cycle


E lmac/Varian,
San Carlos, CA 94070

Perkin-Elmer,
Norwalk, CT 06852

ESCO Products,
Oak Ridge, NJ 07438

Optical Industries, Inc.,
Costa Mesa, CA 92626

Laboratory fabricated.


ITEM








detection system was one of dynamic range. The analog system, coupled

to a computer, allowed a dynamic range of only 1024 discrete levels.

The photon counter has a dynamic range of about 10 However, in this

study a range of only 105 was normally used. The second advantage of

the digital system is one of convenience, i.e., the photon counter

needs no adjustment between ranges while the analog system had to be

switched through different ranges.

So, although a comparison was sought, it was felt that the best

system available should be developed with the equipment on hand. When

the results are discussed (see Chapters IV and V) possible advantages

due to the analog/digital detection systems will be noted.













CHAPTER IV

THE HADAMARD TRANSFORM SPECTROMETER (HTS) SYSTEM



Hadamard Transform Spectrometry

The multiplex method of Hadamard transform spectrometry (HTS)

makes use of conventional grating spectrometer technology. In place of

a single exit slit, which is used to isolate a narrow bandpass (spectral

interval) region of the electromagnetic spectrum, a multi-slit mask is

used to allow portions of a much wider spectral region to reach the

detector. The mask is specially constructed to block some spectral

Intervals and allow others to pass. Therefore, the signal reaching the

detector for a given mask position, i, is given by



J=N
j-N
Sj= ajXJ



where the a .'s (j=1 to N) are either 0 (if the light is blocked at

spectral interval X.) or 1 (if the light is allowed to pass at spectral

Interval X.). When a set of N orthogonal equations is formulated with

various combinations of a..'s, then the individual X.'s can be determined
Ii J
by solving the set of simultaneous equations given by N intensity

readings with N different mask configurations. Hadamard matrices give








the relationships necessary to form independent equations with a set of

N different masks or, alternatively, a cyclic mask, constructed so

that an orthogonal set of coefficients (aij's) can be created by moving

the mask one slot width N times at N positions to form the set of

N equations.



Implementation of the HTS System

A cyclic mask can be designed for any N (N = 2n -1; n any integer)

according to the method of primative polynomials given by Nelson and

Fredman (37).

For this work, N = 2 -1 = 255 spectral intervals are covered at

the exit focal plane. A cyclic mask would then have 2N 1 = 509

transparent (open) and opaque (closed) slots. The order of l's (open

slots) and0's(closed slots) is determined once the first n (8 in this

case) coefficients are given. The first 8 coefficients used for this

mask were 10001110. The ninth coefficient can be found from the first

eight coefficients by modulo 2 addition. (In mod 2 addition 0 + 0 = 0,

1 + 0 1, 0 + 1 = 1 and 1 + 1 = 0 with carry = 1.) Now, if the first,

third, fourth and fifth coefficients are added, mod 2, the result is

the ninth coefficient. Thus the ninth coefficient is 1 + 0 + 0 + 1 = 0.

Each subsequent coefficient can be determined from the previous eight

coefficients in this same manner. The 509 coefficients, implemented into

the mask, are shown in Table 2. The coefficients at the first mask

position are determined from the portion of the mask consisting of the

first 255 coefficients 10001...to 00000 (see underlined section of

Table 2). The coefficients of the second mask position are formed by

shifting, one place to the right, and so are 00011...to 00001, and so

forth until the last (255th) mask position has coefficients 01000...to

00000.














TABLE 2

255 SLOT CYCLIC MASK CODE: "1" DENOTES TRANSPARENT SLOT
AND "0" DENOTES OPAQUE SLOT. NOTE THAT THE 255 SLOT CYCLIC
MASK IMPLIES 509 TOTAL MASK SLOTS. THE UNDERLINED PORTION
REPRESENTS THE SLOTS ILLUMINATED AT THE FIRST MASK POSITION





10001 11000 10010 11100 00001 10010 01001 10111 00100 00010

10110 11010 11001 01100 00111 11011 01111 01011 10100 01000

01101 10001 11100 11100 11000 10110 10010 00101 00101 01001

11011 10110 01111 01111 11010 01100 11010 10001 10000 01110

10101 01111 10010 10000 10011 11111 10000 10111 10001 10100

00000 10001 11000 10010 11100 00001 10010 01001 10111 00100

00010 10110 11010 11001 01100 00111 11011 01111 01011 10100

01000 01101 10001 11100 11100 11000 10110 10010 00101 00101

01001 11011 10110 01111 01111 11010 01100 11010 10001 10000

01110 10101 01111 10010 10000 10011 11111 10000 10111 10001


10100 0000







The Hadamard spectrometer was constructed from a Czerny-Turner

scanning monochromator, a single pass, 0.35 m focal length, f/6.8 mount

with a 48 mm x 48 mm, 1180 lines per mm grating blazed for 250 nm.

The reciprocal linear dispersion with this configuration was approximately

2.0 nm/mm. The slits are straight-edged and bilaterally adjustable in

width from 5 to 2000 pm and in height with values of 12, 5, 3, 1 or

0.5 mm. The folding mirror at the exit slit was removed, and a

laboratory-fabricated translation stage was mounted at the front panel

with the front edge of the slide assembly (as shown in Figure 4)

mounted at the exit focal plane. A field stop, 0.510 in x 0.5 in,was

aligned to allow approximately 12.5 nm on either side of the exit focal

point to fall on the mask which was connected to the slide assembly.

The mask consisted of a copper-nickel bimetallic strip 1" x 3" x 0.010".

The 255 slot cyclic mask (509 total slots) was inscribed on the

blmetallic strip with each slot having a width of 0.002 in (50 pm) and

a height of 0.387 in; the total mask code length was 1.018 in. The

slide assembly was spring loaded in the translation stage and was driven

by a 40 turns-per-in micrometer which in turn could be stepped in

4acrements of 1/19200 in by a 480 steps-per-revolution stepping motor.

The detector was a 30 mm diameter end-on photomultiplier tube

with S-13 spectral response, and it was operated at 750-1000 V. The

photoanode current was amplified by a current-to-voltage converter and

measured by a lock-in amplifier, which was tuned to the phase and

frequency of the chopper described in Chapter III. The output of the

lock-in amplifier was directed to the analog input of the PDP-11

'computer (see Appendix If for a description of the computer system).

A block diagram of the complete HTS system is shown in Figure 4. The

components used in the system are listed in Table 3.



























V



V
e-J



1







L.



0
C
u









0
L.





L









-C









0
E
L0





-o
Im









I-
I-













TABLE 3

COMPONENTS OF THE HADAMARD TRANSFORM SPECTROMETERa


DESCRIPTION
(MODEL NUMBER)


509 Total Slot
255 Cyclic Code


Monochromator


Photomultipl ier


Stepping Motor


Photomultiplier
Power Supply

Lock-in Amplifier


EU-700


EMI 9526B


HDM-12-480-4


EU-42A


804


SOURCE


Dynamic Research Corp.,
Wilmington, MA 01887
Under license from
Spectral Imaging,
Concord, MA

Heath Co.,
Benton Harbor, MI 49022

Gencom Div.,
Plainview, NY 11803

USM Corp.,
Wakefield, MA 01880

Heath Co.,
Benton Harbor, MI 49022

Keithley Instruments,
Cleveland, OH 44139


aNote: Other components (excitation source, flame, computer system)
are listed in Tables 1 and 12.


ITEM


Mask








The Fast Hadamard Transform (FHT)

The process of accumulating data in an orthogonal fashion in

order to form a set of linearly independent simultaneous equations is

begun when the atomic fluorescence system, including the light source

and flame cell described in Chapter III, and the spectrometer and

electronics described in the last section, is aligned and adjusted.

The monochromator is set so that the center of the desired 25 nm span

is displayed on the wavelength counter. The computer program is loaded

and started with the command "G". The computer steps the mask to the

first position and takes a series of 255 readings from the lock-in

amplifier by means of the analog-to-digital converter (ADC). These

readings are averaged, and the average is added to or subtracted from

a double precision (two 16 bit words) 255 point data buffer storage

array according the following scheme. The first eight coefficients,

corresponding to the first eight slots of the mask as it is currently

positioned, are determined in the same manner as the coefficients for

the mask were calculated in the previous section. For example, for the

first mask position, the first eight coefficients are 10001110. These

binary digits are designated b0 to b The double precision accumulators

are numbered 1 to 255 and are determined as eight bit binary numbers

a0 to a7 (00000001 for the first accumulator number). Corresponding

bits are added, mod 2, according to (a0 + b ) + (al + bI) +...+ (a7 + b7);

the sum determined will be 1 if the number of matching set (1) bits in

these two eight bit words is odd, and the average signal at this slot

position is then added to the respective accumulator. If, on the other

hand, the number of matching set bits is even (0), then the average

signal is subtracted from the patricular accumulator. It can be seen

that for the first code word (10001110), there are no matching l's for







the first storage location (00000001), and so the signal is subtracted

from the first accumulator.

The accumulator is then incremented by one and the new set of

bits, representing the new accumulator number, is matched with the code

word; the average signal is then added to or subtracted from the next

accumulator (10001110 has 1 matching set bit with 00000010 so the first

signal is added to the second accumulator). The code word remains the

same (until the mask is stepped to the next location), the accumulator

number is increased by one and the add/subtract process is repeated

until the average signal at this mask position has been added to or

subtracted from each of the 255 accumulators.

At this point, the code word describing the first eight slots of

the next mask position is determined in the same manner as used to

determine the coefficients in the last section. Briefly, the bits

b0 + b2 + b + b are added, modulo 2, and the result is used as bg.

A shift, one to the right results in the eight bit word b1 to b8.

These bits are used as the new code word. The mask is then stepped one

slot width to the new mask position, and the data gathering and

kaddition/subtraction process is repeated.

After 255 mask steps, the spectral signal values are stored as

Double precision, octal (base 8) integers with any noise portion of the

signal resulting in a baseline fluctuation about zero. To eliminate

problems caused by negative numbers being routed to a unipolar digital-

to-analog converter (DAC), each signal value is digitally offset until

no negative numbers remain in the data buffer.

A final permutation is needed to obtain the actual signal versus

wavelength spectrum. The real channel in a monotonically increasing

spectrum (denoted by X ; i = 1 to 255) is related to the stored








channel (Sk) by an eight bit binary number which can be found according

to the following procedure.

The intensities of the first eight spectral elements (Xi; i = 1

to 8) are contained in the storage channels (accumulators) Sk as follows:
Xi = Sk
1-1
where; k = 2 and i = 1 to 8.


Thus X1 = S;

X = S 2;

X3= S4;
X4 8g;



x8 = S128'


Subsequent spectral intensities (Xi; 9 < i < 255) may then be found in

storage locations calculated from the binary representations of the

previous eight storage locations. Spectral intensity Xi(9 i 255),

with binary representation (x.i, x1,2 x.3 x,4 x, x.6 x., x. 8)
1,1 x2 i,3 i,4 x,5 i,6 i 7 ,8
will be found in accumulator Sk where the binary representation of

k Is (s1 s2 s3 s s5 s 7 s8) and where,


< (mod 2)
s. = x + x + x + x
s = i+j-9, + xi+j-9,3 + i+j-9,4 i+j-9,5



where s. is the value of the first binary bit of the Sk accumulator and
J
x + 1 s the value of the first binary bit of the (i+j-9)'th spectral
Xl+j-9,1
interval and the other x-terms are correspondingly defined. For example,

for X9 (the ninth spectral) interval, the eight pervious accumulators






33
in binary notation are


X1 = 00000001, i.e. X1 = x1,1 x1,2 x1,3 x1,4 x1,5 x1,6 xl,7 x1,8'
X2 = 00000010,
X3 = 00000100,
X .= 00001000,
X = 00010000,
5
X = 00100000,
X7 = 01000000,
X8 10000000, i.e. X = x8,1 x8,2 x x x8,5 8,6 x8,7 x8,8'



Thus for i = 9


s x +x + x + x = x +x1 +xI +x1
1 X9+1-9,1 +9+1-9,3 9+1-9,4 9+1-9,5 1,1 +1,3 1,4 +X1,

s1 0 +0 + 00+ 0 0

s2 x 2,1 + x2,3 + x2,4 + x2,5 0+0 +0 + 0= 0,
s x3,1 + 3 + ,4 + 3,5 0 + 0 + 0 +
s 54 +X +X +X = 0+0+0+1=1,
s4 4,1 *4,3 4,4 4,5
s5 = x +x +x +x = 0 + 0 + 1 + 0 = 1,
5 5,1 5,3 5,4 5,5
s = x +6,1 x + x + x = 0+1 +0 +0= 1,
s = x +x +x +x = 0+0+0+0=0,
7 7,1 7,3 7,4 7,5
s8 x8,1 + x8,3 + x8,4 + 8,5 1+ + 0 + 0 =1.


Therefore, k = (00011101) = 358 = 2910 so X9 = S29 and so the ninth
spectral element is found in the 29th sequential storage location.
Each subsequent location can therefore be determined from the eight
binary words describing the eight previous storage locations.








The storage locations are determined, and each stored value is

divided by (N+1)/2 or 128 in the present case (N = 255). These final

values are stored in a sequential (increasing with wavelength) data

buffer area of the computer. At this time, the transform is complete,

and the digital data are available for processing.

It can be seen that the FHT can be accomplished in this way with

N x N additions (or subtractions), and N simple divisions. For this

reason, the FHT can be programmed on any computer without the need for

a hardware multiply/divide or a slow, software multiply/divide package.

This one-step arithmetic process is the basis for the order of magnitude

Increase in speed of the FHT (56) compared with the fast Fourier

transform (FFT) which requires N log2 N multiplications (or divisions)(57).

Because the spectral intensity values are stored, they are

available for digital manipulation. The data can be read out on an

oscilloscope or an x-y plotter. Alternatively, the entire set of

signals can be punched on paper tape for storage. Also, the data can be

smoothed, and the baseline averaged. Peaks can be located and integrated,

and comparisons with calibration standards or previous spectra can be

made. Multiple scans can be generated and added or subtracted.



Analytical Procedure with the HTS System

Stock aqueous solutions of 1000 pg ml of the metals listed in

Table 4 were prepared from reagent grade chemicals. Appropriate serial

dilutions of the combinations of elements were made in-the concentrations

shown. The spectrometer was manually scanned to the center wavelength

shown In Table 4 for the wavelength range desired.

The system was then energized. The illuminator was started and

allowed to stabilize, the flame was ignited and adjusted, and the














TABLE 4

ELEMENTS INVESTIGATED WITH THE
HADAMARD TRANSFORM SPECTROMETER SYSTEM


ELEMENTS


Ni, Co, Fe

Mn, Mg

Cu, Ag

Cr


CENTER WAVELENGTHa


240 nm

285 nm

330 nm

359 nm


CONCENTRATIONS


g9 mi -1
,g ml1


pg ml"1

pjg ml"1
mu ml"I


- 500 jg ml"1

- 200 pg ml-

- 200 mg mli-

- 200 jig ml-1


a
Wavelength setting of grating spectrometer.








computer program was loaded and readied. Solutions, including a blank

and the standards, were aspirated each in turn and the data gathering

process was initiated by means of software (the keyboard). The current

amplifier and lock-in were set to give 10 volts per amp and 30 mV full

scale, respectively. Thus, the current measured and amplified by the

lock-in was about 3 nA. The settings were adjusted if necessary on a

subsequent run if the readings during a run exceeded full scale out

from the lock-in or if no reading during a run exceeded 10% full scale

out. The maximum S/N in the output resulted when there was a maximum

difference in the reading obtained as the mask was stepped across the

exit plane. All of the adjustments to the electronics were performed

to maximize this difference in readings. The photomultiplier tube

dc voltage was initially set at 800 V but was also adjusted in the range

750 V to 850 V to maximize the difference in readings. The results from

the FHT were available about 30 s after the start of a scan via the

oscilloscope display. The results were typed out as intensity vs.

channel number and also were plotted via the x-y recorder.



Results and Discussion

It was intended that analytical curves (log intensity vs. log

analyte concentration) be obtained by gathering digital data from the

printout of the intensity of the channels as a function of the

concentration of analyte introduced into the flame. After an exhaustive

study of this possibility it was determined that this was not feasible for

two reasons; (i) the concentrations which allowed signal-to-noise ratios

of greater than about 10 were found to be on the non-linear portions of

an analytical curve; lower concentrations of analyte, which would fall








in the linear portion of the working curve would not give S/N ratios

-greater than about 2 or 3; (ii) all variations in electronic parameters,

in-order to increase differences in the readings as the mask was

stepped, resulted in vastly different background noise levels. For

these reasons, the x-y plots of the spectra were used to estimate the

limits of detection for the elements of interest.

One problem became evident during the preliminary operation of

the system. With the electronic sensitivity used in these studies,

i.e., about 3 nA current and 100 ms time constant with the lock-in, a

blank solution resulted in considerable noise in the baseline as

depicted in Figure 5. The background, which is reproducible in peak-to-

peak noise intensity but not in spectral distribution, was probably due

to slight fluctuations in scattered radiation in the flame. Since about

-oe-half of the mask slots are transparent, a total spectral region of

approximately 12.5 nm of the background is passed at each mask position.

Any noise (due to the source) on the background is passed to the detector.

This effect, which is a direct consequence of the multiplexing effort,

ams a major cause of-the poor performance of the HTS.

Iieiesolution capability of the HTS system is demonstrated by

the ultielement AFS spectra shown in Figure 6. 200 pg ml-1 each of Mn

sand Mg were aspirated. Entrance slit widths of 200 pm and 50 jur resulted

-in the spectra shown. In each case, a digita) filtering technique was

used to average the noise on the baseline in order to better demonstrate

.the resolution possible. The Mn triplet at 279.5, 279.8 and 280.1 shows

baseline resolution with 50 jim entrances it width, and the spectral

bandpass is less than 0.3 nm per channel with this slit width.

In Figures 7 to 10 results are given of Hadamard scans of

wavelength centered as listed in Table 4. In Figure 7 the AFS spectrum


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is given for 300 Pg ml"1 Ni, 200 jug mll Co and 200 ug ml"1 Fe; in

Figure 8 the AFS spectrum is for 10 jg ml'" each of Mn and Mg; in
-1
Figure 9 the AFS spectrum is for 100 ag ml- each of Cu and Ag; in

Figure 10 the AFS spectrum is for the Cr triplet from 100 ug ml"1 Cr.

Each of the spectra was scanned in 28 s with an entrance slit of

100 jm. From results such as these, estimates of limits of detection

for these eight elements were made and are given in Table 5. Also

listed there are the detection limits found for those elements using

a single channel system in a stationary mode, and the same type of

150 W xenon continuum source (53).

A comparison of these results show that the HTS system results

in detection.1imits some orders of magnitude less than for a stationary,

single channel system. It can be estimated that the loss in S/N due to

the extremely wide spectral region for the background scatter is at

least one order of magnitude. This result was estimated from the

background noise levels in a single channel scanning system with

similar time constant and sensitivity. The root-mean-square (rms)

noise ratio is aboutTO for the HTS/single channel system.

All of the detection limits listed in Table 5 are for similar

signal levels from all fluorescent lines in the wavelength region under

investigation. This situation would not normally occur in real samples

where vastly differing concentrations of elements could lead to large

differences in signal levels. To investigate potential problems in

multiplex systems for the analysis of mixtures of different concentrations,

the Mg-Mn mixture shown in Figure 11, i.e., a mixture of 50 jg ml"1 Mg
-1
and 25 jug ml Mn, was measured. The Mn signal is clearly evident.

When the ratio is increased from 2:1 to 3:1 (60 jig ml-1 Mg and














TABLE 5

LIMITS OF DETECTION (LOD)a FOR THE ELEMENTS
ANALYZED BY THE HADAMARD TRANSFORM SPECTROMETER SYSTEM


WAVELENGTH


232.0

240.7

248.3

279.8

285.2

324.7

328.1

357.9


LOD(this work)
j9 ml 1


40

25

25

5

1

5

2


LOD(reference 53)
jig ml1


10

10

5.0

0.3

0.04

I

0.1


The limit of detection is defined as the concentration of
analyte which gives a signal twice the rms noise observed
when a blank is measured.


mote:


ELEMENT

























04



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20 pg ml-1 Mn), the noise level increased, and the signal from the Mn

was no longer clearly visible. (Figure 12). This effect of the

multiplex disadvantage is certainly a major limitation of this type of

spectrometric system.














CHAPTER V

SINGLE CHANNEL SCANNING SPECTROMETER (SCSS) SYSTEM



Preliminary Discussion

Most conventional spectrometric systems are able to scan the

wavelength which is presented to the detector. In order to be able to

scan at speeds greater than about 0.2 nm/s all that is needed is a

means of rapid data handling and some method to determine the wavelength

accurately and reproducibly during the scan. These functions are easily

performed by means of a computer, and so, with a computer available,

the scanning technique is easily implemented.

Because only one spectral interval is measured at a time, only

conventional problems of spectral interference are encountered. The

spectral bandpass of the instrument must be chosen so that possible

spectral interference will be of low probability. At the same time,

the dispersion of the instrument should be chosen such that the time

necessary for scanning the grating is as short as possible.

An obvious advantage to a single slit scanning system is that the

range of wavelength which can be covered during a scan is only limited

by the data handling capability of the system. However, in order to

compare this system with the HTS only 25 nm wavelength scans are

appropriate.






50

Description of SCSS Experimental Setup

The AFS excitation source and atomization flame cell described

in Chapter III were used in the SCSS experimental setup as well as in

the HTS setup. Only the dispersion device and detection system are

changed from the HTS system.

A 0.3 m, f/5.3 modified Czerny-Turner mount monochromator with

a 50 mm x 50 mm, 600 grooves/mm grating was used as a dispersion device.

This monochromator has individual bilaterally adjustable straight slits

variable in width from 5 um to 2000 um and in height from 0 to 20 mm.

The reciprocal linear dispersion with the specified grating was 5.41 nm/mm

and the resolution with 10 um wide 4 mm high slits was 0.12 nm.

Throughout this work, the slits were set at 75 jum wide and 10 mm high

to correspond closely to the HTS system bandpass. With these conditions

the bandpass was approximately 0.3 nm.

The normal scanning motor/gear system for this monochromator was

disengaged, and a stepping motor was coupled directly to the shaft of the

precision screw which rotates the grating. The stepping motor, which

makes 480 steps per revolution (0.750 per step) can be driven at speeds

to about 1200 steps second-1. Because one revolution of the precision

screw scans the monochromator 10.0 nm, the resolution capability of the

monochromator/stepping motor system was 480 steps/10 nm = 4.8 steps/0.1 nm

or about 0.02 nm per step. Maximum stepping speed used in this

application was about 1000 steps s" so that the maximum slewing rate
-1
was about 20 nm s

A high gain, low dark current photomultiplier tube with S (Q)

spectral response (EMI 6256S) was used in a photon counting mode within

a magnetically and electrostatically shielded housing at the exit slit.

The tube was operated at -1450 V from the photon counting ratemeter/power








supply described later. The dark count with this system was 70-100

counts per second (cps).

The photon counting system consisted of a fast amplifier/discriminator,

ratemeter/power supply, and a digital synchronous computer. The

amplifier/discriminator was powered by the ratemeter/power supply low

voltage power out. Output from the discriminator was sent through the

ratemeter to the digital synchronous computer. The ratemeter was used

as an intermediate analog output device and power supply for the

photomultiplier tube and the amplifier/discriminator. These components

were operated according to the manufacturers instructions.

The digital synchronous computer (DSC) is essentially a photon

counter with two high speed 8 digit scalers, an arithmetic processing

unit and timing and control signals and functions. In a mode called

CHOP, It is able to function as a "lock-in photon counter" by use of

the two scalers in conjunction with a mechanical chopper. When a

synchronizing signal indicates that the data signal to be measured is

-present at the photomultiplier tube, the DSC uses a scaler designated

as "DATA". Switches and a clock are available for setting the

:observation time, in microseconds, for which the DATA scaler is to be

used. After that observation time, the scaler is shut down and the

-DATA is stored. The synchronizing signal next indicates that the

background signal is present and the counts from the background are

-routed to a scaler designated "BACKGROUND". This scaler accumulates

counts for the same observation time as the DATA scaler. The DSC has

thumb-wheel switches on which the operator can set a "preset number" of

chopper cycles. After each DATA-BACKGROUND cycle, the preset N counter

is checked to see if the number of chopper cycles set there has been


I








finished. If not, further cycles are started and the total of DATA

and BACKGROUND are stored in the appropriate scaler. If the preset N

counter has been accomplished then the results are displayed on light

emitting diodes on the control panel of the instrument in floating point

notation (3 significant digits and an exponent). The information displayed

can be either the DATA, the BACKGROUND, or the sum or difference of

these two counts. At the same time, a "ready" light is lighted, and

If the instrument is in the automatic cycle mode, the process is

started again (see Figure 13 for a timing diagram of this process).

The components of the SCSS are listed in Table 6 and a block diagram of

the system is shown in Figure 14.

The DSC is Ideally suited to computer control because two

connectors at the rear of the Instrument allow all of the functions of

the DSC to be controlled by TTL level signals from a computer interface.

Also, the data is available as TTL levels in binary coded decimal (BCD)

notation. Synchronization signal lines are provided to allow communication

between the DSC and a laboratory computer. For a more detailed

description of the DSC/computer interface, see Appendix II.

The software for the SCSS is designed for simple control/data

acquisition. Briefly, the computer steps the grating 0.1 nm and then

counts for 4 chopper cycles. The fluorescence information (difference

between the DATA and BACKGROUND scalers) is picked up after each cycle,

and, at the end of 4 cycles, the sum is stored in a data buffer. The

computer steps the grating 0.1 nm again and the data gathering is

repeated until a wavelength scan of 25.6 nm is complete. The data can

be displayed or printed or normalized to values between 0 and 1023 for

plotting by means of an x-y recorder. The routines are quite easily















Sync
s signal





Light at
Flame




Data
Register




Background
Register





Figure 13.--


I I I I I


I I I I I I
I I I I I I
II IIS I

I I I I I I


I I I I I I
i t I I

I I I I I





Timing diagram showing when the DATA and
BACKGROUND registers of the Digital Synchronous
Computer are open with respect to the source
modulation.



















-o




4.
41



4A
C


O
























4-
0
L
4.






4c





S





u


I






I
S
0


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-G












0
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-o


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0 0 C.
T 8 pi
I

IX I














TABLE 6

COMPONENTS OF THE SINGLE CHANNEL SCANNING SPECTROMETERa


DESCRIPTION
(MODEL NUMBER)


SOURCE


Monochromator


Photomult Ipl ier


Photomultiplier Housing
Amplifier/Discriminator
Digital Synchronous Computer
Power Supply/Ratemeter


Stepping Motor


218


EM I 6256S


1151
1120
1110
1105


HDM-12-480-12


GCA/McPherson Instr. Co.,
Acton, MA 01720

Gencom Div.,
Plainview, NY 11803

SSR Instrument Co.,
Santa Monica, CA 90404


USM Corp.,
Wakefield, MA 01880


Other components (excitation source, flame, computer system)
are listed in Tables I and 12.


ITEM








written and, since no transformation is necessary, almost all the computer

time is used in data acquisition.



Operation of the SCSS System

When the system is in operation (flame and optics aligned and

adjusted, chopper and electronics energized and computer program

loaded), the monochromator is initially positioned at 190.0 nm. The

wavelength region to be scanned can be approached by slewing. The

keyboard command "F" will accept an octal (base 8) number at 0.1 nm

steps to be slewed forward from 190.0 nm. The digital display will

show thewavelength at all times during all operations. When the

starting wavelength is reached, a "G" keyboard command will initiate

the scan. The scan time is 31 s for a 25.6 nm scan. Upon completion

of the scan the TTY will type "READY", and the fluorescence intensity

vs. wavelength will be displayed on the oscilloscope. This may be

plotted on the x-y recorder by typing "P", or the set of intensities

may be printed with the command "W". Plot time on the x-y recorder is

65 s and printing. timeon the high speed printer is 28 s. Also, the

data may be normalized so that the intensities are between 0 and 1023

for the 10-bit digital-to-analog converter. This routine, on an "A"

command, will divide or multiply the entire data buffer by 2 as many

times as necessary until the data points all have a magnitude of 1023

or less. The normalized data are stored in a separate data buffer

(number 1) and a further buffer (number 2) can be used to save ("S"

command) whichever data buffer is currently displayed. The buffer on

the display can be changed by the commands 0, 1 or 2 corresponding to

the data buffer of interest. The plot and print commands operate on the

buffer which is currently displayed.


-,W~








When the operation is finished with the data from the current

scan, a scan from the present location can be achieved with commands

"C" (continue) followed by "G". The next 25.6 nm wavelength region will

then be scanned. Alternatively, the operation can begin the same scan

over again with "B" to rewind and "G". The rewind time is 1.3 s.

A "control R" command at any time will slew the monochromator to the

original starting wavelength (190.0 nm).

Because of the limited dynamic range of the DAC-display system,

the most useful results are obtained from printing the original results

(data buffer 0). If the display is normalized, then the ratio between

lines in the wavelength region is maintained, and this ratio can readily

be observed. The printed data may be corrected for off-wavelength

background and plotted in an intensity vs. concentration analytical curve.



Analytical Procedure with the SCSS System

Stock aqueous solutions of 1000 jig ml-1 of the metals listed in

Table 7 were prepared from reagent grade chemicals. Appropriate serial

4dilutions of the combination of elements shown in Table 7 were made in

the range 0.1 jg ml-1 to 100 g ml-1m

When the system was energized, the command was given to slew

rapidly to the starting wavelength for the particular element combination

as-given in Table 7. A blank solution deionizedd water) was aspirated,

-and the scan was initiated with the "G" command. After the scan was

complete, both a plot of fluorescence intensity vs. wavelength, and a

digital printout was requested. After this was accomplished, the

nxmoochromator was slewed back to the starting position, and a standard

-solution was aspirated; the intensity values were plotted and printed

each in turn. Three replicate analyses of each standard solution were














TABLE 7

ELEMENTS ANALYZED BY THE SINGLE CHANNEL SCANNING
SPECTROMETER BY GROUPS WITH WAVELENGTH RANGE AND
CONCENTRATION RANGE


WAVELENGTH


RANGE (nm)


CONCENTRATION
RANGE (pg ml"')


Cd

Co, Fe

Mn, Pb, Sn, Mg

Ag

TI


210.0

230.0

265.0

320.0

355.0


- 235.5

- 255.5

- 290.5

- 345.5

- 380.5


0.1 100

1.0 100

0.03 100

0.1 100

0.1 100


ELEMENT


Zn,

NI,

Au,

Cu,

Cr,


Fc~fw


-,






59

performed. The printed results were used in the preparation of

analytical curves. The average, off-wavelength background was subtracted

from the peak intensity, and the average of the three replicates was

plotted vs. the concentration of the analyte.



Results and Discussion

Some of the results from the SCSS system are shown in Figures

15 to 20. In Figure 15 the background (obtained in 31 s) is given

for the wavelength range from 265.0 nm to 290.5 nm while aspirating

deionized water into the flame. In Figure 16, the spectrum is given
-1
for 1 jug ml each of Zn and Cd. This figure shows the effect of

normalization on small peaks (of less than one-half full scale) as each

of the readings is multiplied until at least one spectral interval is

more than one-half full scale. In Figure 17, the AF spectrum for
-1
10 jig ml each of Ni, Co and Fe is given. Five elements can be

w-easured in the wavelength range 265.0 nm to 290.5 nm as shown in

Figure 18; 30 pg ml- each of Au, Mn, Pb, Sn and Mg give discernable

signals with the SCSS-. The signals from Mn and Mg are extremely large

-t 30 jug ml" and they cannot be plotted on the same scale with Au, Pb

and Sn at this same concentration. Also, 1 jg ml-1 each of Cu and Ag

givealarge S/N ratios as seen in Figure 19. The resolution of this

-system is demonstrated in the slight separation of the 327.4 nm line

of Cu and the 328.1 nm strong line of Ag. The signal at approximately

336-um appears in the spectra in this region and has tentatively been

assigned to the molecular fluorescence of NH in the flame (58). Finally,

the Cr triplet and the resonance line of TI are shown at a concentration
-l
of 1 jug ml in Figure 20. This figure has also been scale expanded by

a factor of 2 for clarity.

































































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The analytical working curves for the 14 elements studied with

the SCSS system are shown in Figure 21 and Figure 22. The shapes of

the curves are expected for AFS with a continuum source (59). The

curves for Pb (measured at 283.3 nm) and Sn (measured at 284.0 nm) do

not have the characteristic slope of 1, probably because of the strong

Influence of the Mg fluorescence (measured at 285.2 nm). The limits of

detection, defined as the concentration of analyte which would result

in a signal two times the rms noise on the background, for the 14

elements are listed in Table 8.

The background in the 230.0 nm to 255.6 nm wavelength region

was seen to contain significant flame fluorescence background (see

Figure 23). The effect of the background on elemental analysis in this

region is shown clearly in Figure 24, which gives the spectrum of

30 ug ml" each of Ni, Co and Fe. The molecular species involved in

this background is believed to be CO (58).

The S/N of any spectral interval of the SCSS should, according

to Chapter II, be independent, as far as the resolution and stray light

capability of the olptTcal system allows, of the signal in any other

spectral Interval. To check this premise, and as a comparison with the

HTS system, dissimilar concentrations of elements within a wavelength

scan were aspirated, and plots made of the intensity vs. wavelength.

In Figure 25, the lack of any interelement effects of a mixture of

10 jig ml"1 Zn plus 1 ug ml"1 Cd and a further mixture of 10 jug ml"

Zn and 0.3 jg ml"1 Cd is shown. The results for Cd are unchanged and

are also the same as if equal concentrations of Zn and Cd were used.

If 700 jag ml"1 Co is combined with 10 jpg ml1 each Fe and Ni, the Co

fluorescence lines are extremely intense (Figure 26) but, except for a

rise in the baseline, the Ni and Fe signals remain the same as when






67









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TABLE 8

LIMITS OF DETECTION (LOD'S) FOR THE ELEMENTS
ANALYZED BY THE SCSS SYSTEM


WAVELENGTH


213.7

228.8

232.0

240.7

248.3

267.6

279.5

283.3

284.0

285.2

324.7

328.1

357.9

377.6


LOD(this work)
ug ml"


0.3

0.1

0.6

0.2

0.4

1

0.01

0.2

3

0.002

0.02

0.02

0.04

0.06


LOD(reference 53)
jg ml"


7

0.08

10

1

5



0.2

20

5

0.04

0.2

0.05


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76
the concentrations of all three were the same. In Figure 27, 800 jug ml"'
-1
TI has essentially no effect on the signal from 3 Jg ml- Cr. However,

100 pg mil Cu raised the background substantially in the vicinity of

the 327.4 nm Cu line and the 328.1 nm Ag line. Even here, as shown in

Figure 28, no effect on even the 328.1 nm Ag line is noticed if the

Increase in background is taken into account. It is apparent that,

within the limits of the dispersive system, no adverse effects of

spectral interference are noticed with the SCSS system.














CHAPTER VI

COMPARISON OF MULTIPLEX AND SCANNING TECHNIQUES



The Hadamard transform spectrometer (HTS) and the single channel

scanning spectrometer (SCSS) share almost every hardware feature and the

design and construction of these two instruments is similar with respect

to time and costs. The HTS system does require a relatively expensive,

precision mask for coding the spectral interval intensities. Furthermore,

the multiplex method requires.some mask alignment precautions to assure

that the mask is in the correct optical position. Otherwise, the

complete systems are similar in operating characteristics. Because the

4TS system used a shorter dynamic range analog lock-in amplifier, it

mas necessary to change scales to obtain the optimum S/N benefit from

:the system. However, a photon counting system could eliminate the problem

4n the HTS system as it did in the SCSS.

Software considerations show that the HTS system software is

somewhat more complex than the SCSS software. Because some logical

functions and comparisons must be performed to determine the addition/

subtraction process and further decisions are necessary to complete the

final permutation of the stored values, so that a monotonically increasing

.spectrum results (see Chapter IV), the HTS software requires both

-moderate assembly programming effort and some time during the scan.






78

The SCSS, on the other hand, is essentially straightforward data

accumulation and stepping functions. In each case, the final data are

stored in digital form within the computer, and are available for

various filtering, integration or comparison purposes.

The wavelength range of the HTS is determined by the dimensions

of the mask and the dispersion of the grating. Likewise, the resolution

is limited to one point for each channel determined and in this study.

The wavelength range was about 25.0 nm with maximum resolution of about

0.1 nm per channel. If more than one wavelength region is to be

studied, the grating must be manually slewed from one region to the

next. Indeed, this function could be incorporated under computer

control, but this would involve more hardware and software considerations.

The SCSS is essentially limited only by computer memory storage

locations and in the present case more than 4000 data points could be

stored. The total number of points accumulated in a scan is software

controlled. Also, the resolution capabilities of the SCSS are limited

to 48 points per nm, a factor of about 4.8 more than with the HTS. The

number of points per nm is also under software control, and so, in the

SCSS, both the wavelength range and the resolution can be controlled by

the software.

From these considerations, it can be noted that the HTS system

is essentially a SCSS with a few more problems and complexities added

on and a little less versatility. The use of the multiplex HTS system

in the area of multielement atomic fluorescence analysis can only be

Justified if this method can improve on the sensitivity of rapid AFS

analysis.

A comparison of the results of the two methods shown in

Chapters IV and V is probably best seen in the limits of detection (LOD's)








obtained for similar elements by the two methods. Because the analysis

time was almost the same for the two techniques, the ratiasof LOD's are

.good estimates of the S/N ratios of the analytical lines determined in

a multiplex compared to a single channel mode. The HTS resulted in

LOD's worse by a factor of 50 to 500 in the eight elements common to

the two systems as seen in Table 9. The other 6 elements, studied with

the SCSS, could not be determined with the HTS (except for TI which was

not attempted with the multiplex method) because the noise in the

multiplex method severely limited the sensitivity of that system.

Because the systems were designed to maintain as many similarities

-as possible as far as excitation source, flame cell, optics and

4monochromator speed are concerned, it may be assumed that instrumental

influence can account only for a factor of 2 to 5 in the S/N ratios of

the two systems. The S/N considerations, described in Chapter II,

expect at most a degradation of S/N by a factor of 0.7 in the HTS system

if the background is large, and only shot noise is considered. The last

part of Chapter II considers. the effect of proportional noise

(fluctuation or scintillation noise (52)) in multiplex.systems. It can

dteA determined that if the noise due to scatter from the flame

or wolecular fluorescence from the flame is the major source of noise

in the tfTS system used in this study, then the S/N ratio of the HTS

system may be degraded by as much as 16 (the square root of the increase

of the number of spectral intervals measured) when compared with the

-SCSS. This, combined with other small factors, may account for the

4-atios in the LOD's found in the two systems.

The increase in the intensities measured with a multiplex

method is fundamental in the expected increase in S/N. When this

increase in signal is enough to allow noise, proportional to a high














TABLE 9

COMPARISON OF LIMITS OF DETECTION (LOD'S)
WITH THE HTS AND SCSS SYSTEMS.


HTS
RATIO
SCSS


67

125

63

500

500

250

100

125


ELEMENT


HTS LOD
ug ml-1


40

25

25

5

1

5

2

5


SCSS LOD
ug m"1


0.6

0.2

0.4

0.01

0.002

0.02

0.02

0.04


I ~








background level, to prevail over the random, photon noise of lower

signal levels, then the multiplex method has serious disadvantages

compared with a single channel system. This seems to be the case with

multielement atomic fluorescence flame spectrometry.

It is appropriate to consider the premise that a wavelength

scan is actually necessary in multielement analysis. Measuring the

intensity at wavelengths where no analytical line can appear is

certainly a waste of analysis time (unless an off-wavelength intensity

is desired) which could be put to better use by increasing the

measurement time (and thereby the S/N ratio) at spectral intervals of

interest. Overcoming the analyst's preference for an intensity vs.

wavelength spectrum is of primary importance to rapid multielement

analysis.














CHAPTER VII

PROGRAMMED SLEW SPECTROMETER (PSS) SYSTEM



Preliminary Discussion

The conclusion of Chapter VI leads naturally to the suggestion

that analysis time can be reduced substantially if intensities are

measured only at the wavelengths of interest in multielement atomic

fluorescence. Such a programmable slewing system would rapidly and

accurately slew to specific wavelengths, make measurements for some

time and then slew to another wavelength. In this way, little time

would be wasted between spectral intervals of interest. The

signal-to-noise relationships for the single channel scanning spectrometer

would still hold for the individual spectral intervals. If the time

spent slewing to the wavelength is considerably shorter than the

measurement time, their S/N ratios can be increased by spending more

time with useful measurements. For the shot noise limited ce, the

S/N is proportional to the square root of the measurement time. Now, if

there are five spectral intervals of interest in a 25.0 nm wavelength

region and the slewing rate is 20 nm s1, the intensities at the five

wavelengths can be measured for 5 s each, and the total analysis time

would be.about 26 s. The S/N of each of the spectral intervals would

Increase by a factor of (5/0.1) or about 7, as compared to the

scanning system where the spectral intervals are measured for 0.1 s each.








The equipment for such a programmable monochromator is

completely contained in the single channel scanning spectrometer. The
-1
computer is able to slew the grating at 20 nm s by use of the stepper

motor. All that is necessary is additional software for the computer.



Description of Software

An assembly language program was developed to accept a list of

elements from the teletype and convert these atomic symbols to stepper

motor steps necessary to slew from 190.0 nm to the analytical wavelength.

The element symbols can be typed in any order the computer sorts them

according to increasing wavelength. After the elements are specified,

the computer .asks if a background scan should be performed and if

desired the computer will store the background intensities at each

analytical wavelength while a blank solution is being aspirated. This

background will be subtracted during subsequent cycles. At each spectral

interval, the computer gathers the DATA, BACKGROUND, SUM and DIFFERENCE

for each chopper cycle (see Chapter V) and sums them individually for

255 chopper cycles (*about 5 s ). At that time, each of these readings

is printed out, with the atomic symbol of the element at that wavelength,

by a high speed printer. If a background had been stored, it is

subtracted before printing, and if no background subtraction is made,

a symbol is printed denoting that the reading is uncorrected. The

monochromator is slewed to the next spectral interval, and the process

is continued. Alternatively, a command will allow the computer to wait

at the wavelength and take more readings, continuing only on command

from the keyboard.

A separate routine is used in conjunction with a mercury penlight

for al Ignment of the monochromator/stepper/computer system. The penlight








is placed in the monochromator optical path and the slits are closed to

5 jim. On command, the computer slews the wavelength to within 5 nm of

the 253.7 nm line of mercury. It then scans rapidly until the intensity

reading reaches a peak and starts to fall off. The computer then

considers this point to be one step beyond the mercury line and proceeds

backwards until it has set the monochromator at exactly 190.0 nm. This

procedure takes only a few minutes and allows excellent alignment of

the system.



Analytical Procedure with the PSS System

A solution of 50 ug ml"1 each (and listed in Table 10) of the

elements analyzed by the SCSS was prepared from 1000 ug ml" stock

solutions. Silver was not included because AgCI precipitate was formed

by the reaction of Ag+ with the chloride from HCI used in preparing

stock solutions of some elements. Serial dilutions were prepared from

the mixture until the least concentrated solution contained 0.01 jg ml"1

of each element.

The programmed slew spectrometer (PSS) was initialized, the

source started and allowed to warm-up, the electronics energized and

the computer program was loaded and started. Before the analysis, an

alignment procedure, described in the previous section, was performed

with a mercury penlight. Then, the 13 elements were entered on the

keyboard and the analysis was begun.

A background run was stored while aspirating deionized water.

Then each of the standard solutions was aspirated, and the system slewed

the spectrometer, measured the intensities and printed the results for

each element of interest. Ten complete replicates of the above

procedure were done for the entire range of standards. The analysis














TABLE 10

LIMITS OF DETECTION (LOD'S) WITH THE PSS


WAVELENGTH


213.7

228.8

232.0

240.7

248.3

267.6

279.8

283.3

284.0

285.2

324.7

357.9

377.6


LOD(this work)a


0.05

0.01

0.2

0.06

0.08

2

0.003

0.06

0.1

0.0005

0.002

0.006

0.01


LOD(ref. 53)a


7

0.08

10

1

5



0.2

20

5

0.04

0.2


atmits of detection are measured in pg ml-1


ELEMENT







time for each of the solutions was 86 s. The wavelengths used for each
o
of the elements of interest are listed in Table 10.



Results and Discussion

The printed results were analyzed and growth curves from the

averages of results are depicted in Figures 29 and 30. As expected,

the results are very similar to the results obtained for the SCSS with

some additional extension towards lower limits. The limits of detection

for the 13 elements are listed in Table 10.

The analysis time of 86 s represents a factor of 2 better than

If the SCSS were used to cover the wavelength range covered in the PSS.

(213.7 nm to 377.6 could be covered in 164 s at 1 nm s-1 with the SCSS.)

When the increase of sensitivity of about 7 is also considered, it can

be seen that the PSS is more than an order of magnitude better as far

as speed-sensitivity is concerned. If an intensity vs. wavelength

spectrum is not necessary then the PSS is the system of choice for

multielement AFS.

Normal precautions must be made with regard to spectral overlap

and precise background corrections. For example, the large signal from

Mg fluorescence at 285.2 nm affects the intensity observed for Sn at

284.0 nm and Pb at 283.3 nm. This is the probable cause of the non-unity

slope of the analytical curve observed in Figure 29 for Pb and in

Figure 30 for Sn. Alternative spectral lines should be considered if

this condition is evident in the real analytical situation. Also, it

may be more appropriate to sample an off-wavelength spectral interval

intensity rather than store a blank for the background correction.

These functions can be implemented with suitable software but have not

been considered in this study. The results for Au at 267.6 nm are







87








45

4J
3

N














-o





C C
EO



LS

b. ,,=
r, ,o
06


4 0



eO








I l I I a.

um
45



















































































P3


(sjuno3) 7VN91S 33N33S3fOl7nd


(-


.-W






89

somewhat poorer than the results obtained with the SCSS (2 ug ml vs.

1 ug ml-1 respectively). The mixture of elements, prepared for the

PSS, had some finely, divided metallic precipate after standing for a

-period and analysis of this precipate showed it to be Au. This type

of interference must be carefully avoided and a dedicated study of

multielement standard solution preparation is certainly needed.

The tabulated results, printed in digital form, are free from

operator bias and require no judgment as to baseline or peak values.

With further software refinements, it would be possible to have the

computer determine the analytical curves for each element of interest

and then calculate the concentrations of the elements in unknown

samples and present the results as a finished report. The assembly

language programming of this level of software sophistication is a

,matter best left to computer programmers.













CHAPTER VIII

COMPARISON OF THE THREE SYSTEMS



Three spectrometric systems have been constructed and evaluated

for the purpose of achieving rapid analysis of multielement samples.

In each case, the best equipment available was used for the construction.

This procedure resulted in the best possible devices, but unfortunately,

disallowed an accurate comparison of the techniques on their own merits.

Nevertheless, the experimental results lead to several conclusions.

First, Hadamard transform spectrometry in the UV-visible region is

very sensitive to the high backgrounds found in flame spectrometry and

to the time-varying fluctuations in the background. The single channel

scanning spectrometer and programmed slew spectrometer systems are not

as sensitive to this type of signal fluctuation, because only one channel

and one Instant in:time is used in the latter systems. Second, if HTS

systems are to function with the theoretical advantage in thb f"-visible

region, then every effort should be made to keep the background as low

as possible, and also the dynamic range and response of the detector

should be as large and as fast as is possible* The photon counting

system used in the SCSS would probably make an ideal detector for a

future HTS system. Finally, and most important, is the consideration of

the necessity of a wavelength scan. If a scan is not necessary, then a


90








system of slewing, stopping and measuring is more appropriate and

versatile than the SCSS for multielement atomic analysis. This approach

is only an extension of the original procedure of sequential analysis;

but with the same type of equipment as used in the scanning methods, it

is possible to devise a system which will determine elemental concentrations

at any desired wavelength and in less time than would be spent scanning

over informationless spectral regions. The limits of detection found

for the three systems studied in this work are tabulated in Table 11.

It is readily apparent that the PSS is faster and more sensitive than

either the HTS or the SCSS. The S/N relationships can even be programmed

so that the measurement phase of the cycle is a time-dependent function

of the counting rate. This would allow for more time to be spent

measuring low level signals while less time could be spent where the

signal level is high. Because of the versatility in wavelength

selection and S/N relationships, this slewing system is undoubtedly

Vhe most useful multielement measurement system available at the

present time.














TABLE 11

LIMITS OF DETECTIONa (LOD'S)
FOR.THE THREE SYSTEMS STUDIED


COMPARED
IN THIS WORK


LOD-HTS

-
jjug mil






40

25

25


LOD-SCSS
jig ml-1


0.3

0.1

0.6

0.2

0.4

1

0.01

0.2

3

0.002

0.02

0.02

0.04


LOD-PSSb
ug mi-1


0.05

0.01

0.2

0.06

0.08

2

0.003

0.06

0.1

0.0005

0.002



0.006

0.06


ELEMENT


Zn

Cd

NI

Co

Fe

Au

Mn

Pb

Sn

Mg

Cu

Ag

Cr

Ti



aNote:


b


The limit of detection is defined as the concentration of
analyte which gives a signal twice the rms noise observed
when a blank is measured.

The PSS is two times faster than the HTS or the SCSS.













APPENDIX I

AVERAGE SIGNAL-TO-NOISE RATIOS
FOR THE UV-VISIBLE SPECTRAL REGION (39).



The Fellgett advantage (F is the gain in signal-to-noise (S/N)

ratio for equal analysis time) applies over the entire spectral region

of interest when a multiplex method is compared with a single channel

(SC) method in the IR spectral region. Consider the specific case of

4Hadamard transform spectrometry (HTS) measuring N spectral intervals

in T s analysis time. Each of the spectral interval intensities is

measured for one-half the total analysis time,e.g., T/2 s. A scanning

system measures each spectral interval for T/N s. Therefore, the

increase in measurement time for the HTS case is (T/2)/(T/N) = N/2.

For a signal with only random errors (as compared with systematic errors)

.an increase in measuring time is the same as an increase in the number

of independent measuring samples. The accuracy of such independent

-4mesurements is increased by the square root of the ratio of the

measurement times for the HTS and SC systems e.g., (N/2).

In-the IR region,the increase in precision is due to the fact

that the major source of noise is in the detector and is completely

independent of the signal level. At each spectral interval, there is

n-average gain in S/N of (N/2)2 in the HTS compared to the SC. In the

t~-visible region, low noise photomultipliers are used, and the major