atomic absorption spectroscopy with a resistively heated ......atomic absorption spectroscopy with a...
TRANSCRIPT
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Atomic Absorption Spectroscopy With A
Resistively Heated Carbon Furnace
by
MAHMOUD CHAMSAZ, M.Sc., D.I.C.
A thesis submitted for the degree of Doctor of
Philosophy of the University of London.
May, 1978.
Department of Chemistry, Imperial College of Science and Technology, South. Kensington, London S.W.7.
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To my wife
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Summary
This thesis describes the development and application
of a resistively heated graphite furnace for trace element
analysis by atomic absorption spectrometry (AAS).
The first chapter is concerned with the history of
AAS and the development of non-flame atom cells. The
second chapter describes the theory of atomic absorption.
First the theoretical aspects of absorption spectroscopy
are briefly discussed, and then the theory of atomization
processes together with the interferences encountered in
non-flame atom cells will be considered.
The construction and subsequent development of a
continuously-operated graphite furnace and the associated
nebulizer and desolvation system is described in chapter
three.
The last chapter is concerned with trace element
analysis. Three different methods of sampling are
examined. The first is continuous sample introduction
for the determination of Cd, Ca, Co, Pb, Mg and Zn. The
second method describes the determination of difficult
to atomize elements (Mo and Al) using discrete sample
introduction. The last section investigates the
advantages of a deposition technique and these are illus-
, trated by Cd, Mo, Mg and Ca.
III
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ACKNOWLEDGEMENTS
I would like.to thank sincerely my supervisor,
Professor T.S. West, for the opportunity to gain research
experience whilst working under his guidance.
I would also like to thank Dr. R.A. Chalmers and
Dr. I.L. Marr for allowing me to work in their laboratory
and for their help during my period in Aberdeen.
I am particularly indebted to my immediate supervisor,
Dr. B.L. Sharp, for his encouragement, useful discussion
and great concern throughout my research programme.
The research work presented in this thesis was
carried out in the Chemistry Department of the Imperial
College of Science and Technology and the University of
Aberdeen from October, 1974 to May, 1978. It is entirely
original except where due reference is made. No part of
this work has previously been submitted for any degree.
IV
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Chapter I
CONTENTS
Page Introduction
1.1. History 1
1.2. Atom Cells 2
1.2.1. Furnaces 3
1.2.1.a Low Temperature Furnaces 3
1.2.1.b Graphite Furnaces 4
1.2.2. Filaments and Open Cells 12
1.2.2.a Wire Filaments 12
1.2.2.b Graphite Filaments 15
1.2.2.c Sample - Boats 16
1.2.3. Cathodic Sputtering Cells 17
1.2.4. Plasmas 20
1.2.4.1. Non-flame-like Plasmas 20
1.2.4.1a Arcs 20
1.2.4.1b Sparks 21
1.2.4.2. Flame-like Plasmas 22
1.2.5. Lasers 23
1.3. Conclusion 24
Chapter II Theory of Atomic Absorption Spectrometry
2.1. Principles 26 2.2. The Intensity of a Spectral
Line 26 2.3. The Width of Spectral Lines 27
2.4. Variation of Atomic Absorption with Concentration
2.4a The Absorption Coefficient, K 28
2.4b The Total Absorption Factor, AT 28
V
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Page
2.4c The Absorbance, A 29
2.5. Theoretical Aspects of Atomization Process 29
2.5.1. = Thermodynamic Approach 29
2.5.2. Kinetic Approach 33
2.5.2a Atomization under Increasing Temperature 34
2.5.2b Atomization under Isothermal Conditions 39
2.6. Interferences with Electrothermal Atomizers 42
2.6.1. Physical Interferences 43
2.6.1a Sample Introduction Inter- ferences 45
2.6.1b Memory Effects 45
2.6.2. Spectral Interferences 45
2.6.2a Line Overlap Effects 45
2.6.2b Emission Radiation Effects 46
2.6.2c Scattering Effects 46
2.6.2d Molecular Absorption Interferences 46
2.6.3. Chemical Interferences 49
2.6.3a Anion/Cation Interferences 49
2.6.3b Pyrolysis Losses 49
2.6.3c Condensation 49
2.6.3d Carbide Formation 50
2.6.3e Nitride Formation 51
2.6.3f Ionization 52
Chapter III Instrumentation
3.1. Graphite Furnace Atomizer 52 System
3.2. Power Supply 53
3.3. Temperature Measurements 55
V I
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3.4.
3.5.
3.6.
3.7.
Purge Gas
Stopped Gas Flow
Pyrolysis Treatment
Nebulizers
Page
55
58
59
62
3.8. Sample Introduction System 65
3.9. Spectral Light Source 72
3.10. Detection System 73
Chapter IV Determination of Elements by Graphite Furnace AAS
4.1. Continuous Sample Introduction 'Method 75
4.1.1. Optimization of Parameters 76
4.1.1.a Optimization of Monochromator Slit-width 76
4.1.1.b Optimization of HCL Current 79
4.1.1.c Optimization of Nebulizer Gas Flow Rate 79
Optimization of Temperature of the Spray Chamber 79
4.1.1.e Optimization of Purge Gas Flow Rate 82
4.1.1.f Optimization of Atomization Temperature 82
4.1.2. Sensitivity, Detection limit and Reproducibility 88
4.1.3. Interference Effects Study 89
4.1.4. Results and Discussion 89
4.1.4.1 Cadmium 89
4.1.4.1a Introduction 89
Analysis 90
4.1.4.2 Calcium 94
VII
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VIII
Page
4.1.4.2a Introduction 94
4.1..4.2b Analysis 94
4.1.4.3 Cobalt 98
4.1.4.3a Introduction 98
4.1.4.3b Analysis 99
4.1.4.4 Lead 104
4.1.4.4a Introduction 104
4.1.4.4b Analysis 104
4.1.4.5 Magnesium 105
4.1.4.5a -Introduction 105
4.1.4.5b Analysis' 109
4.1.4.6 Zinc 112
4.1.4.6a Introduction 114
4.1.4.6b Analysis 114
4.2. Discrete Sample Introduction Method 118
4.2.1. Optimization of Parameters 118
4.2.1.a Optimization of Atomization Temperature 118
4.2.1.b Optimization of Purge Gas Flow Rate. 120
4.2.2. Results and Discussion 120
4.2.2.1 Molydenum 120
4.2.2.1a Introduction 120
4.2.2.1b Analysis 120.
4.2.2.2 Aluminium 125
4.2.2.2a Introduction 125
4.2.2.2b Analysis 125
4.3. Aerosol Deposition Method 128
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Page
4.3.1. Optimization of Parameters 128
4.3.1.a Optimization of Deposition Time 129
4.3.1.1) Optimization of Deposition Temperature 129
4.3.2. Results and Discussion 133
Conclusion 140
References 142
IX
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CHAPTER 1
INTRODUCTION
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1
1.1. History
Isaac Newton is regarded as the founder of the science
of spectroscopy, following his analysis of the continuous
solar spectrum in 1666. Later in 1802, Wollaston dis-
covered several dark lines in the solar spectrum by passing
the sun light through a slit. These lines were also
observed by Fraunhofer in 1817. In 1823 Fraunhofer built
the first transmission grating and measured the exact
wavelengths of these lines. The basic principal of atomic
absorption was published by Kirchoff in 1860 (1). Using
the atomic lines discovered by Fraunhofer, he was able to
deduce the presence of certain elements in the solar
atmosphere. Working with Bunsen, the use of spectral lines
for determining alkaline metals in a flame was reported (2).
The first analytical use of atomic absorption was reported
by Woodson (3) in 1939 who determined mercury in the
atmosphere. The first paper to realize the general
practical of an atomic absorption spectrometry (AAS) as an
analytical technique was published independently by Walsh
.(4) and Alkemade and Milatz (5) in 1955. Alkemade
constructed an apparatus in which the emission of an
element introduced into a flame was employed as a source
of resonance radiation. The analyte solution was
nebulized into a second flame and the decrease in the
radiation intensity from the first flame was measured.
Walsh showed the advantages of AAS over emission
spectrometry (ES) and proposed the absorbance method of
recording the signal. He proposed the design of an
apparatus for performing analysis and later in 1957 he and
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his colleagues published their first results on the
experimental developments of the technique (6). Since
then AAS has been extensively used. and has provided a
sensitive and selective technique for the analysis of
trace elements. The sensitivity and precision of AAS
is limited by the characteristics of the excitation
source and the atom production technique.
1.2. Atom Cells
This thesis is concerned with a graphite furnace
atomizer and its application to AAS. It is therefore
relevant to discuss the development of the different
types of atom cells used in atomic spectrometry.
Flames have been widely used as atomizers for the
production of atoms in both AAS and atomic flu3rescence
spectrometry (AFS). They do, however, suffer from some
disadvantages. Flame cells are not suitable for the
atomization of solid samples and the volume of sample
needed for the analysis of solutions is large. Flame
background absorption and emission at the resonance line
cause considerable noise and lower the precision of the
technique, and finally, the precise control of the
chemical environment is not possible in flame cells. It
is those limitations which have led to the development of
non-flame cells. Winefordner (7) in a study of typical
graphite cells has shown that the concentration of the
atoms can be at least 500 times higher than that in a
flame, giving rise to an increase of 10-100 times in
sensitivity for both atomic absorption and atomic emission.
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In non-flame cells the chemical environment is controlled
by the use of .an inert gas which promotes the formation of
free atoms. Careful design of non-flame cells is
necessary to limit both emission and background absorption.
Many types of non-flame cells have been developed,
these may be categorised as follows: Furnaces, Filaments,
Cathodic sputtering cells, Plasmas and Lasers.
1.2.1. Furnaces
Furnaces•yield high analytical sensitivity and a
number of different types have been described.
1.2.1a Low Temperature Furnaces
The maximum temperature which can be achieved with
these devices is around 1500°C. As a result, memory effects
may be significant and because of the low working temperature,
chemical interferences are more severe than in flames.
However, the high sensitivity obtainable to some extent
offsets these disadvantages. Mislan (8) has demonstrated
the use of a 36cm silica tube of 2.5cm i.d. as an atom cell.
The tube can be heated to a maximum temperature of 1250°C
by a wire wound resistance furnace. An indirect nebulizer
spray chamber was used to transfer the sample solutions to
the absorption tube. Cd was investigated and excellent
detection limits were obtained. Choong and Long-Seng (9)
have used an 80cm long silica tube heated to 1200°C for the
study of absorption spectra of silver vapour. Hudson (10)
has employed a stainless-steel absorption cell heated by a
resistance wire for AA measurements on sodium vapour.
Tomkins and Ercoli (11) have reported the use of a tantalum
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tube as an absorption cell. The tube could be operated
to a temperature of 1400°C and enabled them to study the
spectra of barium, calcium, thallium and radium. Black
et al (12) have described the construction of a platinum
tube furnace for use in AFS. The sample is nebulized
into the platinum tube which is heated electrically up to
a maximum temperature of 1600°C. They studied the atomic
fluorescence of Cd, Zn, Hg and Fe and showed that'this type
of furnace can be effectively used for studies on volatile
elements.
1.2.1b Graphite Furnaces
The use of high temperature graphite furnaces enables
the efficient atomization of a wide range of elements to
be achieved. King (13) in 1905 was the first to use an
electrothermal atomizer, in this case for observing the
emission spectra of elements. The arc-heated furnace
consisted of a carbon tube 16mm.o.d. and 5mm i.d.
held inside a carbon block. A hole was drilled at the
centre of the tube for admission of a carbon electrode.
An arc was formed between the furnace wall and the carbon
electrode giving rise to temperatures of 2200°C. Later
in 1908 King (14) constructed a resistively heated furnace,
The furnace was constructed around a graphite tube 15-20cm
long, 16mm o.d. and 4mm i.d. and is shown schematically in
Fig. 1.1. The tube was held at the centre of a brass
cylinder 40cm in length and 10cm in diameter. The furnace
was heated by a 200 amp. current from a 5KW transformer at
25V. During the study of the emission spectrum of
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f
a. Carbon tube
b. Brass cylinder
c. Supports for the carbon tube
d. Quartz window
e. Copper leads
f. Hydrogen gas inlet
Fig. 1.1. Schematic diagram of a resistively heated carbon furnace designed by King.
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titanium, working temperatures of 2500°C were recorded
for the inside of the tube (15,16). The furnace was
later modified and was used for the study of the emission
and absorption spectrum of iron (17) and determination of
oscillator strengths (18). The King furnace was improved
and employed for vacuum work by Paul (19) and by Codling
and co-workers (20,21) and has been the standard device
for the production of absorption spectra of metal vapours.
L'Vov (22-27) was the first to -propose a pulsed
method of atomizing samples in a graphite furnace. The
graphite furnace used in his early work was similar to
the arc atomizer of King. It consisted of a graphite tube
10cm long, lOmm external diameter and 3mm internal diameter.
A carbon electrode was used for sample introduction and an
auxiliary electrode for arcing. The furnace was pre-
heated by a 10KW transformer prior to introduction of the
sample electrode. A d.c. arc. was automatically formed
for 3-4 seconds between the sample electrode and the
auxiliary electrode as soon as the sample electrode was
moved into the furnace. The graphite tube was lined with
tantalum foil in order to reduce the loss of atomic
vapour by diffusion through the porous carbon. The whole
assembly was contained under an aluminium cover evacuated
and filled with argon to the desired pressure. Because of
the inefficiency of heating the furnace by the d.c. arc,
L'Vov and Lebedev (28) modified the furnace to enable
resistive heating of the electrode. The modified furnace
is schematically shown in Fig. 1.2. The absorption tube
was a graphite cylinder 3 - 5cm in length
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a
a. Electrode with the sample
b. Graphite tube
c. Graphite contacts.
Fig. 1.2. Electrothermal atomizer designed by Lvov.
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with 2.5-5mm internal and 6mm external diameter. The
furnace was heated by an a.c. current from a 4KW trans-
former at 10V to a maximum temperature of 3000°C. .Samples
in the form of solid or liquid were placed on an auxiliary
carbon-rod electrode 6mm in diameter heated by an a.c.
current from a 1KW transformer. The atomization system
was enclosed and puryed with an inert gas. The rod was
heated for 2-3 seconds after the tube had reached its
required temperature and the atomic absorption signal was
then recorded. The graphite tubes were coated with a layer
of Pyrolytic graphite which resulted in low gas permeability,
high heat conductivity and resistance to oxidation. Using
liquid samples of 2-5 /21 L'Vov (29) obtained atomic
absorption sensitivities for 37 elements ranging between
10-10 -14 to 10-14 g. L'Vov and Khartsyzov (30) studied the
application of the graphite furnace to the determination of
elements having their resonance lines below 190nm. Employ-
ing an argon-purged graphite furnace with lithium fluoride
lenses and windows, high frequency discharge lamp sources
and a vacuum monochromator, they were able to measure the
atomic absorption. of sulphur, iodine and phosphorus with
sensitivities of the order of lx10-1 0
g, 3x10-11
g and
3x10-12
g respectively. This furnace has also been used
for the determination of oscillator strengths and the
Lorentz widths of atomic lines (31,32).
In 1965 Massmann (33,34) simplified the L'Vov furnace
and constructed a graphite furnace for AAS. Tie furnace
consisted of a resistively heated graphite tube and was
mounted as shown in Fig.l.3. The tube was 5.5cm long,
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a. Graphite tube
b. Steel holders
c. Sample inlet port
d. Mounting holder
e. Insulation
Fig. 1.3. Schematic diagram of a resistively heated graphite furnace designed by Massmann.
(side view).
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6.5mm i.d and 1.5mm wall thickness. A hole, 2mm in
diameter, was drilled through the tube wall at the centre
to enable the introduction of liquid samples (5-200p1).
The graphite tube was water-cooled and operated in an
atmosphere of argon to prevent it from being oxidized.
The tube required 400 A at 101.7 to heat it to its maximum
temperature of 2500°C. Later Massmann (35) aevised a
cell for atomic fluorescence measurements. A cap-
shaped graphite cuvette, 4cm long, 5.5mm i.d. and 1.5mm
thickness was employed which could hold sample solutions
of 5 to 50p1. The cup was held virtically between two
stainless steel electrodes in an enclosed chamber. The
solution was placed into the cuvette and its open top and
the emission was viewed through a slit cut into the wall.
Solid samples up to lmg could also be used without any
background absorption effects. Massmann (36) has
reported atomic absorption and atomic fluorescence results
for a number of elements with good sensitivities.
Manning and Fernandez (37) have constructed a graphite
furnace similar to that of Massmann. The tube, 5cm long
and 9.5mm internal diameter, was heated by a 400 A current
at 0-10V to temperatures of 2500-2600°C and could accomodate
solid samples. Employing a three stage heating cycle,
they were able to perform the direct determination of
copper and strontium in milk. This analysis is difficult
by flame methods, without sample pre-treatment. The use-
fulness of the Massmann type furnace has been demonstrated
in the analysis of biological material (38-41) and water
(42-45).
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11
Woodriff and Co-workers (46-50) have constructed and
used a graphite tube furnace for AAS. The tube was 15cm
long, 9mm outside diameter and 7mm inside diameter and
supported inside a graphite shield tube 29cm long and
12mm inside diameter with 4mm wall thickness. The
graphite tube was heated by a current from an arc welder
to temperatures up to 3000°C. Liquid samples were
nebulized and introduced to the furnace through the side arm
by a stream of argon. A modified version of the furnace
was adapted for discrete sampling of liquids and solids.
The samples were placed in a graphite cup and introduced
through the side arm. This furnace has not been widely
used in commercial atomic absorption spectrometers because
of its large size.
Morrison and Talmi (51,52) have constructed an
inductively heated graphite furnace for the direct analysis
of solids or evaporated solutions by. AAS and AES. The
furnace, a graphite crucible of 22mm o.d., 16mm i.d. and
11cm long, was heated to temperatures of 2500°C by
consuming 4.5KW of power at 3 MHZ. The induction furnace
was used to vaporize and atomize the sample while excitation
of the atoms was achieved by the helium plasma produced by
the RF field. The absolute sensitivities for a number
- of elements were inferior to those given by Woodriff (47),
Massmann (35), West (53) and L'Vov (54) by 1-4 order of
magnitude. However, the larger samples analysed in the
RF furnace compensates for this disadvantage whenever
dealing with real samples.
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Headridge and Smith (55) have employed an induction
furnace for the determination of cadmium in solutions and
zinc-base metals. The graphite tube, 15mm i.d., 38mm
o.d. and 7.5cm long was heated to 1900°C by a six turn
induction coil coupled to a 6KW induction generator. The
same authors have reported an improved furnace design
yielding greater sensitivity (56). The furnace consisted
of a vertically mounted graphite tube, 12cm long, 38mm
o.d. and 13mm i.d., closed at the lower end and having
two hollow side arms at right angles to the tube to
provide a light path. The graphite was surrounded by
a quartz jacket and heated by the induction coil to a
temperature of 2400°C. The furnace was flushed with
argon to avoid oxidation of the graphite. Cadmium was
determined in solution and sinc-based alloys with sen-
sitivities of 0.62 ng and 0.4 ng respectively.
1.2.2. Filaments and Open Cells
1.2.2a Wire Filaments
Bunsen (57) in 1859 described the use of a platinum
loop for the sampling of various metals. Metal powders
were placed on the platinum loop and introduced into a
Bunsen flame; the emitted light was dispersed and the
spectra of the metals were recorded. Several techniques
have been developed recently which use an electrically
heated filament for the vaporization of, solution samples.
With these devices the signals are transient and are
obtained as the atomic vapour passes trhough the absorption
or fluorescence light path.
12
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Brandenburger (58) and Brandenburger and Bader (59-61)
have reported the use of a copper filament for the
determination of mercury and a platinum filament for the
determination of cad,m.ium, zinc, lead, thallium, copper,
silver and gold. The wires were coated with the element
of interest and on heating, the elements were vaporized
into the atomic absorption light path and measured. They
have claimed the method to be 100 to 10,000 times more
sensitive than with a flame atomizer.
Bratzel, Dagnall and Winefordner (62) have employed
a filament technique similar to that of Brandenburger and
Bader for atomic fluorescence studies of volatile elements.
A platinum loop was used to vaporize the sample solutions.
The vaporized element was then swept into the excitation
light path and its atomic fluorescence was measured. Using
1 Al samples, detection limits of 10-7, 10-8
and 10-15g
were reported for gallium, mercury and cadmium respectively.
West et al. (63) have reported the construction of a
tungsten filament atom reservoir for measuring trace amounts
of zinc, lead, copper and silver by AAS. They employed a
cylindrical tungsten filament 6cm long and 2.2mm in diameter
with its centre ground to locate 1 Al of sample solutions.
They compared this method with a carbon filament atom
reservoir and reported improved sensitivities. The
increased sensitivities may have been due to less pene-
tration of the solution into the filament. A study of
interference effects showed that the tungsten filament
removed or greatly reduced the interferences. This was
attributed to the greater ability of tungsten in transfer-
. ing heat to the atoms thus minimizing condensation which is
13
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responsible for many interference effects.
Tantalum and Molybdenum filaments have been employed
for the atomic absorption analysis of copper, nickel and
cobalt (64). In contrast to tantalum, molybdenum did not
react with these elements and gave reproducible results
with high sensitivities, equal or superior to those
obtained with the Massmann furnace (44). However,
nitrate ions at concentrations higher than 0.01M reacted
with Mo and caused permanent damage to the filament. For
the elements studied, the molybdenum filament showed a
substantial decrease in the observed interference effects
when compared with a graphite filament.
Newton and Davis (66) have described an electrically
heated tungsten-alloy wire loop atomizer. The solution
could be introduced either directly with a 5plpipet, or by
submerging the metal wire into the solution for a certain
time, or by electrodeposition. Using different methods of
sampling, detection limits were reported for 19 elements
and compared with results obtained using flame and graphite
atomizers. Newton.et al.(67) have also employed a tungsten-
rhodium wire loop for the determination of cadmium and
lead by AAS. The loop was soaked in the solution for a
certain period of time and the metal ions were pre-
concentrated onto the surface. The loop was then
electrically heated and the atomic absorption signal
measured. Using this technique, detection limits of
- 4x10 14g and 2x10
-11g were reported for cadmium and lead
respectively. This method of sampling gave up to 100
times increase in sensitivity for some elements over the
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15
standard flameless wire loop technique.
Williams and Piepmier (68) have constructed and
operated a tungsten filament atomizer for the determination
of calcium, chromium, copper, iron, magnesium, manganese
and tin. The filament, obtained from a light bulb, was
heated by a 6V power supply at 4 A and sheathed with a
flow of argon. Using 3/11 sample solutions, sensitivities
comparable with those of conventional methods were
obtained.
Mounce, Dagnall, Sharp and West (69) have reported
the construction of a tantalum loop atomizer for the atomic
fluorescence determination of bismuth in a non-dispersive
system. The tantalum wire, 0.25mm in diameter, was
sheathed in argon. The flow rate of the argon was found
to have a profound effect on the sensitivity of the method.
A detection limit of 3.2x10-11g was obtained with a re-
producibility of 6.5%. The important features of metal
filament atomizers are low background emission in the U.V.,
a non-reactive evaporation surface and a low power require-
ment.
1.2.2b Graphite Filaments
West and Williams (70) have reported the construction
of a simple and yet sensitive atom cell for use in both AAS
and AFS. It consisted of a graphite filament, 20mm long
and 2mm in diameter, heated up to 2500°C by passing a current
of about 100 A at 5V. The two filament supporting elect-
rodes were water cooled and the whole assembly was housed
in a chamber purged with argon. The detection limit for
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16
magnesium and silver was 10-10 g for AAS and 10-16 and
3x10-11
g respectively for AFS. These results represented
gains in sensitivity of 3-7 orders of magnitude over
flame methods. Alder and West (71) modified the original
atom cell in which an open filament with inert gas shield-
ing was used. Employing this device; atomic fluorescence
(72-75) and atomic absorption (76-81) measurements on a
range of elements were reported.
Amos (82) has described a furnace termed the "Mini-
Massmann" which was similar to that of the West and
Williams filament cell with two modifications. A trans-
verse hole, 1.5mm in diameter, was drilled into the rod
which acted as a cavity and smaller liquid samples (0.5
to 1/41) could be accommodated. The argon or nitrogen
shielding gas was replaced by hydrogen which on ignition
formed a diffusion flame. This device has been used for
the determination of various elements in blood and lub-
ricating oils (83,84).
Winefordner el al.(85) have constructed an electrically
heated graphite filament similar to that of West and
Williams. The rod had a cavity for locating the sample.
They have reported atomic absorption determinations of
arsenic, chromium, copper, iron, nickel, lead, silver, gold
and maanesium in aqueous solutions, blood and lubricating
1.2.2c Sample Boats
Donega and Burgess (86) have described a device which
used sample boats cut to 50mm x 6mm size from lmm tantalum
-
17.
or tungsten foil, or 5mm graphite sheet. The boats were
capable of holding 50 to 100 )l of solution and were heated,
by a variac at 12V and 30-50 A to 2200°C in less than 0.1
second. The boats were supported by two copper rods and
enclosed in an inert atmosphere maintained eta pressure of
between 1 and 300 torr.
Hwang et al.(87) in a more thorough study of the
tantalum ribbon atomizer, have reported the sensitivities
for 36 elements. Suzuki et al.(88) .have employed a
similar cell for the trace analysis of aluminium, chromium,
copper, iron, magnesium and manganese. The metal strip
was flushed by an inert gas and powered by a low voltage
(0-110V) transformer at 20 A. Nickel, tungsten, tantalum
and platinum were investigated as the strip material.
Small signals were obtained with platinum and both platinum
and nickel could not be used at high temperatures. Tantalum
and tungsten showed similar results but tantalum was
preferred because it was easier to fabricate. Continuum
emission from the tantalum above 400 nm, limited its
application to those elements having their resonance lines
at longer wavelengths. The advantage of this type of
atomizer is the low power required for electrothermal
heating.
.1.2.3. Cathodic Sputtering Cells
The sputtering process is a very convenient and
efficient method of producing an atomic vapour directly
from a metanic sample. Paschen (89) in 1916 developed the
hollow-cathode tube as a spectral line source, and in 1959
Russell and Walsh (90) suggested its use both as a line
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18
source and an atom cell in AAS. Later in 1960, Walsh et
al.(91,92) constructed a hollow-cathode tube and a sputt-
ering chamber for atomization of metallic samples. The
metal sample in the form of a cylindrical hollow-cathode,
4cm long and 12mm i.d. was fitted into a spring clip in
the lid of the stainless steel sputtering chamber. The
tube was then sealed and filled with argon to lmm Hg
pressure and the discharge initiated. The chamber which
was fitted with silica windows was then placed in the
light path of'an atomic absorption spectrometer. The cell
was used for the determination of silver in copper (92).
The technique is restricted to metal samples as the cathode
has to be prepared from the sample itself.
Goleb and Brody (93) modified the Walsh cell and
produced a device in which the cathode was made of aluminium.
Sample solutions were evaporated onto the inside wall of the
cathode. Using this technique, they were able to detect
microgram quantities of refractory and non-refractory
elements.
Ivanov et al.(94) have described the construction of
a graphite hollow-cathode sputtering cell. The sample was
evaporated onto a fine molybdenum wire which was then placed
in the central axis of the hollow-cathode. Copper and
calcium were investigated with this device. A pulsed dis-
charge has been used to produce atomic vapours in a neon-
filled hollow-cathode lamp (95). Copper and magnesium were
determined in this way without any interference from the
emission spectra.
Gandrud and Skogerboe (96) have reported the use of
-
19
a hollow-cathode discharge cell to detect nanogram amounts
of silver, arsenic, calcium, cadmium, mercury, antimony,
selenium and zinc. Sample solutions were evaporated onto
graphite or aluminium discs which were then placed on a
brass rod cathode. The cell was run at 3 torr of argon,
and 50 mA discharge current. Kirkbright and Wilson (97)
modified this device and were able to detect u.03,pg of
iodine using the 183 nm line.
Massmann (98) has constructed a hot hollow-cathode
cell for the analysis of solid samples by atomic absorpt-
ion. It consisted of a graphite tube, 30mm long, 7mm i.d.
and 9.3mm o.d., supported on a small cylindrical graphite
electrode. The cathode assembly was mounted on a molybdenum
rod 2mm in diameter. The water-cooled, earthed, brass base
plate of the assembly housing acted as the anode. The
cell was fitted with silica windows and purged with argon.
A discharge was operated at 1-10 torr and powers up to 1KW.
Detection limits were reported for the determination by AAS
of silver, antimony, zinc, copper, cadmium, magnesium,
manganese and chromium in 30mg samples of relatively
volatile matrices.
Hot hollow-cathode cells have been found to have
advantages over cool cathodes. With cool cathodes, the
ccalLinuum radiation from the cathode and also the spectrum .
emitted by the sample interferes seriously with the
absorption measurements. However, hot hollow-cathode cells
can only be employed for the determination of relatively
low volatility elements as they can not be heated above
.2000°C. Because of the low pressure inside the hollow-
-
cathode, the residence time of the atomic vapour in the
absorption volume is short and therefore the method is
not. suitable for detection of small amounts of elements.
1.2.4. Plasmas
A plasma is defined as a mass of vapour or gas in
which a significant fraction of the atoms or molecules
are ionized. A classification of plasma types has been
given by Sharp (99). He divided plasmas into two types,
flame-like and non-flame-like plasmas, and these are brief-
ly' discussed in this section.
1.2.4.1 Non-flame-like Plasmas.
In this type of plasma, the discharge is confined to
a column joining the current-carrying electrodes. Arcs
and sparks are examples of this type of plasma.
1.2.4.1aArcs
The arc is a sensitive source capable of detecting
elements below the limits of detection of a spark. An
arc is an electric discharge which has to be initiated
either be momentary' mechanical connection across the
electrode gap or by means of an auxiliary spark. The
necessary components for a d.c. arc are a direct current
power supply, a variable resistor and a discharge gap.
The d.c. source has to be capable of furnishing of voltage
from 50 to 300V at 1 to 30 A. When a substance is placed
into the arc, the high temperature of the arc (3000 to
8000°K) causes volatilization and excitation of the atoms.
High voltage a.c. arcs have also been used which employ a
potential difference of 100V or more. A.C. arcs have been
20
-
21
found to be steadiex and more reproducible than the d.c arcs.
Belydev (100,101) have employed a d.c. arc for the
determination of cadmium and silver in graphite matrices.
A graphite electrode, heated by a 6 A arc, was used to
raise the temperature and hence to atomize the elements.
The technique was ,recommended for high and average
volatility elements and the detection limits were 10 to
100 times better than by flame emission. Kantor and
Erdey (102) have used a time-resolved a.c. arc atomizer
and high intensity spectral lamps for AAS. Marinkovic
and Vickers (103) coupled a conventional nebulizer-spray
system to an arc and used the system for the determination
of aluminium, boron, magnesium, vanadium and tungsten and
achieved sensitivities comparable to flame AAS.
1.2.4.1b Sparks
An a.c.spark is capable of producing much higher
excitation energies than the a.c. arc with less heating
effect. The circuitry for sparks is simple and very
similar to that of an a.c. arc. The spark is produced
by connecting a high-voltage transformer (10-50KV) across
the two electrodes. The spark is more reproducible and
more stable than arc. Because of the lower heating effect,
it is well suited for the analysis of low melting materials
and can be readily adapted for the analysis of solutions.
Robinson (104,105) has applied a spark initiated in a
flame and in a nebulized analyte solution in an attempt to
produce free aluminium atoms for AAS. The later system
was found to be effective and aluminium was determined
with a sensitivity of 3 PPm at 394.4 nm.
-
22
1.2.42 Flame-like Plasmas
This type is characterised by existing with a signific-
ant portion of the discharge external to the main core into
which power is coupled. There are two main types of
flame-like plasma; the d.c. arc plasma and the high
frequency plasma.
A plasma torch is a device designed to heat gases to
very high temperatures by taking the advantages of the high
conductivity of an ionized gas. The conventional d.c.
plasma torch requires electrodes for carrying energy to
the gas and is characterised by operating at high current
(100-1000 A) and low voltage (10-100V). Employing an
electrode coupled plasma torch atomizer, Friend and
Diefenderfer (106) have reported the determination of two
refractory elements, aluminium and lanthanum, by AAS.
Their results show that the formation of refractory oxides
is eliminated when a plasma system is employed. The d.c.
plasma torch has not been extensively utilized in AAS.
The inductivity-coupled radio frequency plasma torch
(ICP) was first described by Reed (107-109) in 1961. In
a typical radio frequency plasma torch developed by Fassel
(110), argon is introduced tangentially into the annular
tube gap between the two outer quartz producing a low-pressure
region at the end of the inner tube. Argon is also
introduced into the inner tube producing a low-velocity
laminar flow on which the plasma operates. A third
injector tube is added for the introduction of'sample into
the centre of the plasma. The induction coil, a water-
cooled copper pipe, is coupled to a RF generator giving
-
23
2 to 30KW output at 5 to 40 MHZ. A graphite rod is
introduced into the coil which is. conductively heated
and produces thermal electrons ( a tesla spark may also
be used). The free electrons are accelerated by the
magnetic field causing a further breakdown in the gas.
The neutral argon is then heated to temperatures of up
to 10,000°K on collision with the energetic charged
particles. The usefulness of the ICP as a practical
source for analytical spectrometry has been demon-
strated by Fassel et al.(111) and Greenfield (112).
Although usually used in emission mode, the determination
of aluminium, calcium, magnesium, neodymium, rhenium,
titanium, tungsten, vanadium and ytterbium by AAS with
good sensitivities have been reported. ICP's have
also been applied to AAS by several other workers (113-115).
The high temperatures available with ICPrs minimises the
chemical interferences caused by the formation of stable
refractory compounds. The much greater energy available
in the plasma makes it possible to vaporize refractory
solid samples directly. The background radiation is
also markedly less than with flame.
1.2.5. Lasers
The development of lasers in recent years has made
, it possible to directly atomize solid samples for atomic
absorption analysis (116,117). The technique of atom-
izing samples by a pulsed laser beam has been described
in detail by Karyakin et al.(118-120). A parallel beam
of light generated by a laser is focussed onto the sample.
The intense heat produced by the pulsed energy (a few
-
24
joules to a few tens of joules) raises the temperature
to 5000-10,000°C and on evaporization, emission or
absorption is observed in the atomic vapour.
Hagenah et al.(121) have described the effectiveness
of a laser pulse as an atomization source and have reported
the most detailed experimental results on the use of lasers
in AAS. They were able to determine copper, silver,
calcium in graphite with sensitivities at the RPM level.
1.3. Conclusion
Electrothermal atomizers have provided a considerable
improvement in sensitivity for many elements by AAS. This
is due to the transfer of the complete sample aliquot to
the atomizer thereby overcoming the inefficiency of a
nebulization system and avoiding the large dilution of
atomic population which occurs in the flame cell. Graphite
furnaces have been found to be superior to filaments.
Filaments are more susceptable to interferences as there is
no energy available above the filament to be imparted to
the atoms leaving the heated surface. In furnaces, the
atomic vapour is confined within the tube extending the
residence time of the free atoms and hence improving the
sensitivity. They also produce a slow rise time signal
amenable to modulated a.c. amplification and the use of
- strip chart recorders. In filaments, the transient nature
of the signals requires faster electronics and a rapid
response recorder. The power consumption for furnaces
is larger than that of filaments. However, furnaces
allow larger sample solutions or solids to be analysed.
The discrete sampling technique can only be applied with
-
small sample aliquots, and often this results in poor
reproducibility. When furnaces are used with continuous
sampling, reproducibility is much better but the sensit-
ivity is considerably decreased due to the inefficiency
of the nebulization process. However, for many elements,
these sensitivities are better than those obtained by the
flame technique. In addition, techniques such as "depos-
ition" can be employed to improVe sensitivities (see
chapter 1V).
25
-
CHAPTER II
THEORY OF ATOMIC ABSORPTION SPECTROMETRY
-
26
2.1. Principles
Atomic absorption spectrometry is defined as the
absorption of radiant energy from an external light source
by atoms. By receiving energy from radiation of a
particular wavelength, atoms become excited and reach a
higher energy level. The transitions between low and
high energy levels produce the atomic spectrum of, the
element. For most elements, the characteristic
absorption wavelength is the resonance line which results
from transitions from the ground state to the lowest excited
state. The resonance line is the strongest in the
absorption spectrum because it involves the lowest energy.
Further information concerning the theory of the resonance
radiation and excited atoms may be obtained in the book by
Mitchelland Zemansky (122). '
2.2. The Intensity of a Spectral Line •
The intensity of a spectral line produced by emitting
or absorbing atoms depends on two factors. Firstly, with
a given number of atoms in each energy level depends on the
intrinsic properties of the atom; the greater the probab-
ility of a transition, the more intense is the corres-
ponding spectral line. Secondly, the number of transitions
depends on the number of atoms available at the initial
energy level from which the transition occurs. The
relative population of an atom's level is an important
factor in the determination of the relative intensity of the
spectral lines: The Maxwell-Boltzmann law can be easily
adapted for the calculation of relative population of the
-
27
quantized levels. Walsh (4) has shown that the number of
atoms in the first excited state is only a small fraction
of the number of atoms in the ground state. The ratio
of atoms in the excited state to the ground state becomes
appreciable only at high temperatures and for transitions
of long wavelengths. The fraction of atoms in the higher
excited states is even less, hence the number of absorbing
atoms in the ground state can be considered to be equal
to the total number of free atoms. As a result, the
temperature rarely affects this equality unless there are
excited states just above the ground state. An under-
standing of the relative population of atomic states is
necessary in the selection of appropriate absorption
lines.
2.3. The Width of Spectral Lines
Even the sharpest spectral line has a finite width and
this has an important consequence in the application of both
AAS and AFS. The width of an absorption line is governed
by the following factors:
a) Natural broadening due to the finite lifetime of
an atom in the excited state (--10-8
sec.) which ranges
between 10-5 - 10-4 nm.
b) Doppler broadening due to the random thermal
motion of atoms relative to the observer which is of
the order of 10-3
- 10-2 nm for the spectral lines of
most elements.
c) Lorentz broadening due to the collision of emitt-
ing or absorbing species with foreign atoms or mole-
cules.
-
28
d) Holtsmark or resonance broadening due to the
collision of like atoms.
e) Stark broadening due to the external electric
fields or charged particles.
f) Self absorption broadening which is important
only for lines having their lower level at or near
the ground state. Self absorption happens as a
result of absorption of radiation by the present
atoms. Self reversal is a particular case of self
absorption in the presence of a temperature gradiant.
2.4. Variation of Atomic Absorption with Concentration
The relationship between atomic absorption and atomic
concentration has been fully published (122-124) and will
be breifly discussed here. The theory of AAS can be best
described in terms of three quantities; the absorption
coefficient, the total absorption factor and the absorbance.
2,4a The Absorption Coefficient, Kv
The absorption coefficient, K , is defined by
o -K L I = I (a ./) (2-1)
whereIci andIare the incident and transmitted intensities
of radiation of frequency 1) passing through an absorption
cell of having a path length of L. The dependence of Kv on
frequency is often represented by the Voigt function (125)
which includes both Doppler and Lorentz broadening.
2.4b The Total Absorption Factor, AT
The total absorption factor is defined in terms of the
total radiation energy passing through the cell and is
given by:
-
29
I -I AI AT = 9 = • : Io
I
(2-2)
where AI x 100 is defined as the percentage of absorption I
2.4c The Absorption, A
Absorbance, A, is defined by the following expression:
I A =.log
A is related to AT
by :
= log ( 1 ) 1-A
T
(2-3)
(2-4)
Absorbance can easily be calculated, assuming K is p
constant, and provides a linear relationship with atomic
concentration over a wide concentration range.
2.5. 'Theoretical As ects of Atomization Process
This thesis is concerned with a continuously operated
graphite furnace, and is thus relevant to discuss briefly
the theoretical background to the atomization process.
Most of the work on furnace atomic absorption has been
carried out on practical analysis and only a few authors
have investigated the process of atom formation. Two
approaches have been proposed to study the mechanism.of
atomization and these are discussed in this section.
2.5.1. Thermodynamic Approach
In order to understand the thermodynamic approach,
the various reactions occuring in an atomizer must be
considered. It has been found that with carbon furnace
0
and can be determined experimentally.
-
30
atomizers, oxy-anion salts of metal solutions are prefered
to those of the halide. This is because the halide
solutions tend to give a higher degree of molecular
volatilization and so the degree of atomization is decreas-
ed. ' Most oxy-anion salts are decomposed to the metal
oxide on heating.
Maessen and Posma (126) have assumed that atomization
occurs by dissociation of the metal oxide. The dissociat-
ion of a metal oxide, MO, can be shown by the following
equation:
MO M 0 2 2
The free energy of this system, AG, is given by
-AG = RT Ln K
(2-5)
(2-6)
where R is the gas constant, T the temperature and K the
equilibrium constant. The degree of dissociation of the
metal oxide,v, is related to partial pressure of oxygen by:
K 104-. = p
KP + 4P(02)
(2-7)
By knowing AG, K and hence pk.can be calculated. In carbon
atomizers, however, there are two more reactions to consider:
C + 202 CO (2-8)
CO + 202 CO2 (2-9)
,These reactions control the partial pressure of oxygen and,
hence, the degree of dissociation of the meatal oxide. From
a knowledge of partial pressure of oxygen they predicted that
the order of atomization for cobalt, lead, gold and zinc
would be Zn4:Pb4Au4Co.
-
The most extensive studies into the mechanism of
atomization have been carried out by Campbell and Ottaway
(127). These authors have suggested that in the carbon
furnace atomizer, carbon plays a role in the direct
reduction of metal compounds to produce atoms. One good
example is magnesium sulphate which is converted to
magnesium oxide at 890°C and sublimes without decomposition
to gaseous molecules of magnesium oxide at 2770°C.
Magnesium, however, can be determined with very good
sensitivities using a carbon furnace at temperatures of
about 1550°C. This indicates that carbon is effective in
the direct reduction of metal compounds to atoms.
A possible mechanism of reduction of metal oxides to
atoms by graphite is shown in equation (2-1o).
MO(s) + C(0- ---0=C0(g) + M(g) (2-10)
The energy released in this reaction.would seem to be
sufficient for the production of metal atoms in the gaseous
state. Assuming that the reduction of metal oxides by
carbon is rapid, Campbell and Ottaway have proposed that the
temperature at which free metal atoms are formed can be
determined. This can be achieved by calculating the
temperature at which the free energy, AGo, for the react-
,ion (2-10) becomes zero. They have calculated the lowest
temperatures for the production of atoms for 27 elements.
Table (2-1) shows the thermodynamic reduction temperatures
and appearance temperatures together with corresponding
melting and boiling points. The appearance temperature is
defined as the lowest temperature at which a substantial
31
-
Table 2.1. Thermodynamic Reduction Temperatures and • Appearance Temperatures for Various Elements Together with Melting and Boiling Points for Comparison
Oxide Element m.p.,K Element b.p.,K Temperature Appearance at which AG° temperature becomes negative, K of element,
Pb0 600 2042 1000-1100 1,000
Al203 932 2720 2400-2500 2,300
Cu20 1356 2855 1800-1900 1,730
Fe304 1812 3160 1700-1800 1,750
Na20 371 1163 1200-1300 1,230
Ni0 1728 3110 1700-1800 1,800
Cr2 03 2 76 2915 1800-1900 1,800
ZnO 693 1.181 1200 • 1;100
Co0 1768 3150 1800 1,720
Sc203 1673 2750 2400-2500 2,450
Cd0 594 1038 800 850
V203 2190 3650 2400-2500 2,350
Si.02 1683 2950 2300 2,300
Ba0 983 1910 2300 2,200
TiO2 1 1950 3550 2400-2500 2,420
Li02 454 1604 1900-2000 2,100
Mn304 1517 2314 1600-1700 1,600
Ag20 1234 2450 1200-1300 1,150
K
-
Table 2.1. continued
Oxide Element m.p.tK Element b.p.,K Temperature Appearance
at which AG° temperature becomes' negative, K of element, K
Sr0 1043 1640 2300 2,100
Hg0 234 629 Below room temp.
B203 2300 4200 AG°react still+ve
Nb0 2770 5200 3000
Sn02 505 2960 1350 1,800
K20 336 1039 1100 1,550
Sb203 903 1910 1100-1200 1,550
Ca0 1123 1765 2400-2500 1,800
Mg0 923 1390 2100-2200 1,550
-
34
population of atoms is produced in.the carbon furnace.
These workers have found good agreement between
experimentally determined appearance temperatures and the
ones predicted by thermodynamic calculations for 22
elements out of 27 elements.
Aggett and Sprott (128) have compared the appearance
temperatures of various elements in both graphite and
tantalum atomizers. Comparison of these temperatures
indicates whether or not oxide reduction by carbon is
the cause of atom formation. For the majority of elements,
the minimum atomization temperature from tantalum was
within 100°C of the atomization temperature from carbon.
Of the 15 elements studied, only the minimum vaporization
temperature of cobalt, iron and tin from carbon were signif-
icantly lower than that from tantalum. It was suggested
that for these four elements, the free atoms are formed
through the direct carbon reduction of the metal oxides.
To check this conclusion, the most likely reaction between
the metal oxide and graphite was postulated and the free
energy for that reaction was calculated at the temperature
when atoms were first observed from - carbon furnaces. For
aluminium, cadmium, calcium, magnesium, manganese and zinc,
the free energies were positive and the most likely •
mechanism of atom formation would be the dissociation of
metal oxides. For cobalt, iron, nickel and tin, however,
the free energies were negative indicating that reduction
of the oxide by carbon is thermodynamically feasible. It
was concluded that for these elements, carbon acts in the
reduction of metal oxides to free atoms.
-
35
Czobik and Matousek (129) have also investigated the
mechanism of atom formatibn in the carbon furncace. They
have reported the effect of various anions as acids•on the
atomization temperature of silver, cadmium, copper, nickel,
lead, tin and zinc. Of the anions investigated, only
phosphate affected the atomization temperatures. Addition
of phosphate resulted in an increased atomization temperat-
ure for the elements Cd, Zn, Ag and Pb which atomized at
a lower temperature than tin and had no effect for elements
Cu,Ni and Cr which atomized at higher temperatures than
tin. Two mechanisms of atom formation were then suggested.
The first mechanism involves reduction ofmetal oxides by
carbon and is only applicable to compounds which can form
oxides at temperatures lower than those required for
reduction process to occur e.g., Cu, Ni, Cr. The second
mechanism is thermal dissociation of metal compounds and is
applicable to compounds of higher thermal stability which
decompose at temperatures higher than those required for the
reduction process e.g.,Ag, Cd, Pb, Zn.
The thermodynamic approach has been sucessful in
explaining some of the observed effects in electrothermal
atomizers, however, it does not explain why the elements
Al, Ca, Cr, Mo, Ti and V form thermodynamically stable
carbides at and below the temperatures at which the free
energy for reaction (2-10) is zero. Neither does it
provide any indication of atomization rates or prediction
of absorption peak shapes.
2.5.2. Kinetic Approach
The models which have been proposed for studying the
-
kinetics of atomization can be classified into two groups,
(a) atomization under increasing temperature and (b) atom-
ization under isothermal conditions.
2.5.2a Atomization Under Increasing Temperature
L,Vov (130) has proposed a general kinetic approach
for the production of metal atoms in a graphite cuvette in
which the sample is vaporized under constantly increasing
temperature conditions. This method can also he applied
to rod and filament atomizers as well as easily atomized
elements at high temperatures in a furnace.
The rate equation for the change in the number of
atoms in a graphite furnace can be written as:
• dN _ dt -I' ) n2(t)
(2-11)
Where dN is the rate of change in the number of atoms, N, dt
present in the gaseous state in the atomizer at time t,
nI(t) the number of atoms entering the system at time t and
n2(t) the number of atoms escaping from it at time t.
Because it is assumed that the evaporation of the substance
occurs at a constantly increasing temperature, atom
formation is a linear function of time and is given by:
n (t) = At
(2-12)
Where A is a constant. The integration of n1(t) from
time 0 to t i , the atomization time, would be equal to the
total number of atoms in the sample, No, thus:
I n i (t) dt = No (2-13)
or n .(t) = (2-14) t1-
36
-
Assuming that atoms are removed from the furnace by
diffusion, then:
37
n ) = 2(t • t2 (2-15)
Where t2 is the residence time of atoms in the system. By
substituting equation (2-14) and (2-15) into equation (2-11)
the following equation is obtained:
dN =2Not dt t 2 t2 (2-16)
Solution of equation (2-15) describes the number of atoms,
N, in the system at time t.
For t < t1
0
e t12
t12 t2
Or
At time t =
vaporized,
t92
= 2No - 1 e
-t
(2-17)
(2-18)
t2 )
completely
- t
t- 2 t2 (
tl, when the sample has been
Nt reaches its maximum value:
2 2 1 - 1 + :- 2No e t2 Nt=t t, 1 t2 1
For t>ti equation (2-15) must be modified to account for
the fact that the sample introduction step has been completed
and only simple diffusional decay follows, hence:
dN =-N dt t
2
2 t - 1
(2-19)
N =
ft
2No t2 t2
2 t to dt = 2No
-
-t
1
+
(2-18)
(2-20)
(2-21)
into equation
-t tl-t
t2
_ t2
can
equation
t1 t7 t2
(2-22)e
to give the number
2
be used
t
Solving this equation yields:
38
Equation (2-17) and(2-22)
of atoms, N, in the system at time t.
Torsi and Tessari (131-134) have developed a kinetic
modal for determination of atomization energies using a
carbon filament atomizer. They have obtained an
atomization energy of 405 KJ mol-1
(95 Kcal mol-1
) for
the production of Ni atoms, which suggests that the atomic
species are produced as a result of sublimation of the
metal.
Johnston, Sharp, West and Dagnall (135) have
employed a similar approach to examine the nature of atom
production from a non-mechanistic viewpoint. They have
assumed that the vaporization of solid analyte and, hence,
the equilibrium concentration of gaseous analyte species
are governed by a simple Boltzmann factor. They have
N
t dN
Ntl
Nt = Nt e 1
Substituting for Nt from
(2-21) gives:
2
N = 2No t2
t1 2
-
associated various energies with the vaporization process
which are then characteristic of the analyte. These
energies are related to either the heat of vaporization
of the metal or to the metal oxide bond dissociation
energy, which ever is larger.
2.5.2b Atomization Under Isothermal Conditions
Fuller (136), has measured the variation in atomic
concentration in a graphite furnace under isothermal
conditions. The rate equation for the change in the
amount of atoms can be written as:
dN dt• ni(t) -n2(t) (2-23)
The notations are the same as used earlier. The rate
of formation of atoms was found to follow first order
kinetics and is given by:
nl(t) = Kl(No-Nt) (2-24)
Where. No is the initial amount of element, Nt the amount
of element atomized up to time t and K1 the rate constant
for the atomization process.
Integration of equation (2-24) yields:
= N0(1-e -;.K1t
(2-25)
Substituting for Nt into equation (2-24) gives (2-26)
-K1t ni(t) = KiNoe
(2-26)
The rate of loss of atoms from the furnace is
dependent on the amount of atoms present in the furnace at
time t and is given by:
39
-
n2(t) = K2N
(2-27)
Where K2 is the rate constant for the removal of atoms
from the furnace and is proportional to the flow rate of
inert gas through the furnace. Substituting equations
(2-26) and (2-27) into equation (2-23) produces equation
(2-28) which on intenration gives the concentration of
atoms at any time.
-K1t
dN dt
=K1Noe K2N
(2-28)
-K1t -K2t
K1 N No e e ) (2-29) K2-K1
Fuller's approach is applicable to furnace atomizers,
particularly at low temperature and for elements which
are difficult to atomize.
Kinetic measurements can be used to postulate the
mechanism of atom formation. There are four possible
mechanisms which fit simple kinetics for the formation
of atoms in a graphite furnace.
1) MO(s/l) Fast 1 M(s/l) + 2 02
M(s/l) Sl M(9) ow
2) --- MO(s/1)+ C Fast--0- M( §/1) + CO
M(s/l) ----o- Slow M (g)
3) MO(s/1) ------a- Slow M(s/1)
2. + 0
M(s/l) Fast M(g)
-
41
4) MO(s/1)+ C Slow
m (s/l) + CO
Fast M(s/1) (g)
Fuller (136) has employed the kinetic approach to
investigate the mechanism of atom formation for copper.
Kinetic measurements were carried out under the same
conditions in the presence of carbon and in the absence
of carbon but with the presence of a different reducing
agent. If the results are identical, then the most
probable mechanism would be 1,2 or 3, if, however,
atomization does not occur or it'occurs but at different
rates, then mechanism 2 and 4 would be most probable.
Experiments were carried out with plain carbon tube and
tantalum-lined tubes. The results showed that the product-
ion rate of copper atoms was considerably greater with the
tantalum liner present. From the results it was concluded
that atomization normally occurs through reduction by
carbon and that this process is the most probable rate
controlling step in the production of copper atoms. The
mechanism of atom formation for copper was therefore
concluded to be as follows:
- Slow 0+(s.--•- ?Cu CO 2 - (s/l)
Fast Cu (g) (s/l)
Kinetic theories of atomization can be used to
investigate the method of signal measurement. The common
methods of measuring signals in AAS are the measurement of
-
At the time t = t1, when the sample is completely atomized,
the number of atoms reaches its peak value, Npeak' Using t1
equation (2-17) and assuming then t2
42
an equilibrium absorbance in the flame technique (4) and
peak absorbance in the non-flame technique (137). An
alternative method has been described by Massmann (138).
The theoretical advantage of this method has been shown by
L'Vov (130).
Considering equation (2-17) as the value of t increases
the magnitude of number of atoms tends towards an equilib-
rium va.lueN , given by: equil t2
= Nequil N — o t1 (2-30)
= Npeak No (2-31)
In this case the peak value corresponds to the total
number of atoms in the sample, No.
The integration method is based on measuring the
integral value Q of atoms during the entire period of
production of atoms. The relationship between Q and
the number of atoms, N, is given by:
t3 QN Ndt
(2-32)
0
where t3 is the length of time during which the signal is
recorded. By substituting equations (2-17) and (2-22)
into equation (2-32), and after integration, provided that
t3 - t1 4 t2 then QN
= No t2 (2-33)
-
The ratio of the magnitude of the signal to the total
number of atoms can be used as a measure of absolute
sensitivity. For the equilibrium method of signaL
measurement
t2 Nequil t2 for — < 1 No
t1 t,
(2-34)
Thus it can be seen that rapid introduction of sample
into the system and also long residence time results in
higher sensitivity.
For the peak method:
N
peak - t1
-
44
particularly advantageous.
2.6. Interferences with Electrothermal Atomizers
Electrothermal atomizers exhibit considerably more
susceptibility to interferences than the equivalent flame
techniques. Many interference effects have been reported
but only a few authors have explained the cause of these
interferences.
Employing a carbon rod atomizer, Amos et al.(139)
have studied the interference effects encountered in
determination of lead and compared their results with carbon
tube atomization. It was shown that chemical and spectral
interferences were decreased when the carbon tube atomizer
was used. A decrease in the carrier gas flow rate also
resulted in a slight decrease in the chemical interferences
due to the increased residence time of the atoms in the
reducing environment. It was concluded that the short
residence time of the atoms produced by a carbon rod was
responsible for the higher level of interference.
Robinson (140.) has reported the construction of a
carbon hollow T atomizer. The atomization takes place in
the stem of the atomizer and the absorption measurement is
carried out along the cross piece. This design yielded
greater atomization efficiency together with complete
destruction of the solvent resulting in reduction of chemical
interferences and removal of molecular absorption.
The addition of salts (141) and acids (142-145) has
been used to overcome some of the interferences observed
in graphite atomizers. The integration method of signal
measurement has been found to be effective in removing
-
vaporization interferences (146-148).
The interferences encountered in furnace atomizers
may be classified into three types:-
physical, spectral and chemical.
2.6.1. Physical Interferences
2.6.1a Sample Introduction Interferences
Because of the temperature gradient existing along
most atomizers, sample size and positioning of the sample
is of great importance when discrete sampling is used.
In the case of continuous sample introduction, the physical
process which occurs during nebulization have an important
effect on the sensitivity of determination. Signal
integration can be used to reduce some of variations
resulting from variable sampling.
2.6.1b Memory Effects
Incomplete atomization of an element causes
accumulation of that element on the atomizer with sub-
sequent enhancement in the analytical signals. This type
of interference occurs with elements which form stable
refractory oxides. The use of higher atomization temper-
ature or longer atomization time can minimize this inter-
ference. Unlike furnaces, memory effects are rarely
- observed with filaments.
2.6.2. aectral Interferences
2.6.2a Line Overlap Effects
This kind of spectral interference is very rare in AAS.
Fassel et al. (149) have reported that spectral line inter-
45
-
ferences may occur when there is a significant overlap of
the Primary Source emission line profile with the
absorption line profiles of any interfering species in
the atom cell. These interferences are usually avoided
by selecting an alternative interference free absorption
line.
2.6.2b Emission Radiation Effects
These are produced by emission from the heated
atomizer and other elements in the sample. Light
emission from the carbon follows ah approximately black-
body curve and therefore the effect is more serious when
measurements are made in the visible region of the spectrum.
This interfering emission can be largely eliminated by
using a modulated source lamp and a lock in amplifier
detector. The signals from atom cell are not modulated
and are therefore rejected by the measurement system.
2.6.2c Scattering Effects
Large concentrations of matrix vaporized during the
atomization stage, can lead to scattering of the incident
light beam. Light scattering was first demonstrated by
Willis (150) and thought to be the cause of light losses
observed in AAS. Later, Koirtyohann and Pickett (151)
demonstrated that molecular absorption rather than
scattering was responsible at least in part for the light
losses reported by Willis (150).
2.6.2d Molecular Absorption Interferences
This type of interference is caused by molecular
species vaporized during the atomization stage. Like
46
-
47
scattering, molecular absorption results in false absorbance
signals which must be corrected. There are three methods
of background correction; the use of a continuum source,
the adjacent line method and use of the Zeeman effect.
The use of a continuum source was first described by
Koirtyohann and Pickett (152). This involves measuring
the non-atomic absorption signal at an adjacent wavelength
using a deuterium arc. Hydrogen and deuterium hollow
cathode lamps have also been employed as continuum sources.
This technique, however, can not be used above 350 nm due
to the lack of intensity of the source. The adjacent
line technique invloves measuring the absorbance at a
non-absorbing line which is adjacent to the analyte
resonance line. The Zeeman effect employs a magnetic
field to split the spectral lines from the light source to
produce non-absorbing lines outside the atomic absorption
profile. It has recently been employed (153) to correct
molecular absorption inEaectrothermal atomizers.
Alkali halides are the most serious cause of molecular
absorption interferences (154,155). Employing a graphite
furnace, Wilson and Kirkbright (156) have reported the
determination of iodine from iodine - containing salts at
206.1 nm. When they used a deuterium lamp to correct for
_non-specific absorption, the iodine absorption signal was
not observed. A further study by Kirkbright et al.(157)
indicated that the absorption signal observed at 206.1 nm was
due to molecular potassium iodide vapour produced in the
graphite furnace. Solutions of sodium and potassium
sulfate, nitrate, chloride, bromide and iodide were also
-
48
investigated. For sulphates and nitrates of sodium and
potassium, no absorption was observed in the range 190 nm
to 360 nm. It was then suggested that on heating,. these
salts are decomposed to the corresponding metal oxides.
This is then followed by the reduction of the metal
oxides by carbon as demonstrated by Ottaway et al. (127).
Yasudu and Kakiyama (158) have described,the absorption
spectra observed for the halides, sulfates and nitrates
of the transition metals. The resu.lts showed that the
gaseous metal halides were formed and vaporized in the
atomizer at temperatures of 300-500°C. Metal sulfates
and nitrates were decomposed to the metal oxides on heating.
This was confirmed by observing the absorption spectrum of
sulfur dioxide and nitric oxide when metal sulfates and
nitrates were heated to about 300-400 and 100-150°C
respectively.
Ediger (159) has reported a method of interference
modification using a chemical agent. For the determin-
ation of cadmium in the presence of 0.15mg sodium chloride,
the non-atomic absorption signal was so large that even
with background correction the cadmium signal was not
observed. Addition of ammonium nitrate to the furnace
reduced the non-atomic absorption signal considerably.
- This was due to coversion of the sodium chloride to a
more volatile compound. Employing this technique, Ediger
et al.(160) were able to determine copper in sea water with
a graphite furnace. The detection limit was found to
be 10-3
PPm compared, with 0.03 PPm obtained by direct
injection of sea water (43).
-
2.6.3. Chemical Interferences
2.6.3a Anion/Cation Interferences
As mentioned earlier, the presence of oxyanions does
not seem to interfere with elemental determinations. With
halides, on the other hand, a loss of element can occur
through vaporization of the molecular halide. Losses of
lead as volatile PbC12 and Pb Cl have been reported (161,
162) in determination of lead in the presence of chloride.
It was found that treating sample with hydrogen and ashing
at temperatures of about 730°C removed the chloride inter-
ference. Cation interferences are more complex and have
not been generally explained.
2.6.3b Pyrolysis' Losses
When using discrete sampling, a pyrolysis stage is
often employed prior to atomization. As a result, the
sample matrix is broken down or vaporized giving rise to
reduced interference effects. Fuller (163) has reported
losses of copper and nickel during the pre-atomization
stage at temperatures above 600°C. It was suggested
that metal oxides are formed on heating which are then
reduced to the corresponding metals by carbon from the
furnace. The metals are then lost through evaporation.
Losses of mercury (164), lead, cadmium, berylium and
vanadium (165) have also been reported.
2.6.3c Condensation
After atomization of an element, the atoms cool
quickly by entering the cooler gas layer above the atom-
izer and condense rapidly. In the presence of other
49
-
50
elements which are atomized or vaporized at similar
temperatures,.the analyte element becomes occluded with the
interfering elements on condensation. This type of inter-
ference is of importance mainly with filament atomizers
because of the steep temperature gradient above the
. atomizer surface. West et al.(73,74) have demonstrated
that many inter-element interferences happen as a result
of a gas-phase interaction. It was suggested that
absorption measurements under limited field viewing
condition should minimize the interference effects (76).
A horizontal slit was placed across the primary source
which shielded the region where most inter-element effects
would likely occur. Jackson and West (78) reported that
the only interference observed with limited field viewing
was from elements of similar volatility to the analyte
atoms.
2.6.3d Carbide Formation
Losses of atomic vapour may occur as a result of
the formation of involatile carbides between the analyte
and the incandenscent graphite. Pyrolytic coating of
the graphite tube has been found to reduce carbide formation
considerably (166) and hence enhances the sensitivities for
many elements. Sturgeon and Chakrabarti (167) in a
study of the mechanism of atom loss in graphite furnace
showed that the pyrolytic coating considerably reduced
the diffusion losses of atoms through the graphite walls.
Mo and V were investigated and the results showed that the
rate of loss of molybdenum and vanadium atomic vapour from
-
51
uncoated tubes was increased by 21% and 12% respectively, •
over that obtained with coated tubes.
Although carbide formation has been found to be
disadvantageous, it can be employed to passivate tubes and
therefore prevent the analyte forming a carbide. Fisher
et.al.(168) have reported a 10 fold signal enhancement for
the determination of beryllium after the carbon was treated
with zirconium. The zirconium carbide on the surface of
the carbon prevented formation of beryllium carbide and
hence improved its sensitivity. These authors have also
reported enhancement factors of 2.6, 1.8 and 2.5 for
manganese, chromium and aluminium respectively when the
furnace was treated with lanthanum.
2.6.3e Nitride Formation
If nitrogen is used as the purge gas, some elements
may form thermally stable nitrides (169) and hence the
sensitivity is reduced. L'Vov (170) has demonstrated
that a loss of Al can occur by reaction with nitrogen
in the presece of incandescent graphite to form thermally
stable aluminium nitride.
Sturgeon et.al.(171) have reported a 60 per cent
reduction in aluminium sensitivity when nitrogen was
substituted for argon as the sheath gas. Atomization
of aluminium in a tantalum of tungsten lined atomizer,
however, produced equal sensitivities for both argon and
nitrogen gases. Substituting pyrolytic graphite for
standard graphite decreased the above difference by 25 per cent.
It was suggested that carbon plays a role in the formation
-
of the nitride as shown in equation (2-37)
Al203(s)+ 3C0 (2-37) + 3C
(s) + N2(g) 2A1N(s)
Aluminium nitride is stable up to 2500°K.
2.6.3f Ionization
Ottaway and Shaw (172) have investigated the
possibility of ionization interferences in carbon furnace
AAS and AES. Barium was chosen as a test element and
signals observed for atomic absorption and emission and
also ion absorption. Addition of caesium to barium
suppressed the ionic absorption signal. However, in
contrast to flame atomization (173), the suppression of
ionization did not produce any increase in the atomic
absorption signal. The reason was thought to be that
the ion population is negligible compared with the atom
population. From the results obtained they conclude
that ionization interferences would not be significant in
carbon furnace spectrometry.
52
-
CHAPTER III
INSTRUMENTATION
-
53
3.1. Graphite Furnace Atomizer System
The graphite furnace used in this study is a
modification of the device described by West and Williams
(70). A schematic diagram of the furnace is shown in
Fig.3.1. The whole assembly is mounted on a stainless
steel base and housed in a glass chamber fitted with a
vertical stem at the top centre for sample introduction and
another one at the side as the exhaust outlet. The space
between the electrodes is enclosed. by two insulating
syndanio plates at the side and another at the top with a
hole at the centre for sample introduction. These plates
were found to disintegrate gradually at high temperatures
and so stainless- steel plates were used to line and
protect their inner surface from high temperatures.
The graphite tubes 20mm long and 5mm internal dia-
meter were machined from Ringsdorff high purity graphite
rods. A hole of 3.5mm in diameter was drilled at the
centre of the tube to provide introduction of the
sample aerosol into the tube. In order to obtain good
electrical contact between the electrodes and graphite
tube, graphite rings were machined to hold the tube.
The electrodes used for this system were rather bulky
and caused condensation of water on their surfaces. These
electrodes were later replaced by two L-shaped electrodes
which were much thinner than the early designs and showed
no condensation.
3.2. Power Supply
When operating a graphite furnace continuously, a
-
54
b
a
C
e
a. Graphite. tube
b. Stainless steel electrodes
c. Stainless steel base
d. Water inlet
e. Water outlet
f. Argon inlet
Fig.3.l. Graphite furnace system used for this study (side view).
-
55
large power is usually needed for heating. In this study
the power was supplied by wiring three transformers (12V
and 100 A) in parallel which could provide a maximum
current of 300 A at 12V. Two copper strips 75cm long
and 2.5cm wide with a thickness of 3mm were used for
transferring the electrical power to the electrodes. The
voltage supplied to the furnace could be varr:;ed from 0 to
12V by a variac transformer (270V and 20A) connected to a
30A single phase a.c. power supply.
3.3. Temperature Measurements
An optical pyrometer was employed to measure the
temperature of the graphite furnace for this study. This
was achieved by focussing the radiation from the inner wall
of the tube at the viewing lens of the optical pyrometer.
At voltages less than 4V, however, there was no significant
radiation from the tube and a thermocouple had to be used.
The thermocouple was a chrome-alumel type and was inserted
inside the tube and in contact with it during temperature
measurements. The maximum measurable temperature with
this type of thermobouple was 1,000°C and so measurements
were carried out up to 4.5 volts. The results are shown
in Fig.3.2. The data were obtained while nebulizing
distilled water at an uptake rate of 1.9 1.min-1.
3.4. Purge Gas
An inert gas was continuously flushed through the
graphite furnace in order to maintain a non-oxidizing
environment and hence increase the lifetime of the graphite
tubes. The three gases commonly used for this purpose are
-
3200
2800
2400
2000 -
1600 -
1200
800
400
2 4 6 8 10 12
Voltage, V
Fig.3.2. Temperature vs. voltage curves for the furnace. (a) Thermocouple measurements, (b) optical pyrometer measurements.
U 0
Tem
pera
ture
,
56
-
.57
argon, nitrogen and hydrogen. It has been xeported that
argon yields the maximum sensitivity for the determination
of lead (87). Argon has the lowest specific heat and
thermal conductivity when compared with the other gases.
It seems that the thermal properties of the purge gas
have a significant effect on sensitivity for many elements.
Manning et al.(37) ih investigating the choice of gases
for analytical measurements reported a three times reduct-
ion in aluminium sensitivity with nitrogen as the purge
gas. Other workers (166) have also reported a signif-
icant reduction in aluminium sensitivity by substituting
nitrogen for argon. This may be due to formation of
involatile aluminium nitride.
Amos et al.(139) have reported the use of an argon-
hydrogen mixture which yielded a reduction of interference
effects together with improved sensitivity for many
elements. On heating, the hydrogen ignites spontaneously
and burns as alargon-hydrogen-air entrained diffusion flame.
It has been shown that the temperature of the flame around
the graphite does not exceed 500°C. It was concluded
therefore that the flame temperature does not contribute
significantly to the atomization process. The enhance-
ment was attributed to the highly reducing environment
caused by the presence of hydrogen which promotes higher
atomization efficiency. A 20-fold improvement in alumin-
ium sensitivity was obtained when hydrogen was introduced
into argon in the ratio 2 argon to 1 hydrogen.
It was mentioned earlier that oxygen must be excluded
from the graphite atomizer to avoid oxidation of the graphite.
-
58
Some workers (174,175), however, have added oxygen