Title: Vitreous carbon tube furnace for atomic fluorescence spectrometry, by Charles J. Molnar
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Title: Vitreous carbon tube furnace for atomic fluorescence spectrometry, by Charles J. Molnar
Physical Description: xiii, 129 leaves. : illus. ; 28 cm.
Language: English
Creator: Molnar, Charles John, 1948-
Publication Date: 1974
Copyright Date: 1974
 Subjects
Subject: Fluorescence spectroscopy   ( lcsh )
Chemistry thesis Ph. D
Dissertations, Academic -- Chemistry -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
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Thesis: Thesis -- University of Florida.
Bibliography: Bibliography: leaves 122-128.
General Note: Typescript.
General Note: Vita.
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Bibliographic ID: UF00098343
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000580810
oclc - 14087769
notis - ADA8915

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VITREOUS CARBON TUBE FURNACE FOR ATOMIC
FLUORESCENCE SPECTROMETRY










By

CHARLES J. MOLNAR


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











UNIVERSITY OF FLORIDA

1974













DEDICATION


This dissertation is dedicated to all my friends,

and especially to Jude, the best friend I have ever known.














ACKNOWLEDGMENTS


The author is especially grateful to his research

director, Dr. James D. Winefordner, for his advice and

encouragement in the course of this research. The

scholarly assistance of the members of his committee is

also greatly appreciated.

Dr. James D. Winefordner's postdoctoral fellows and

graduate students will always be remembered with gratitude

for their constructive suggestions and invaluable help.

The author also wishes to acknowledge the technical

advice and assistance of Francis D. Ottinger and Arthur P.

Grant, as well as the other staff members of the Machine

and Instrument Shop.

An expression of thanks is due Gail G. Pokrant for

all her help with this and other projects.














TABLE OF CONTENTS


ACKNOWLEDGMENTS . .

LIST OF TABLES . .

LIST OF FIGURES . .

ABSTRACT . . .

Chapter


I. INTRODUCTION . . . . . . . . .

II. THEORETICAL CONSIDERATIONS . . . . .

Types of Atomic Fluorescence Transitions.

Intensity of Atomic Fluorescence . . .

Fluorescence Quenching Considerations . .

Noise Considerations . . . . . .

Atom Generation and Signal Measurement. .

Concentration of Analyte in the Atom Cell .

Limit of Detection Considerations . . .

III. EXPERIMENTAL . . . . . . . . .


Reagents . . .

General Layout of the

Sources of Excitation

Optical System . .

Electronics . . .

Nebulizers . . .


. . . . . .

Experimental System .

. . . . . .

. . . . . .

. . . . . .

. . . . . .


PAGE

iii

vii

viii

xi


1

9

9

9

19

20

21

24

28

31

31

31

31

38

40

40










Chapter Page

Desolvation Chamber . . . . . . 45

Vitreous Carbon Tube Furnace. . . . 48

Nebulizer, Desolvation Chamber, and
Vitreous Carbon Tube Furnace Mount .. . 53

Procedure for Evaluation of Experimental
Characteristics of the System. . . 53

Gas flow rates ........... 53
Sample solution flow rates ...... 54
Mean particle diameter . . . .. 54
Desolvation time . . . . ... 56
Scattered radiation. . . . .. 59
Rise velocities. .......... . 59
Efficiency of sample transport ..... 61
Temperature of furnace and gaseous vapor 61

General Procedure for Making Atomic
Fluorescence Measurements. . . . 64

IV. RESULTS AND DISCUSSION . . . . ... 66

Atomic Fluorescence Signal-to-Noise
Optimization . . . . . ... 66

Definitions of Analytical Parameters. . 69

Continuous-Sampling Mode Results. ... . 70

Limits of detection . . . . . 70
Linear dynamic range and sensitivity . 73
Correction for scattering. . . . 80
Precision. . . . . . . 82
Atom population profiles . . . .. 82

Pulsed-Sampling Mode Results. . . .. 86

Limits of detection. . . . . ... 86
Size of sample . . . . . . 87
Linear dynamic range and sensitivity . 92
Precision. . . . . . . . 92












Chapter Page

V. SUMMARY AND FUTURE WORK . . . . ... 96

Summary. . . . .... . * * 96

Future Work. ........... .... . 98

Desolvation chamber considerations. . 98
Atomic emission considerations. .... .. 103
Atomic absorption considerations 107
Stop-flow considerations . . 108
Conventional resistively heated
non-flame cell . . . . . .108

APPENDICES

I. DEFINITION OF SYMBOLS .......... 111

II. ROTAMETER CALIBRATION . . . ..... . 117

LITERATURE CITED . . . . ..... 122

BIOGRAPHICAL SKETCH. ......... . . . 129













LIST OF TABLES


Table Page

1. Stability of Aerosol Production with High
Pressure Nebulizer . . . . . .... 55

2. Comparison of Residency Times in Atom Cells. ... 60

5. Experimental Conditions and Analytical Results
for Measurement of Several Elements by Means of
the Continuous-Sample Introduction Tube Furnace
Atomic Fluorescence Spectrometric System . . 71

4. Comparison of Non-Flame Atomization Atomic
Fluorescence Spectrometric Limits of Detection 72

5. Experimental Conditions and Analytical Results
for Measurement of Several Elements by Means of
the Pulsed-Sample Introduction Tube Furnace
Atomic Fluorescence Spectrometric System . . 88

6. Comparison of Flame and Non-Flame Atomization
Atomic Absorption and Fluorescence Limits of
Detection and Plasma Torch Limits of Detection 90













LIST OF FIGURES


Figure Page

1. Physical process leading to atomic absorption,
atomic fluorescence, and atomic emission
spectrometry . . . . . . . .. 3

2. Types of atomic fluorescence . . . ... 10

3. Schematic diagram of the atom cell . . . 13

4. Atomic fluorescence growth curve with a line
source of excitation . . . . . ... 17

5. Atomic fluorescence growth curve with a
continuum source of excitation. . . . ... 18

6. Schematic diagram of continuous- and
pulsed-sample introduction tube furnace atomic
fluorescence spectrometric system . . 33

7. Cutaway view of the EDL thermostating unit. . 36

8. Schematic diagram of nebulizer I. . . ... 43

9. Schematic diagram of nebulizer II . . ... 47

10. Schematic diagram of the stainless steel
desolvation chamber . . . . . . .. 49

11. Schematic diagram of vitreous carbon tube
furnace . . . . . . . . . . 51

12. Gas temperatures in the desolvation chamber at
various heights above the nebulizer . . .. .58

13. Temperatures of non-flame cell at various
points. . . . . . . . . . .. 63

14. Atomic fluorescence analytical curve for Te in
the continuous mode of operation. . . . 74

15. Atomic fluorescence analytical curve for Sn in
the continuous mode of operation. . . . 75

viii











Figure Page

16. Atomic fluorescence analytical curve for Ag in
the continuous mode of operation . . ... 76

17. Atomic fluorescence analytical curve for Bi in
the continuous mode of operation . . ... 77

18. Atomic fluorescence analytical curve for Pb in
the continuous mode of operation . . ... 78

19. Atomic fluorescence analytical curve for Tl in
the continuous mode of operation . . .. 79

20. Decay of atom populations with height above the
vitreous carbon tube atomizer for two elements
as measured by atomic fluorescence spectrometry. 84

21. Fluorescence signal versus sample size for Bi,
Sn, and Co with pulsed mode of operation . . 91

22. Atomic fluorescence analytical curve for Sn in
the pulsed mode of operation . . . . . 93

23. Atomic fluorescence analytical curve for Co in
the pulsed mode of operation . . . ... 94

24. Atomic fluorescence analytical curve for Bi in
the pulsed mode of operation . . . ... 95

25. Fluorescence signal versus time after
introduction of three pulsed samples of 10 ul
of 30 pg ml-1 Sn at 303.4 nm . . . . . 99

26. Schematic diagram of a modified stainless steel
desolvation chamber. . . . . . . ... 101

27. 2.3 Log concentration in the atom cell versus
time after introduction of the pulsed sample
for two volumes of exponential dilution flasks
with identical initial amounts of analyte. .. 102

28. Emission signal versus gas temperature at
grazing incidence to the vitreous carbon tube. 105

29. Emission signal versus external H2 flow rate . 106












Figure


30. Ar nebulizer flow rate versus rotameter
reading . . . . . . . . .

31. External Ar flow rate versus rotameter
reading at 60 pounds. . . . . .

32. External H2 flow rate versus rotameter
reading at 12 pounds. . . . . .

33. Internal CH4 flow rate versus rotameter
reading at 8 pounds . . . . . .


Page


. . 118


. . 119


. . 120


. 121














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


VITREOUS CARBON TUBE FURNACE FOR ATOMIC
FLUORESCENCE SPECTROMETRY

By

Charles J. Molnar


Chairman: James D. Winefordner
Major Department: Chemistry

Resistively heated non-flame cells previously used

for atomic absorption and atomic fluorescence spectroscopy

are reviewed. A versatile new type of resistively heated

non-flame cell usable in both the continuous introduction

and the pulsed introduction modes is described, and

analytical figures of merit in atomic fluorescence

spectrometry are given.

The theory of the conversion of the analyte to an

atomic fluorescence signal is reviewed, along with some

reasons why non-flame cells in atomic fluorescence

spectrometry should attain better signal-to-noise ratios

than flame atomic fluorescence spectroscopy. The theories

of transient and continuous signals are discussed, and

methods to obtain maximum signal-to-noise ratios for each

are described.











The design, construction, and evaluation of the new

versatile resistively heated non-flame cell are discussed.

The non-flame cell system consists of a very efficient

pneumatic nebulizer operating at high pressures with low

sample consumption for introduction of a fine sample aerosol

into a vitreous carbon tube furnace via a desolvation

chamber. Atomic vapor of several metals (Sn, Pb, Te, Ag,

Tl, and Bi) is produced by continuous pneumatic nebulization

of an aqueous sample through a vitreous carbon tube furnace,

and atomic fluorescence is excited with single-element

electrodeless discharge lamps. The high efficiency of the

system is considered along with precision, sensitivity,

linear dynamic range, and limits of detection for the elements

examined. The decay of atom populations above the furnace

outlet using either an Ar-H2 diffusion flame sheath or an Ar

sheath is determined, and the results are discussed.

Pulsed pneumatic nebulization of aqueous samples,

containing various metals (Ag, Bi, Cd, Co, Mg, Pb, Sn, Te,

Tl, and Zn), into a vitreous carbon tube furnace produced

metal vapors which are excited with single-element electrode-

less discharge lamps, and the resulting atomic fluorescence

is measured. This mode of operation improves the absolute

limits of detection; no deleterious effects to the linear

dynamic ranges resulted. The concomitant increase in the

life of the vitreous carbon tube is also stressed. The











atomic fluorescence intensity versus different sample sizes

for a few elements is determined.

Additional advantages of the present non-flame cell

system over some past non-flame cells are that operator

error due to sample placement is eliminated, and that the

conversion of the atomizer from the continuous mode to the

pulsed mode of sampling is achieved by simply altering the

means of sample injection.

Some areas of future research are discussed.

Preliminary results suggest that the present resistively

heated non-flame cell offers atomic emission results

competitive with some flames. A method for the improvement

of the transient signal is discussed. The use of this

system for atomic absorption is discussed, along with a

possible alteration. Some other areas of future research

are discussed.


xiii













CHAPTER I


INTRODUCTION

Atomic Absorption Spectroscopy (AAS), Atomic

Fluorescence Spectroscopy (AFS), and Atomic Emission

Spectroscopy (AES) all depend upon the efficiency of

conversion of the sample into the atomic species of

interest (the analyte) and the ability to differentiate

optically the radiation absorbed or emitted by the analyte

(the analytical signal) from all other optical radiation

(background). In fluorescence, excitation is a result of

absorption of radiation from an external source, whereas

in emission excitation is primarily a result of col-

lisional transfer of thermal energy stored in vibrationally

excited gaseous molecules, for example, N2 in flames.

Flames have been in widespread use as atomization

cells because they are convenient, reliable (barring

flashbacks), and relatively inexpensive. Also, there is a

wide variety of flames and commercially available burners

which result in high signal-to-noise ratios (S/N) for many

elements in the parts per million (ppm) to parts per

billion (ppb) concentration range.











The efficiency of flames in converting analyte in the

sample solution to gaseous atoms is severely limited for

many reasons. To understand the inefficiency (for some

elements) of flames as atomizers, one must first have

examined the processes which lead to flame atomization, as

shown in Figure 1.

With premix burners, the efficiency of transferring

the liquid analyte to the flame as a dry aerosol is about

4-5 per cent [1]. The total consumption burner has a larger

efficiency of transferring the analyte to the flame as dry

aerosol, but has a poorer solute vaporization efficiency [1].

Processes which lead to low atomic concentrations in the

flame atomization cell include analyte dilution due to the

large flow rates of the fuel and oxidant needed to support

flames and gas expansion upon conbustion. Reactions

occurring in the flame lead to the formation of many flame

gas radicals which may either combine with the analyte to

form stable compounds monoxidess of the analyte species

resulting from 0 radicals) or may emit radiation resulting

in appreciable background in spectral regions of interest

(such as OH, CN, etc.). In some of the higher temperature

flames, there occurs significant ionization of some elements,

such as Na, K, Cs, Ca, Sr, etc.

Ideally, atomization devices would have high

efficiencies of sample transport, low background due to









Analyte


I Nebulization

Wet Aerosol

SDesolvation

Dry Aerosol

I Volatilization

Vapor


-4e


atoms


ions


molecules


Fig. l.--Physical process leading to atomic absorption,
atomic fluorescence, and atomic emission
spectrometry.


Liquid











emission, and a gaseous environment which would lead to

efficient conversion of analyte vapor to analyte atoms.

L'vov has been a pioneer in the field of resistively heated

non-flame cell spectroscopy; he has used a graphite tube

furnace contained in an inert atmosphere for extensive AAS

studies [2-6). A complete review of his work is included

in his text [7]. Massmann developed a resistively heated

graphite cuvette atomization cell suitable for AAS and AFS

[8,9]. The AAS system has been developed commercially by

Perkin-Elmer; the Perkin-Elmer non-flame cell AAS system

has been used to analyze many different elements in complex

matrices such as milk [10], sea water [11], oils [12], and

blood plasma [13]. Kahn and Slavin [14] increased the

sensitivity of this system by stop-flowing the Ar sheath;

this system was then further improved by automation of

sample introduction by Pickford and Rossi [15], which im-

proved precision from 5-10 per cent to 1-2 per cent.

Samples have also been atomized from carbon fila-

ments into an inert carrier gas by means of resistive heating.

West and Williams constructed the first graphite filament

atomization device [16] for both AAS and AFS. The filament

was first enclosed within a Pyrex cell fitted with quartz

windows and purged with Ar. Later, the Pyrex cell was re-

moved and a laminar flow of Ar was used to provide an inert

atmosphere about the atomization filament. A number of











elements have been studied with this atomization device [17-

27]. Limited field viewing has led to reduced matrix

effects [21,22]. Applications have been reported for

oil-based solutions [263, Co in soils [28], Mo in organic

matrices [27], and V in titanium dioxide [29]. Winefordner

et al. described a similar atomization device which was

evaluated extensively in AAS and AFS and was used for the

analysis of trace metals in both aqueous and oil-based

samples [50-54]. The latter non-flame cell was operated in

both an Ar and an Ar/H2 sheath. Studies were carried out

on the decay of atomic populations with height above the

graphite filament by AAS [52] and AFS [35]. Temperature

programming of the filament was used to separate temporally

the signals of Ag and Cu in a single cycle [34]; temporal

resolution was applied to the analysis of Ag and Cu in jet

engine oils.

Amos [35,36] modified the West filament system by

drilling a lateral hole through the filament and creating

the "Mini Massmann" furnace; he compared the results of AFS

to AAS for several elements. Amos found AFS with the

graphite filament to result in better limits of detection

for many elements than did AAS with the .tube non-flame cell;

an Ar/H2 diffusion flame was used to shield the atomized

elements. The "Mini Massmann" furnace has become the com-

mercial Varian non-flame atomization device on which











application studies for elements in oils [37], Se in natural

waters [38], and Ag and Au in metallurgical samples [39]

have been reported. Bratzel and Chakrabarti [37] measured

the temperature of the atomic vapor produced in the Varian type

of non-flame cell and found that thermal equilibrium did

not exist, which is not surprising in a system containing so few

molecules to aid the attainment of equilibrium.

Brandenburger and Bader were the pioneers in using

resistively heated metal filaments as atomization devices

[40]. Bratzel, Dagnall, and Winefordner employed a

resistively heated Pt filamert for use as an atomization source

for AFS with excellent results for some volatile elements [41].

Hwang et al. [42] used a resistively heated tantalum strip in

an enclosed inert atmosphere to analyze Pb in blood serum.

They also reported the determination of thirty-seven elements by

AAS [43].

Few resistively heated non-flame cells with continuous-

sample introduction have been reported. Woodriff et al.

[44 46] employed either continuous pneumatic nebulization

of the cold aerosol into a resistively heated graphite tube

furnace or discrete sampling via sample placement with a

microliter syringe into a resistively heated graphite tube.

Applications using the discrete sampling mode were performed

on Ag in snow [47], Pb in air [48], and Pb in fish [49].

M. S. Black et al. [50] evaluated two types of platinum tube











furnaces with continuous pneumatic nebulization for AFS; no

applications of this system to real samples were described.

M. K. Murphy, S. A. Clyburn, and C. Veillon developed a

unique pyrolytic carbon tube furnace for use in AFS [51];

this system resulted in sub-parts per billion (ppb) limits

of detection for some elements. The latter two non-flame

cells utilized a Veillon-Margoshes sample aerosol injection

system [52].

Non-flame cells other than the resistively heated

non-flame cell types have been employed in various types of

atomic spectroscopy, but they will not be discussed here.

The interested reader may consult the excellent reviews of

non-flame cells by Winefordner [53], Kirkbright C54], and

Winefordner and Vickers [55] for a thorough discussion of

these, as well as the previously mentioned resistively

heated non-flame cell studies.

The purpose of this research project was to develop

an efficient, versatile non-flame atomization cell for use

in atomic fluorescence spectroscopy. This entailed the

development of a new high pressure, low sample consumption

pneumatic nebulizer which was coupled to a vitreous carbon

tube non-flame cell via a desolvation chamber into which

the analyte was introduced either in the continuous or

pulsed modes. The resulting system was evaluated as an

atomizer for atomic fluorescence spectrometry. Analytical









8

figures of merit are given for several elements, and the

possible use of the atomizer for emission spectroscopy is

also discussed.













CHAPTER II


THEORETICAL CONSIDERATIONS


Types of Atomic Fluorescence Transitions

Resonance fluorescence transitions involve identical

lower and upper levels in the excitation-de-excitation

processes. Direct line fluorescence involves only the same

upper level in the radiational excitation-de-excitation

processes. Stepwise line fluorescence involves different

upper levels in the radiational excitation-de-excitation

processes. If the fluorescence energy exceeds the excita-

tion energy, the fluorescence is termed an Anti-Stokes

process, and if the reverse is true, the fluorescence

transition is then called a Stokes process. A few types

of atomic fluorescence have been shown in Figure 2. For

a more extensive discussion of the types of fluorescence,

and where they have been observed, one should consult the

article by Omenetto and Winefordner [56].


Intensity of Atomic Fluorescence

The intensity of atomic fluorescence emitted from the

analyte depends principally upon: the intensity of the

exciting radiation; the concentration of the analyte in the

9






(2)


A-


if'


2


--- I


--


(3)


2

I


0


(4)


Fig. 2.--Types of atomic fluorescence.
(1) Resonance fluorescence.
(2) Anti-Stokes direct line fluorescence.
(3) Stokes direct line fluorescence.
(4) Stokes stepwise line fluorescence.










atom cell; the efficiency of the conversion of absorbed

radiation into the emitted radiation (quantum efficiency);
and the ratio of the excitation source half-width to the

absorption line half-width. If it is assumed that the
source excitation is imaged on the atomic vapor exiting
from the atomizer (atom cell), that the atom cell completely

collects all source excitation, and that the solid angle of
the monochromator is filled by the fluorescence of the atom
cell, then the most general equation for the integrated
fluorescence radiance (intensity) BF, for an isolated
spectral line is
RAY' Ll'
BF = (-) II-1

-1 -2 -1
where BF is the fluorescence radiance (erg sec cm sr-1),

L is the fluorescence path length (cm), 1' is the atom cell
height (cm), As is the total atom cell surface area (cm ),
EA is the total irradiance per unit area absorbed by the
spectral line which results in fluorescence (erg sec-1cm2),
and Y' is the quantum efficiency (watts emitted by fluores-
cence to the watts of the source power absorbed per unit

area).
The irradiance absorbed by the analyte in an atom

cell is given by

EA = J i BSi i II-2
= B~1 K2i











where J. is the solid angle over which the excitation

occurs, ,i is the summation over all i of (Bi Ki), BS is

the incident spectral radiance of the exciting radiation
-1 -2 -1
(erg sec cm sr ), and Ti is the total absorption factor
i
(sec-1) for different absorbing lines i. If, as in this

study, only resonance fluorescence is of interest, then

Equation II-2 may be simplified to


EA = LA BS KT


II-3


where BS and KT are values evaluated at the resonance

frequency.

Winefordner et al. [57-59] have rigorously derived

the expressions for the intensity of atomic fluorescence

when using either a line or continuum source at low and

high optical densities in the atom cell. Figure 3 shows

schematically the atom cell, the source intensity direction,

and the direction in which fluorescence is measured.

Assuming the following conditions for fluorescence from an

atom cell:

1. The sample cell is 'completely illuminated.

2. The fluorescence radiation is collimated and at
right angles to the source radiation.

3. The atomic concentration is homogeneous throughout
the sample cell.

4. The temperature is homogeneous throughout the sample
cell.

5. Only resonance fluorescence contributes to the
fluorescence radiance.









I













--f-- '---)





Atomic
Vapor


Fig. 3.--Schematic diagram of the atom cell.
--4 = source radiation.
+ = fluorescence radiation.
1,1',L = for definitions, see text.










the intensity of fluorescence may be calculated. If a line

excitation source is used, the following equation holds for

the atomic fluorescence radiance, BF (erg sec1 cm-2sr-1) at

low analyte concentrations (no) [57,58] where the total

absorption factor, KT, has been evaluated

= f BY ( LI') II-(
BF = lu nolflu lu s (i- A

where
Klu = modified atomic absorption coefficient for pure

Doppler broadening (cm 2)
no = concentration of the atoms in the ground state

(cm-5)
X1 = fraction of analyte atoms in the lower state

involved in the absorption transition

dimensionlesss)

flu = absorption oscillator strength for transition
1 u, where 1 is the lower state and u is the

upper state dimensionlesss)

lu = factor to account for finite half-width of the
line source compared to the absorption line

dimensionlesss)

S = solid angle of source radiation collected and
impinging upon the flame (sr)
1 = absorption path length (cm).








15


For high optical densities, the following equation is

derived for excitation with a line source [57,58]


BF = 2 BS 1' Y' (--) a
X KF n XFfF S

II-5
where

aF = damping constant for atomic fluorescence

dimensionlesss)
KF = modified absorption coefficient (same as Klu),

but for reabsorption of fluorescence (cm2 )

XF = fraction of atoms in the lower state involved in
the reabsorption of fluorescence dimensionlesss)
f = absorption oscillator strength for reabsorption

of fluorescence dimensionlesss).
If a continuum source of radiation is used to excite

resonance fluorescence, then the atomic fluorescence
radiance, BF, for low analyte concentrations, no, is equal

to [57,58]

JT L11'
BF = C1 lu no X1 lu AD BC (7T
lu 4 A
II-6
where

C1 = X (2 In J,) dimensionlesss)
AND = Doppler half-width of the absorption line (nm)











=lu = absorption line peak for transition 1 -- u (cm)
B k = spectral radiance for a continuum source (erg
lu-1 -2 -1 -1
sec- cm2 sr-l m- ).

At high analyte concentrations, no, the atomic fluorescence

radiance, BF, is equal to [57,58]


B = 2 BcklkD aF 1' Y' (1- )(C ) (l ) II-7

where
-1
C2 = /T- (ln 2) dimensionlesss).

The experimental analytical growth curves for atomic

fluorescence with a line excitation source and a continuum

excitation source have been measured and the low and high

concentration asymptotic regions have been compared with

the above equations by Winefordner et al. [60,61].

Theoretical growth curves are shown in Figures 4 and 5.

It should be noted from these growth curves that atomic

fluorescence spectroscopy is most useful analytically for

low concentrations of analyte, no, in the atom cell because

the sensitivity (slope of the analytical curve) is greatest

in that region, approaching an asymptote of 1 for both

continuum and line source excitation of atomic fluorescence.

For high analyte concentrations, the fluorescence radiance

is independent of analyte concentration for continuum source

excitation and depends upon for a line source

excitation.














Large no


. ~slope =1/2


slope = I


Fig. 4.--Atomic fluorescence growth curve with a line
source of excitation.


I. 10 102
LOG (Concentration of the Analyte)


small no













small no


slope=l


// Large no
/ -


slope = 0


10 102
LOG (Concentration of the Analyte)


Fig. 5.--Atomic fluorescence growth curve with a
continuum source of excitation.











Fluorescence Quenching Considerations

The species in a resistively heated non-flame cell

should have a much lower quenching efficiency for the

excited-state atoms than does a combustion flame, mainly

due to the difference in the gaseous environments (that is,

an inert atomic gas in the non-flame cell (Ar, He, etc.)

versus molecular species in the flame (N2, CO, CO2, etc.)

[53]. This should lead to more intense fluorescence from
a resistively heated non-flame cell than from a flame
(assuming equal atom concentrations and source

characteristics to be the case) by a factor of

Y'
Q = -rY 11-8

where the Y'F is the quantum efficiency of fluorescence
n*F
of the analyte in the non-flame cell and Y'F is the quantum

efficiency of fluorescence of the analyte in the flame.
This ratio has been expressed [53] as

KF + fi (KQi n )n- F
Q = i (KQ 11-9

where kF is the first-order rate constant for radiational
deactivation of the resonance level (sec-1), kQ is the
second-order quenching rate constant for deactivation of
the resonance level by collisions of element Z with a

quencher Qi (cm-3sec-l), and hQi is the concentration of the










quencher (cm-). The concentration of the quenching species
is at a minimum in the inert atmospheres of non-flame cells,
and thus

kF > i (ki ni)n-F II-10

This leads to [53]

kF
k + i (ki n F II-

For many common analytical flames [62-64], and for a

typical inert atmosphere non-flame cell, the ratio Q is
given by

kf
S= 11-12
ki ki n

which leads to the conclusion that the analyte should emit

fluorescence approximately ten times more intensely in the
inert atmosphere of a non-flame cell, compared to a flame
cell (keeping in mind the assumption of the same atom

concentrations in both cells and equivalent sources of
excitation).

Noise Considerations

The major sources of noise in atomic fluorescence
flame spectroscopy are flame flicker noise and phototube
photonoise. With higher temperature, fuel-rich flames,
flame flicker noise may be several orders of magnitude











larger than shot noise, hence increasing the lowest concen-

trations which may be observed by atomic fluorescence flame

spectroscopy. The total noise, Aitot, for various

individual sources of random noise Ail, Ai2...., is


Aitot Ai2 + Ai22 + ... II-13

When using a properly baffled resistively heated

non-flame cell which prevents the black-body radiation

of the hot carbon from reaching the photomultiplier, the

atomic fluorescence measurement is limited by shot noise,

thus providing another advantage of resistively heated

non-flames as atomization devices over flames.


Atom Generation and Signal Measurement

Once the physical processes which constitute atomic

fluorescence and the parameters which govern the intensity

of fluorescence have been understood, one must decide how

atomic population is to be produced and how the analytical

signal should be measured. This was covered in great depth

for transient atom populations by L'vov [65]. If No is the

number of atoms of the analyte consumed, N is the total

number of atoms of the analyte in the atom cell at time, t,

7 is the duration of transfer of atoms into the cell, 2

is the average residency time of analyte in the cell, and

is the time the signal is recorded for, then










dN
S= nl(t) n2(t) II-14
where nl(t) is the number of atoms entering the cell at
time, t, and n2(t) is the number of atoms escaping from
the cell at time, t. For transient sample introduction
nl(t) = No/T1 II-15

and
n2(t) = (V) N 11-16

n(t) = II-17
r2(t)

where W/V was the ratio of the flow rate of the aspiration
gas (1 sec-1) to the volume of the atomization cell (1).
Thus, the equation relating the change of atomic
populations to change in time is

dN No N 1
S71- II-18

It is recognized that for1 7 >> 7 the magnitude of N(t)
approaches a constant value or that [65]
dN
S= 0 11-19
From Equation 11-19, it followed from Equation II-18 that

Ne = ()N II1-20

It has been widely recognized that electronic
circuits may distort the original pulse shape. From the
theory of electronic circuits [65,66], the relationship











between the distorted peak shape i(t) to the original peak

shape, N(t), and the transient characteristics of the

network A(t) is given by


i(t) = N(O) x A(t) + J A(t-7)N'(rT-7) II-21
0

where N'(t) is an integration variable, N'(t) is a deriva-

tive function of N(t) where t has been replaced by the

variable, 7", and N(O) is the value of the function N(t)

at time t=0. The time constant, 7', is defined for a

simple RC circuit by

7 = RC II-22

c
where 7c is the length of time during which the output

signal is within 36.8 (that is, 100 x e-1) per cent of the

input signal and R is the resistance (ohms), and C is the

capacitance (Farads). The transient characteristic of the

circuit passing from unity voltage-to-0 is

A(t) = exp (- 1-) 11-23

and the 0-to-unity voltage transient is

At = 1 exp (- ) II-23a
'C

To obtain the highest value of signal-to-noise,
L'vov [65] experimentally showed that the time constant of

the measurement circuit, 7-, should be approximately equal

to the value of 72, the average residency time of the











analyte in the cell for a transient signal. He also

stressed that the analyst may increase precision of his

analytical measurements by using integration techniques

for signal measurement rather than just peak heights.

For continuous-sample introduction, the magnitude of

N(t) approaches a constant value, or

) = 0 II-24

Thus, when measuring the equilibrium signal from a con-

tinuous-sampling atomization cell, the time constant, 7 ,
C
of the recording device may be adjusted to a larger value

than when measuring a transient signal; the larger 7c

reduces the measured noise through time averaging and also

increases the analysis time, and hence the amount of

analyte consumed. At least five time constants should

elapse before performing any analytical signal measure-

ments, to allow the signal to reach greater than 95 per

cent of its maximum value.


Concentration of Analyte in the Atom Cell

For the pulsed mode of sample introduction, it will

be assumed that the analyte is exponentially diluted.

Therefore, the concentration of analyte atoms in the atom

cell, nt, at any time, t, is [67]


nt = no e-( t/)


II-25










where

t = time (sec)

V = volume of the exponential dilution flask (cm3)

u = flow rate of aspirating gas (cm sec- )

nt= concentration of analyte at time, t, in the atom
cell (cm- )

no= initial concentration at time t = 0 of analyte

in the atom cell (cm-3)

The initial concentration, no, in the atom cell can

be estimated if it is assumed that the entire pulse of

analyte atoms enters the exponential dilution flask and

reaches a homogeneous concentration before any analyte exits

and so [53J

v c NA T -26
n = e- II-26
o VeF

where

v = volume of sample solution pulse (cm )
V = volume of exponential dilution flask (cm3)

c = concentration of analyte in sample solution prior

to aspiration (mol cm 3)

NA = Avogadro's number (mol -1)
6 = efficiency of aspiration dimensionlesss)

p = free atom fraction dimensionlesss)
eF = gas expansion factor dimensionlesss)

ZT = ratio of atoms in ground state to total number in
all states dimensionlesss)








26

For the continuous-sampling flame (or non-flame cell)

system, the concentration, n, of the analyte atoms in the

atom cell (atoms cm -) is related to the concentration, c,

of the analyte (mol 1-1) aspirated into the flame by [53]

n = 1 x O19 F E )Z II-27
QeFnT1x1-2F
where

F = sample flow rate (cm min-1)

E = same as defined above

9 = same as defined above

eF = same as defined above

Q = flow rate of the gas entering atom cell (cm3 sec-1

ZT = the ratio of atoms in the ground state to the total

number of atoms in the atom cell dimensionlesss).

The aspiration efficiency, E, is the ratio of the

number of atoms of analyte entering the nebulizer to the

total number of gaseous analyte atoms contained in the

atom cell. This factor corrects for losses of analyte

solution in the nebulizer and associated tubing, and the

incomplete volatilization of the solid particles in the

atom cell.

The free atom fraction,P, is the ratio of the number

of analyte atoms in any form to analyte atoms in all gaseous

forms in the atom cell. This factor corrects for incomplete











dissociation of the analyte compound, combination of analyte

atoms with flame gas radicals, condensation of the analyte

atoms to form the original compounds, and ionization of

the analyte atoms.

The factor ZT accounts for the proportion of free

atoms contained in the atom cell which are not in their

electronic ground state and may be evaluated with the

Boltzmann equation [68]
n, g,-E
n g e s/kT II-28
= e 11-28
no go

where

n = number of atoms in the ground state dimensionlesss)

ns = number of the atoms in a level with an excitation

energy of Es dimensionlesss)

gs ,g = weighting factors dimensionlesss)
k = Boltzmann's constant (8.614 x 105eV K-)

T = atom cell temperature (OK)

Es = excitation energy of levels (eV)

The total atom concentration will be, nT,

nT = no + n1 + n2 + ... II-29

and so from Equation II-29 and the Boltzmann equation

(Equation II-28)










no [ -E/kT -2/kT
n = io + e + g2e + .'"
II-30
n
= o (B) 11-31
go

where

B = g leE/kT + g2e2/kT + ... II-32

and finally ZT is
g0
ZT = 11-33

Limit of Detection Considerations

A very useful analytical figure of merit for use in
trace element analysis is the limit of detection (LOD),

which is defined statistically so that comparisons may be

carried out between different methods of analysis. The

statistical theory used here was developed by Kaiser [69-71].

The limit of detection, C, is defined as

C = mS II-34
where m is the slope of the analytical curve m =

d(log S) (S is the signal resulting from a concentration

of analyte, C) and is assumed to be constant from the LOD

to 1000 x LOD. The smallest analyte signal measurable

(S) is

S= Ss+b 11-35

where Ss+b is the average limiting signal due to the analyte

plus blank and S, is the average blank signal. Generally,











S is further defined in terms of the fluctuations of the

signal. The standard deviation of these fluctuations is

defined by

s =j b2 +b2 II-36
nb ns

where b is the standard deviation of the blank signal,

s~+b is the standard deviation of the signal plus blank
signal, nb is the number of measurements performed on the

blank signal, and ns is the number of measurements per-

formed on the analyte signal (plus blank). At the limit

of detection, the 6" will approximate i+b. The limiting
0 S+b0
detectable signal is then

s = Z -1im II-37

where Z is the z-statistics of the normal distribution,

that is, a protection factor. If the number of observa-

tions are large (about 20), then Z may be replaced by the

Student t and 6 by s which is the estimated standard
s s
deviation of the signal near the limit of detection. If

ns = 1 and nb 15, then Equation II-56 may be reduced to

C = m tsb 11-38

where sb is the estimated standard deviation of the blank

and if sb =ss. However, if ns = nb, then Equation II-36

may be reduced to












C= mt sb V2 11-39
b
If ns = nb = 1, then Equation II-39 reduces to

C =2 mt sb II-40

In the majority of previous studies [72], the limit

of detection is defined as that concentration or amount

which will give a signal-to-noise ratio of 2 or 3. If the

only major random errors are electrical and optical noises,

then the rms noise is defined as approximately peak-to-peak

noise divided by 5 [73]; thus, the limit of detection, C,

may be evaluated from Equation II-39, where nb = 5 and

t = 3, which gives a confidence level of better than 95
per cent.













CHAPTER III


EXPERIMENTAL


Reagents

Stock solutions were prepared from analytical

reagent grade chemicals of Pb(NO3)2, Na2Te0O42H20, AgNO3,

T12SO4, and K SbOC4HO06.- H20 dissolved in deionized water

to give concentrations of 103 jgml-1. Analytical reagent

grade chemicals of SnO, MgO, and ZnO were dissolved in HC1

and Zn metal, Co metal, and CdO were dissolved in HNO3 and

then diluted to 103 ugm11. With concentrations varying

by about a factor of 3, serial dilutions were performed

with deionized water for concentrations ranging from

3x102 jgml-1 to the concentration resulting in a signal of
2 times peak-to-peak noise.


General Layout of the Exoerimental System

A schematic diagram of the experimental components

in the atomic fluorescence system is shown in Figure 6.

The components of this system are discussed below.

Sources of Excitation

Single-element electrodeless discharge lamps (EDL's)

were prepared from the iodide form of the elements of Co,

31













Fig. 6.--Schematic diagram of continuous- and pulsed-sample
introduction tube furnace atomic fluorescence
spectrometric system.

E Electrodeless discharge lamp (EDL).

B Microwave power supply for EDL.

L1, L2, L3 Lenses.

S1, S2 Apertures.

C Chopper.

F Vitreous carbon tube furnace.

M Monochromator.

P Photomultiplier tube.

V High voltage power supply.

A Preamplifier.

L Lock-in amplifier.

R Recorder readout.








I L C L











Sn, Sb, Te, Pb, and Bi. The elements Ag, TI, and Mg were

prepared in their chloride form, while Zn and Cd were pre-

pared in pure form as single-element EDL's. These EDL's

were operated in the thermostated mode described by R. F.

Browner et al. [74,753. The apparatus used for thermo-

stating the EDL's is shown in Figure 7. It consisted of a

fan (Model NTH2, Rotron Mfg. Co., Woodstock, N.Y.) which

blew room air over a heater coil (Model HG 751, Master

Appliance Corp., Racine, Wis.) whose temperature was con-

trolled with a laboratory Variac (Model 2 PF 1010, Staco,

Inc., Dayton, Ohio). The temperature was monitored with a

chromel-alumel thermocouple placed as shown in the metal

conduit with temperature (OC) indicated directly on a volt-

meter calibrated in temperature units (Model K, Barber-Cole-

man Co., Rockford, Ill.). The thermocouple was calibrated

up to 350*C with a common laboratory thermometer, and

because the temperature was in error by only 5 OC at 350C

and the error increased linearly with temperature, the error

was extrapolated up to the maximum temperature used and

then was corrected for. A change in 20 OC had very little

effect (less than 5 per cent) upon the intensity of the

EDL's when the EDL temperature was optimized.

The EDL was situated, as shown in Figure 7, in a

microwave field focused with an "A" antenna. The EDL was

powered by a microwave power generator (Model PGM-10,















Fig. 7.--Cutaway view of the EDL thermostating unit.
All brass construction.






Thermocouple


Quartz Jacket k- Metal Conduit

Electrodeless
Discharge Lamp ----I Heater
Coil

"A" Type

Mount ----- -,-
SFan










Raytheon Co,, Manchester, N. H.). The thermostated unit

shown in Figure 7 from the bottom of the "A" antenna up was

enclosed in an aluminum box measuring 4.5 in. wide, 7.0 in.

long, and 6.0 in. high, with a hinged lid to facilitate

changing EDL's. A 0.25 in. diameter hole was placed in a

convenient location for the thermocouple to enter and a

1 in. diameter hole was located in such a position that the

EDL radiation could be easily focused through the lenses

upon the atom cell. When operating this unit at high

temperatures (greater than 300 OC) (Model SP2A2, Rotron

Mfg. Co.), a fan was needed to cool the microwave cable at

the "A" antenna junction. The whole unit was mounted on

two micrometer screw threads which gave both vertical and

lateral adjustment to better than 0.1 mm to facilitate

focusing the desired portion of the EDL upon the atom cell.

The single-element EDL's for these studies were

operated at the temperatures and powers for the continuous

mode studies and pulsed mode studies shown in Tables 3 and

5, respectively. The optimum temperatures and powers were

experimentally determined each time the EDL was started by

i. lighting the EDL at a power of 50 W;

ii. adjusting the monochromator to the approximate
wavelength of interest using light reflected
from the EDL to the photomultiplier tube;

iii. while monitoring the intensity on a strip chart
recorder, slowly raising the temperature of the
EDL until the EDL intensity started to decrease
from the maximum;











iv. decreasing the temperature until the maximum EDL
intensity was attained;

v. fine tuning the wavelength;

vi. adjusting the microwave power to a reasonable
(an increase in EDL intensity of 2-fold per 10 W
power) trade off between high power and EDL
intensity (high microwave power will decrease
the EDL lifetime).

The EDL's for all but two elements were operated

satisfactorily in this manner. However, the Sn and Mg

EDL's needed a microwave power of 90 to 100 W (with

temperature programming) to remain lit, and the optimum

intensities were very sensitive to small changes in micro-

wave power. This latter point was especially true for Mg;

with moderate microwave power (50 W) the Mg EDL discharge

was contained in only a small portion of the EDL volume.

Even at 100 W, the discharge still did not fill the EDL

cavity. In addition, several of the EDL's could be seen to

fluctuate in intensity by the naked eye (through UV ab-

sorbing safety glasses, of course). An increase of micro-

wave power usually eliminated this problem.


Optical System

Optical grade biconvex quartz lenses with a focal

length of 2.5 in. and a diameter of 2 in. (Esco Products,

Oak Ridge, N. J.) were used throughout for focusing. They

were mounted on a steel plate with the aid of magnetic

mounts (Model M 8-1, Enco Manufacturing Co., Chicago, Ill.)










and were aligned both vertically and horizontally with a

He-Ne lab laser (Model ML 6805, Metrologic, Bellmaur,

N. J.). For all absorption measurements performed, all

three lenses were aligned to 1800 with the entrance optics

of the monochromator in such a manner that a 1:1 image was

created at all focal points as checked with a tungsten

filament lamp, and in such a way that the monochromator's

mirrors and the diffraction grating were totally illuminated.

For AFS measurements, the two lenses farthest from the

monochromator were aligned vertically and horizontally so

that the total solid angle of source intensity which

impinged on the atom cell could be converted to useful

fluorescence, as shown in Figure 6. Aperture S1 of Figure

6 was 0.5 cm by 1 cm, so that all useful fluorescence from

the atom cell would totally fill the slit width and height

of the monochromator, and as mentioned previously, its

mirrors and gratings. Aperture S2 of Figure 6, was

adjusted so that the collected fluorescence would just fill

the solid angle of the monochromator and also fill all of

its related optics. All measurements were performed with

a 0.35 m f/6.8 Czerny-Turner monochromator (Model EU-700/E,

Heath Co., Benton Harbor, Mich.).











Electronics

The electronic equipment used in these studies is

also shown in Figure 6. The associated electronics included

an RCA 1P28 photomultiplier powered by a high voltage power

supply (Model EU-42A, Heath Co.), a lock-in amplifier tuned

to 667 Hz (Model 391, Ithaco, Inc., Ithaca, N. Y.), and a

preamplifier with variable gain (Model 164, Ithaco, Inc.).

Source modulation was performed with a mechanical chopper

(Model 382, Ithaco, Inc.), and the lock-in output was re-

corded on a potentiometric recorder (Model S. R., Sargent

Welch Scientific Co., Skokie, Ill.).


Nebulizers

Different means of creating a wet aerosol mist from

a solution have been used for atomic spectroscopy. Spin-

ning disks, high voltage sparks, ultrasonic radiation, and

pneumatic devices have all been employed to create these

mists; however, only the latter two have received widespread

attention in atomic spectroscopy. Of these, pneumatic

nebulizers have been the commercially accepted choice be-

cause they have been inexpensive, sturdy, and reliable.

However, they have suffered from the problems of creating a

dispersed mist with widely varying particle diameters, and,

when coupled to a spray chamber, poor aspiration efficiency;

furthermore, they needed high gas flow rates to aspirate










properly, and were reported to have a maximum aerosol

density of 0.03 g of solution/liter of aspiration gas [1,76].

Ultrasonic nebulizers have beenplagued with various

problems such as memory effects and sample transport capil-

lary failure, and so were not used in these studies [76,77,

78]. Such nebulizers, however, function on very low flow

rates of gas and hence result in relatively large solution-

to-gas flow ratios. For further information on nebulization

techniques, the reader is referred to the atomic spectroscopy

review by Winefordner and Vickers [55].

In the present work, it was desired to build a high

pressure, low sample consumption nebulizer using low flow

rates of aspiration gas, although some difficulties were

reported by other workers [1,79]. Such a nebulizer seemed

to be ideally suited to non-flame AAS and AFS work. First,

a miniaturized standard conical design similar to the one

used in pneumatic nebulizers was used with limited success

because the sample transport needle (id 0.005 in.) repeatedly

plugged up. The original design was then slightly altered

to that of nebulizer I, shown in Figure 8. The sample

transport needle (I) used was a 6-in.-long 26 g stainless

steel hypodermic needle (Hamilton Co., Reno, Nev.). This

needle was strengthened by placing a concentric stainless

steel tube around the exterior of the capillary needle, 0.5

in. from the tip until an outside diameter of 0.052 in. was












Fig. 8.--Schematic diagram of nebulizer I.

A Nebulizer cone.
B Rubber 0-ring.
C 0-Ring seating gland.
D Alignment rod.
E Micrometer adjustment.
F Housing ring to hold micrometer.
G Stainless steel needle housing.
H Teflon centering sleeve.
I Hypodermic needle.
J Vertical adjustment plate.
K Nebulizer housing.
L Needle housing.
M Luer lock-luer lock connector.
N Teflon system seal.
0 Retaining screw for N.











Nebulizer


M Aluminum


.K Stainless Steel


STeflon
-0
Rubber/Teflon Septum


I--~-1
1Y











attained. This needle was held firmly in place in the

stainless steel needle housing (F) with the use of a slip-fit

Teflon centering sleeve (H). The transport needle was

epoxied in place to the retaining screw (0) for the Teflon

system seal (N). The nebulizer cone was machined to have a

0.020 in. inside diameter and a convergence angle of the

aspirating gas nozzle to the central solution capillary of

300. This left a clearance of 0.001 in. between the solu-

tion capillary and the aspirating gas nozzle when the former

was centered. The centering of the sample transport needle

was effected with three stainless steel micrometers which

were situated at 1200 about the needle with the aid of a 30

power microscope (Model 70, 266, Edmund Scientific Co.,

Barrington, N. J.). In the continuous-sampling studies,

sample solution was force-fed through the sample transport

needle with the aid of a syringe pump (Model 353, Sage

Instruments, Inc., White Plains, N. Y.).

Nebulizer II was designed for use in both the con-

tinuous- and the pulsed-sampling modes. Nebulizer I con-

tained a dead volume of greater than 0.120 ml between the

syringe and the exit of the sample transport needle, which

caused very severe memory effects when sampling in the

pulsed mode. The dead volume was reduced by shortening the

sample transport needle to 2 in. in length, eliminating the

Luer-lock Luer-lock connection by epoxying and crimping a










male unit to the Teflon tubing, and decreasing the length of

the Teflon tubing from 12 in. to 6 in. The former change

necessitated the complete redesign of the nebulizer housing

and sample transport needle centering apparatus; however,

critical aspiration parameters were not altered, such as

the 300 convergence angle of the aspirating gas nozzle to

the central solution capillary and the clearance between

the solution capillary and aspirating gas nozzle of 0.001

in. Nebulizer II is shown in Figure 9. The overall

dimensions of the nebulizer were 1.375 in. in diameter and

2 in. in length. 10-80 Stainless steel screws (D) were

situated at 1200 around the hypodermic needle to allow

centering of the hypodermic needle (H) in the aspirating

gas nozzle (A).

All other parts, as mentioned previously, functioned

similarly to the parts of nebulizer I, shown in Figure 8,

and will not be discussed further.


Desolvation Chamber

A desolvation chamber, heated with heating tape

(Samox Hi Temperature Tape, Arthur H. Thomas Co., Phila-

delphia, Pa.), connected the nebulizer with the vitreous

carbon non-flame cell. Desolvation chambers constructed of

quartz and glass were found unsatisfactory, due to inability

to form a simple, efficient, gas-tight seal between the
















Fig. 9.--Schematic diagram of nebulizer II.

A Nebulizer cone.

B Rubber O-ring.

C Rubber 0-ring retaining screw for B.

D Alignment screw.

E Nebulizer housing.

F Stainless steel needle housing.

G Teflon centering sleeve.

H Hypodermic needle.

I Teflon system seal.

J Retaining plate for I.











NEBULIZER


sample
M Stainless Steel

0 Teflon


D Rubber/Teflon Septum

I Brass











chamber and the nebulizer. Without a good seal, repetitive

pulsed atomic fluorescence signals were obtained while

sampling in the continuous mode. Thus, the desolvation

chamber shown in Figure 10 was machined from type 304

stainless steel. It was bolted to the top of the nebulizer

with sheet asbestos serving as a gasket and, to some extent,

as a thermal insulation for the nebulizer. The desolvation

chamber had an inside diameter of 1.375 in. and was 3.5 in.

long. The top inside of the desolvation chamber was re-

duced to a radius of 0.688 in. The male unit of the de-

solvation chamber was tapered from 0.297 in. outside

diameter at the top to 0.313 in. outside diameter at the

base to facilitate forming a leak-proof seal to the vitreous

carbon tube furnace. Around the inside rim at the base of

the desolvation chamber were entrance ports to add other

gases than were used to effect nebulization.


Vitreous Carbon Tube Furnace

The vitreous carbon tube furnace is shown in Figure

11. Its outside dimensions are 2.5 in. wide by 2.5 in.

long and 2.0 in. in height. All parts ultimately connect

to the water-cooled main brass furnace housing (F). Both

top and bottom water-cooled electrodes (B) contained a

copper slide (A) to facilitate electrical contact. The

top water-cooled copper electrode (B) had three concentric




























CHI4 v/C CH4

Sample


Stainless Steel

f------


Fig. 10.--Schematic diagram of the stainless steel
desolvation chamber.




















Fig. 1l.--Schematic diagram of vitreous carbon tube
furnace.

A Copper electrical contacts.

B Water-cooled electrodes.

C Vitreous carbon tube.

D Boron nitride insulation ring.

E Asbestos insulation ring.

F Furnace housing.

G Graphite rings for electrical contact.








Carbon


M Copper
SVitreous Carbon
M Brass


Furnace
G


Sample
Aerosol
Asbestos
Z Boron Nitride

It 01











circular arrays of holes (used for inert sheath gases) of

1 mm in diameter at radii from the center of the vitreous

carbon tube of 13 mm, 17 mm, and 19 mm, respectively.

These holes were staggered with each other to minimize 02

and N2 entrainment from the air. The two main electrodes

(B) were electrically insulated from the brass furnace

housing (F) with fiberglass gaskets and nylon screws (the

external surface of the furnace housing was less than 75 C

when the vitreous carbon tube (C) was operating at tempera-

tures in excess of 2000 C). Thermal insulation was pro-

vided with rings of boron nitride (D) and asbestos (E)

between the vitreous carbon tube furnace (C) and the

furnace housing (F).

Perhaps the most critical tolerances in this furnace

are the inside diameters and outside diameters of the upper

and lower graphite rings (G), providing electrical contact,

as well as the thickness of the upper graphite ring (G).

If the rings(G) were not machined to have a snug slip fit

to the vitreous carbon tube (C), the electrical resistance

became so great that the maximum voltage (20 V) from the

power supply (SCR 20-250, Electronics Measurements, Neptune,

N. J.) could not supply enough current to sufficiently heat

the carbon tube. The thickness of the upper graphite ring

(G) determined the resistance of the contact between it and

the vitreous carbon tube (C), and hence the power dissipated











near the atom cell. (It thus was used to influence the

temperature of not only the vitreous carbon furnace, but

also the atom cell.) The lower graphite ring (G) had an

orifice of 0.297 in. in diameter to accept the previously

mentioned male unit of the desolvation chamber.


Nebulizer, Desolvation Chamber, and Vitreous Carbon
Tube Furnace Mount

The nebulizer and vitreous carbon tube furnace were

mounted on a 0.5 in. thick, by 2.0 in. wide by 12.5 in.

long piece of aluminum in such a manner that the former,

with the desolvation chamber in place, could easily be

moved up or down 5 in. and locked in place while the

vitreous carbon tube furnace was kept fixed. This whole

assembly was mounted on two micrometer screw threads which

gave both vertical and horizontal adjustment to better

than 0.1 mm to facilitate imaging the atom cell upon the

monochromator entrance slit.


Procedure for Evaluation of Experimental Characteristics
of the System

Gas flow rates

The rotameters were calibrated for various gas flow

rates at different pressures. These calibrations are given

in Appendix II.











Sample solution flow rates

Flow rates, using the syringe pump and 1 ml disposable

tuberculin syringes (Model 501 S-TB, Sherwood Medical

Industries, Inc., Deland, Fla.), to pump the sample through

the nebulizer, were calibrated versus the coarse flow rate

scale and the percentage flow rate scale of the syringe

pump. The aerosol produced was of a pulsed nature when

samples with very low flow rates were aspirated with an Ar

flow rate of 0.90 1 min1. These results are summarized

in Table 1. A flow rate of 0.12 ml min-1 was chosen as a

nominal sample flow rate for all measurements.

Mean particle diameter

The mean particle diameter of the wet aerosol formed

upon aspiration may be estimated from the empirical equation

which Nukiyama and Tanasawa determined for velocities up to

sonic velocities for a conical pneumatic nebulizer [1].

Their equation for the mean particle diameter, d, in un

was

S 585 () + 57 45 1000 V 1.5
III-1


where













TABLE 1


STABILITYa


OF AEROSOL PRODUCTION WITH HIGH
PRESSURE NEBULIZER


Sample Flow Rate
(ml min-1)
0.50
0.25
0.12
0.06
0.03
0.25
0.12
0.06


Ar Flow Rate
Nebulizer
(1 min-1)
0.90
0.90
0.90
0.90
0.90
0.78
0.78
0.78


Stability of
Aspiration

yes
yes
yesb
yesb
no0
yes
yes
yesb


aMeasured stability by monitoring light scattered from
aerosol droplets emerging from nebulizer.
After a time delay of 10 sec.
CNo = stability of aspiration was inadequate for
analytical work.











} = velocity of aspiration gas

V = surface tension

C = density of liquid
V1 = volume of liquid (1)

Va = volume of aspiration gas (1)

7 = viscosity

It should be noted that if Va = 5000 V1, then for water,

the Nukiyama and Tanasawa equation (Equation III-1) reduces

to [1]

c ^ (G^ III-2

The velocity of the aspiration gas through the nebulization

cone was estimated by dividing the measured flow rate of the

aspiration gas by the area of the ring-shaped orifice. The
-1
value calculated in this manner was 390 m sec- for an

aspiration flow rate of 0.90 1 min-1. Substituting the

speed of sound, 340 m sec- for y in Equation III-2, a
mean particle diameter of 15 .im was calculated. The calcu-

lated velocity of approximately 10 per cent greater than the

speed of sound would further increase turbulence and hence,

tend to decrease the particle size even more.

Desolvation time

The aerosol mist was dispersed and desolvated in the

stainless steel desolvation chamber previously described.

The temperature of the gas passing through the center of











the desolvation chamber ranged from 246 C to 278 OC as

shown in Figure 12, while the heat tape wrapped around the

chamber was measured at 393 C. These temperatures were

measured with 0.015 in. diameter chromel-alumel thermo-

couples, and the resulting voltages were monitored on a

digital multimeter (Model 414, Dynasciences, Chatsworth,

Cal.).

Desolvation time, t, of a particle may be approxi-

mated with the following equation [80].

AC / tn
ts lnL + Cp (T-Tb)/L] II1-3

where
t = desolvation time (sec)

= density of the liquid (g cm-)

A = initial area of the droplet (cm )

Cp = specific heat of the vapor (cal g- C )

X = thermal conductivity of the gas (cal sec-1
cm-1C-1)
T = desolvation temperature (C)

Tb = boiling point of the solvent (C)

L = specific heat of vaporization of the solvent
(cal g-l)
This equation neglects the additional transport of heat to

the boiling droplet by convection, which would significantly

reduce the actual desolvation time. Using Equation 1-3,

a desolvation time of 23 msec for a 15 am diameter sphere










27


180 k


90 F


Fig. 12.--Gas temperatures in the desolvation chamber
at various heights above the nebulizer.


0 2 4 6 8 10

Ht (cm)


270











of water at 250 C was calculated. Also, from the known

nebulization gas flow rate and the volume of the desolva-

tion chamber, a residency time of the sample mist in the

desolvation chamber of 5.3 sec was calculated, neglecting

the additional gaseous volume of the-liquid sample solution.

Thus, desolvation was probably completed in the desolvation

chamber.

Scattered radiation

No scattered radiation (in the absorption mode) was

discernible from water or from a 100 pg ml-1 Zr solution

at the Zn 213.8 nm line which was radiated from a Zn hollow

cathode while absorption measurements were being made at

grazing incidence to the desolvation chamber. The scatter

signal was found to be 2 per cent absorption for scatter

measurements performed with 104 ug ml-1 Zr [81]. The

negligible amount of scatter from large concentrations of

solutes, which are difficult to vaporize, qualitatively

supports the above observations concerning the small mean

particle diameter.

Rise velocities

Comparisons of the rise velocities in the vitreous

carbon tube non-flame to that in various analytical premixed

flames (assuming a primary combustion zone angle of 6) are

given in Table 2 L1]. Ratios of the residency time in the













TABLE 2

COMPARISON OF RESIDENCY TIMES IN ATOM CELLS

Rise Velocitya (m sec-1 )
C-furnace 0.8
H2-02 20.
H2-air 0.4
C2H2-02 20.
C2H2-air 2.
C2H2-N20 1.6


Residency Time Ratiosb
C-furnace:H2-02 50:1
C-furnace:H2-air 1:1
C-furnace:C2H2-02 50:1
C-furnace:C2H2-air 5:1
C-furnace:C2H2-N20 4:1



aRise velocities for the flames were calculated from
Vburn = rrise sin 8, where burn is the burning velocity
(taken from Reference 1).
bResidency time was calculated from tR =

r--- where ho for the flames is the observation height
rise
above the burner taken to be 2.5 cm and h for the furnace
was taken to be the tube length of 5 cm.











vitreous carbon tube to the residency time in a flame prior

to measurement (assuming a distance of 2.5 cm) are also

given in Table 2.

Efficiency of sample transport

The efficiency of the nebulizer and desolvation

chamber was experimentally determined by aspirating 2 ml

of 12.5 ug m1-1 Mg. The chamber was then washed with de-

ionized water of the same pH as the original solution, and

the Mg concentration of this solution was measured via

atomic absorption spectrometry with the graphite filament

non-flame cell [30]. The efficiency was found to be 93 per

cent. The chamber was then rewashed in the same manner to

check for any residual Mg, and no detectable Mg was found.

Temperature of furnace and gaseous vapor

The temperatures in and above the vitreous carbon

tube resulting from various applied currents from a constant

current source were measured with 0.010 in. diameter

tungsten-tungsten 26 per cent rhenium thermocouples (Omega

Engineering, Inc., Stamford, Conn.). The voltages were

monitored on a digital multimeter (Model 414, Dynasciences).

The results are given in Figure 13.














Fig. 13.--Temperatures of non-flame cell at various points.

a Temperature in center of carbon tube.
b Temperature at grazing incidence to carbon tube
with Ar sheath.
c Temperature at grazing incidence to carbon tube
with Ar-H2 diffusion flame.
d Temperature at 1 cm above carbon tube with
Ar-H2 diffusion flame.

e Temperature at 2 cm above carbon tube with
Ar-H2 diffusion flame.

f Temperature at 1 cm above carbon tube with
Ar sheath.
g Temperature at 2 cm above carbon tube with
Ar sheath.











2000

1600

S1200 -

w 800 d

400.-


0 50 100 150 200
CURRENT (A)











General Procedure for Making Atomic Fluorescence Measurements

All electronic equipment (photomultiplier tube,

lock-in amplifier, chopper, etc.) was switched on at least

30 minutes before any experimental data were recorded. The

desolvation chamber required approximately 30 minutes to

reach the steady state in temperature shown in Figure 12,

while the EDL's usually required about 40 minutes to

optimize.

The nebulizer, desolvation chamber, and vitreous

carbon tube furnace were flushed with Ar for approximately

5 to 10 minutes before any atomic fluorescence measurements

were performed. Increasing amounts of current were applied

to the carbon tube furnace until the desired temperature

was reached after the desired flow rates of Ar, H2, and CH4

were adjusted. The sample (or blank) was then aspirated

into the desolvation chamber in either the pulsed or con-

tinuous modes of sampling, and the resulting aerosol mist

was carried to the vitreous carbon tube where atomization

was effected. The vitreous carbon furnace was operated at

a constant current (temperature) until all atomic fluores-

cence measurements were completed.

The continuous mode of sampling was achieved by

force-feeding the solution to the high pressure nebulizer

with a syringe pump (Model 353, Sage Instruments), while

the pulsed mode was effected by forcing the sample into the











nebulizer operated continuously at 60 pounds pressure of

Ar from a 0.5 ml syringe (Model 1000, Hamilton Co.) con-

tained in a repeating dispenser (Model PB-600, Hamilton

Co.).

The excitation radiation passed at grazing incidence

to the top of the vitreous carbon tube. The fluorescence

was collected by the monochromator adjusted to the desired

wavelength. The slit width was adjusted to 750 um for all

studies. The Ar, CH4, and H2 flow rates (see Appendix II)

were monitored by rotameters calibrated with a wet test

meter (Model S-39447, Precision Scientific Co., Chicago,

Ill.).













CHAPTER IV


RESULTS AND DISCUSSION

Atomic Fluorescence Signal-to-Noise Optimization


Five 1P28 photomultiplier tubes were examined for

both dark current and relative gain characteristics at

various applied voltages (600 V 900 V); the gain was

determined with a tungsten filament lamp as the source

operated at a constant voltage (intensity), a wavelength

of 500 nm, and a slit width of 200 nm. The optimum photo-

multiplier tube (of the five measured) was used for all

fluorescence measurements; at the voltage (700 V) found to

give the optimum signal-to-noise ratio, a dark current of

1 x 10-10 A at 700 V was measured.

As mentioned in Chapter III, the intersection of the

center of the excitation optics and the center of the

emission optics (L1 and L2, L3 and M, respectively of

Figure 6) was determined; this intersection could be located

to better than 1 mm using a removable marker. The vitreous

carbon tube furnace alignment was coarsely adjusted in this

manner, and then, using the vertical and horizontal screw

adjusts, the fluorescence signal was finely tuned to a

maximum.











Because ideally a fluorescence signal should be

measured against no background signal, two apertures (S1

and S2 of Figure 6) were used to prevent most source

radiation from being scattered about the room and eventually

finding its way to the photomultiplier tube. Care was

taken to ensure that the aperture did not limit the solid

angle of the monochromator.

No mirrors were used to increase fluorescence

intensity by reflecting the source intensity back upon the

atom cell or in collecting a larger solid angle of the

fluorescence radiation from the atom cell. An improvement

of signal-to-noise (S/N) by a factor of 3-10 by employing

mirrors for this purpose has been reported in atomic

fluorescence flame spectroscopy [82].

The optimum time constant of the lock-in amplifier

(resulting in optimum fluorescence S/N) used in the pulsed

mode was experimentally determined for several elements and

was found to be 0.4 sec.

Fluorescence signal-to-noise for a number of elements

was determined at various monochromator slit widths, and a

750 um slit appeared to be a suitable compromise choice for
all elements.

On the average, about five EDL's for each element

determined were characterized as to their optimum tempera-

ture range, operating power, long term lamp stability,












relative intensities at the wavelength of interest, and

short term noise at the optimum temperature (intensity).

These characteristics were determined as described

previously in Chapter III. All results were recorded, and

the respective lamps were labeled. The figures of merit

for each EDL, for example, optimum temperature range, in-

tensity, operating power, long term stability, and short

term stability were checked for the particular lamp in use

before any atomic fluorescence measurements were performed

to determine if the lamp was still near optimum

characteristics. If not, another EDL was used.

The atomic fluorescence signal-to-noise of Pb and Sn

was examined for vitreous carbon tubes with internal

diameters of 0.188 in. and 0.125 in. The S/N for the

0.188 in. tube was larger by approximately a factor of 2

for Pb and Sn than for the 0.125 in. tube. Therefore, it

was used for all further atomic fluorescence measurements.

The upper graphite ring for electrical connection (G of

Figure 11) was changed in thickness to values of 0.050 in.,

0.060 in., 0.080 in., and 0.100 in., and the resulting

atomic fluorescence intensities were measured for Te, Pb,

and Sn. A thickness of 0.080 in. gave the optimum S/N

ratios when the vitreous carbon tube lifetime was taken

into account, so this thickness was used for all further

atomic fluorescence measurements.











The optimum temperatures of the vitreous carbon tube

furnace were determined experimentally for each element by

comparing fluorescence S/N ratios. The vitreous carbon

tube lifetime was also taken into consideration here;

higher temperatures led to shorter lifetimes of the vitreous

carbon furnace.

The gas flow rates (nebulization Ar flow rate,

external Ar flow rate, external H2 flow rates, and internal

CH4 flow rates) were optimized for each respective element

in the continuous mode study; however, in the pulsed mode

study, average optimum flow rates for all elements were

used. Internal Ar was also added for Ag, Pb, Sn, and Tl

in the continuous mode of operation; however, in all cases,

the fluorescence S/N ratio decreased markedly, and so it

was not further examined. This may be the result of in-

creased atom cloud dilution in the atom cell or decreased

residency times of the analyte in the vitreous carbon tube

furnace.


Definitions of Analytical Parameters

The atomic fluorescence concentration (,jg m-1)

limit of detection (CLOD) was defined as that concentration

(in ug ml-1) giving a signal of 3 x rms noise. Each LOD

was found by alternately running a signal and blank 3-5

times and then extrapolating back from a signal of about











2 x peak-to-peak noise to a signal of 3 x rms noise and

determining the concentration of the lowest signal level.

Atomic fluorescence measurements of the various elements

were determined for concentrations ranging from 103 pg ml-1

to the concentration resulting in a signal of 2 x peak-to-

peak noise with concentrations varying by factors of three.

The upper concentration limit (called UCL) was determined

by the beginning of serious curvature (greater than 4 per

cent deviation from linearity) of the analytical curve.

The linear dynamic range (LDR) was the ratio of UCL/LOD,

and the sensitivity was the slope of the analytical curve.

The precision of the system (relative standard deviation)

was determined for a concentration of 100-200 x LOD.

Continuous-Sampling Mode Results

Limits of detection

The resulting LOD's for the six elements are given

in Table 3, along with the temperature of atomization, vari-

ous gas flow rates, and the linear dynamic ranges (LDR).

The atomic fluorescence LOD's obtained by the present

non-flame cell were compared to the best limits of detection

obtained with a flame and a resistively heated discrete

sampling non-flame cell in Table 4. Both concentration

(in ig ml-1) limits of detection (CLOD) and absolute (in

ng) limits of detection (ALOD) are listed in Table 4. The








TABLE 3

EXPERIMENTAL CONDITIONS AND ANALYTICAL RESULTS FOR MEASUREMENT OF SEVERAL ELEMENTS BY MEANS OF
THE CONTINUOUS-SAMPLE INTRODUCTION TUBE FURNACE ATOMIC FLUORESCENCE SPECTROMETRIC SYSTEM

Ele- Wave- EDL Conditions Tube Furnace Conditions LODe LDRe RSD Sensitivityg
ment length b a Powerb m Ar Flow Ar Flow HFlow
(nm) (C) ( (OC) Rate Rate Rated
Nebu- Ex- Ex-
lizer 1 ternall eternal
(1 min- )(1 min )(1 min-

Te 214.2 430 30 1515 0.78 4.7 1.8 lxlO-2 lxlO4 0.048 0.87
Ag 328.0 530 60 1675 0.78 4.3 1.6 2x10l3 1.5xlO3 0.035 0.98
Pb 283.3 430 30 1675 0.78 4.3 1.6 1x10-2 8x103 0.032 1.0
Sn 303.4 275 90 1800 0.78 5.8 1.8 1xlO2 lxl03 0.045 1.3
T1 377.6 435 45 1675 0.9 4.7 1.6 6xl0-5 3x10 0.036 1.0
Bi 307.7 500 50 1540 0.9 4.7 1.8 2x10l 1x104 0.015 0.96
aTemperature of air flowing by EDL.
bMicrowave power applied to EDL using thermostated "A" antenna.
CTemperature of graphite tube furnace walls.
dFlow rate of Ar and/or H2 around tube furnace exit.
eLOD = limit of detection (see text for definition).
LDR = linear dynamic range (see text for definition).
fRSD = relative standard deviation (taken at a concentration of about lOOxLOD).
gSensitivity slope of Log-Log plots.











TABLE 4
COMPARISON OF NON-FLAME ATOMIZATION ATOMIC FLUORESCENCE SPECTROMETRIC LIMITS
OF DETECTION


This Work Discrete Atomization Flame
CLOD ALOD CLOD ALOD CLOD ALOD
Ele-
ment (ig ml-1) (ng) (pg ml-1) (ng) Ref. (pg ml-) (ng) Ref.
Sn 1xlO2 1x100 2xlO1 lxlO1 33 xO1-1 5x102 83

Pb x 3 5xl 3 10 l1 5xlO3 3x103 36 lxl02 5x101 84
Te 1x102 lx10 6xl0-3 3x101 85

Ag 2xl0- 2x10-2 8x10-4 4x104 33 lxlO- 5x10 86
T1 6x10- 7x101 4xl0-2 2xlO-2 33 8x10lO 4x10 86
Bi 2x105 2x10-1 5xlO3 2x10 85











flame atomic fluorescence ALOD was approximated by assuming

that measurements were performed for 1 minute with a sample

flow rate of 5 ml min1.

In general, the present system operated in the

continuous mode resulted in concentration limits of detec-

tion which did not vary significantly from those concentra-

tion limits of detection found by flame and non-flame

atomic fluorescence. The absolute limits of detection of

the present system operated in the continuous mode were

generally 1-2 orders of magnitude better than flame atomic

fluorescent limits of detection, but about 1 order of magni-

tude poorer (higher) than filament non-flame atomic

fluorescence spectroscopy.

Linear dynamic range and sensitivity

The atomic fluorescence analytical curves for Te,

Sn, Ag, Bi, Pb, and T1 with the present vitreous carbon

tube in the continuous-sampling mode are given in Figures

14, 15, 16, 17, 18 and 19, respectively. The linear

dynamic ranges varied from 1 x 103 for Sn to 1.5 to 104

for Bi, and are listed in Table 4. Bi has a slightly

concave analytical curve; however, it is certainly

analytically useful. The discrete sampling filament

non-flame cell, when measurements were performed by atomic

fluorescence spectroscopy, appears to have significantly

shorter linear dynamic ranges (1 x 102 5 x 10 ) for a

number of elements investigated [34].













4
10

e-

C



.00- 10

I I 0 I





-I 2
10 I 10 10 I(

Concentration (p.g mrl)


Fig. 14.--Atomic fluorescence analytical curve for Te in
the continuous mode of operation.


















S 10
U)


SI02









SI I I
0.1 0 10


Concentration (Ipg mTI)


Fig. 15.--Atomic fluorescence analytical curve for Sn in
the continuous mode of operation.















I 3

4,



(D
c 2



- 10
v,







S-2. 2
10 1 I I0 102


Concentration (/g mi )


Fig. 16.--Atomic fluorescence analytical curve for Ag
in the continuous mode of operation.










5
104


=s io3
S 10 -










c rI I 2
162 1 1 I0 10

Concentration (/pg ml )


Fig. 17.--Atomic fluorescence analytical curve for Bi in
the continuous mode of operation.








I04





10




Co
C1
jU-2< 102









S' I
I I I0 10


Concentration (2g m11)
b. _f
o a/
^,0-










Concentration (ug ml )


Fig. 18.--Atomic fluorescence analytical curve for Pb in
the continuous mode of operation.







79




310




0 2M
4-




L.L




I ( I I
1 I 10 10

Concentration (,pg m11)


Fig. 19.--Atomic fluorescence analytical curve for T1
in the continuous mode of operation.











Correction for scattering

All atomic fluorescence signals were corrected for

any spurious scatter signals by one of two methods, and in

all cases the results of the two methods agreed within

experimental error. If a source line was available (that

is, an Iodine line) which was not a fluorescent line of the

analyte, then the ratio of a non-absorbing line intensity,

Ina, of the source to the primary excitation line, IF, of
the source was calculated.

Ina
R = IV-1


Since only lines in the approximate spectral region of

fluorescence were used, the variation in photomultiplier

spectral response, in spectral transmission of the lenses

and all other optical components, was not corrected for.

The scatter signal, Ss, and the apparent fluorescence

signal, SF, were then measured, and the corrected scatter

signal, S'na, from the atom cell was calculated from
S' S
Sna IV-2
na R

The corrected fluorescence intensity S'F from the atom cell

was then
S' Sp S'na IV-3


for the first method.











The second procedure used to correct for spurious

scatter was

i. measure blank deionizedd water) signal, Sb, at
the temperature of atomization;

ii. measure apparent fluorescence signal, SF, for
concentration of interest;

iii. decrease the temperature of the vitreous carbon
tube furnace to 100 OC and measure any scatter
signal, Ss. The maximum scatter signal was then
assumed to be Ss and the corrected fluorescence
signal S' was calculated from

S'F = SF Ss b I

This method assumes that

i. no appreciable atomization has occurred in the
vitreous carbon tube furnace in step iii, and
hence no atomic fluorescence was measurable;

ii. any scatter signal measured at high temperatures
not measured at low temperatures was due to hot
carbon particles and radical formation;

iii. the scatter signal due to the analyte could only
increase by lowering the temperature of the
vitreous carbon tube due to reduced desolvation
and possible coagulation.

Since this second method may over-correct for scatter, it

was not used in cases where scatter was estimated to be

over 4 per cent of the fluorescent signal (see items i and

iii listed above).











Precision

The precision for Sn, Pb, Ag, Bi, Tl, and Te is

listed in Table 3. The precision obtained is very competi-

tive with all other atomization devices in use for atomic

fluorescence. For all elements examined, the relative

standard deviation is 0.025 to 0.048, which is comparable

to that observed in flame APS and generally better than

that of graphite rod AFS. The present continuous intro-

duction non-flame cell is not subject to the sample place-

ment errors found in the graphite rod atomizers.

Atom population profiles

The method used to measure the decay of atom

population-height profiles has been previously described

[23,53,34]. Atomic profiles as a function of height above

the vitreous carbon tube furnace for several elements were

examined in a similar manner for the present atomization

cell to determine the effect of only an Ar sheath versus

an Ar-H2 diffusion flame sheath on the decay of atom

populations exiting continuously from the present con-

tinuous-sampling vitreous carbon tube furnace. The same

atomization conditions were used for this study as for the

LOD measurements.

The factors contributing to the decrease in the

measured peak atomic fluorescence as the vitreous carbon

tube furnace was lowered include: (i) the loss of atoms

by chemical reaction; (ii) transport of atoms outside the











light path by diffusion; (iii) decrease of the solid angle

subtended by the atoms in the sample cavity; (iv) quenching

of the excited-state atom populations by collisions,

particularly with diatomic and polyatomic species. Even

in the absence of condensation, oxide formation, and other

possible chemical losses, a gradual decrease of the

fluorescence signal from factors (ii) and (iii) would be

expected.

The decay of atom populations with height above the

vitreous carbon tube furnace for Sn and Pb was measured by

atomic fluorescence spectrometry and the results are shown

in Figure 20. In the case of Pb with the Ar-H2 diffusion

flame, the decay of the atom populations was gradual,

reaching 50 per cent at a height of 20 mm above the furnace

exit. With only an Ar sheath, however, the decay of Pb

proceeded somewhat faster, reaching 30 per cent of the

original population at a height of 20 mm. For Sn, the

Ar-H2 diffusion flame had very great influence on the atom

population as a function of height above the furnace tube

exit; at 12 mm above the carbon tube, the fluorescence

signal actually increased 300 per cent, and then it began

to decrease to slightly less than 150 per cent at 24 mm.

With only Ar, the Sn population dropped to 28 per cent of

the original fluorescence signal at 7 mm.





















S250 Sn;Ar-H2


4-J
c

200-



S150 -


a,






50 Pb; Ar H2

Pb;Ar

Sn;Ar


5 10 15 20 25
Ht/mm

Fig. 20.--Decay of atom populations with height above the
vitreous carbon tube atomizer for two elements
as measured by atomic fluorescence spectrometry.
0, Sn, 3034 A; 0 Pb, 2833 A; X, Ar atmos-
phere; *, Ar/H2 atmosphere.











For the two elements (Pb and Sn) studied here, the

atomic population-height profile was less steep for the

present continuous-sample injection non-flame cell compared

to that of the discrete graphite carbon filament atomizer

[33]. For instance, at a height of 5 mm above the atomizer,
the fluorescence signal of Sn with Ar entrainment is about

55 per cent of the initial signal for the present atomizer

as compared to less than 2 per cent of the initial signal

for the graphite filament atomizer. When a H2 diffusion

flame was used with the present system, the fluorescence

signal increased about 3-fold, while with'the carbon

filament atomizers, fluorescence decreased to about 20 per

cent at 15 mm height.

In the case of Pb at 10 mm height above the non-flame

cell, the atomic fluorescence signal was about 60 per cent

of the initial fluorescence signal with either the Ar or

Ar-H2 entrainment sheath, while in the case of the graphite

filament atomizer, the fluorescence signal was less than

30 per cent with a H_ diffusion flame and less than 1 per

cent with only an Ar sheath.

These results tend to indicate that the presence

of the H2 diffusion flame helps to prevent the rapid

attenuation of the atomic populations, and in the case of

Sn, it actually increases the atomic populations at some

heights above the vitreous carbon tube furnace. Previous












studies on a similar type of H2 diffusion flame suggested

that the atomic cell was characterized by a region of an

excess unburned H2, which minimized the entrainment of

02 [30].

In the case of Sn, where the fluorescence intensity

increased to a height of 15 mm over the vitreous carbon

tube, the indication was that H2 was not only protecting

the metal atom cloud from oxide formation, but also was

assisting in Sn-atom production from the molecules formed

previously by reactions such as

MO + H2 --- M + H2

MO + H M + OH

where M was some metal. The explanation for why this

occurs to a far greater extent with the vitreous carbon

tube furnace than with the carbon filament may lie in the

higher temperatures of the furnace, which render the H2

diffusion flame of the former more reactive.


Pulsed-Sampling Mode Results

Limits of detection

The atomic fluorescence limit of detection (LOD)

for this study was defined as that concentration (in ug ml-1)

giving a fluorescence signal of 3 x rms noise of the blank.

The concentration limits of detection, along with the vari-

ous gas flow rates, temperature of atomization, and EDL












conditions for a 10 ml sample size for each element are

given in Table 5. The resulting concentration and abso-

lute LOD's are compared in Table 6 to the best atomic

fluorescence limits of detection obtained by previous

workers with discrete sampling non-flame cells and flames,

as well as to the previous LOD's calculated for the

present system used in the continuous-sampling mode. The

best concentration and absolute limits of detection obtained

by flame atomic absorption, graphite tube atomic absorption,

and the plasmas are also given. Generally the concentra-

tion limits of detection by the present system operated in

the pulsed mode are comparable to most other methods, while

the absolute limits of detection by the present non-flame

cell were generally superior by 1 order of magnitude or

more.

Size of sample

The fluorescence signal did not increase linearly

with increasing sample sizes of 1 pl, 5 gl, and 10 l for

the three elements shown in Figure 21. The figure indi-

cates that the best atomization efficiency is obtained for

1 l and 5 pl samples, while the efficiency drops off

slightly for the 10 pl sample size.




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