Instrumentation
R. E. Sturgeon 1 Metal Ions Group Department of Chemistry Carleton University Ottawa, Ont., Canada K1S 5B6
Factors Affecting Atomization and Measurement in
Atomic absorption spectrometry (AAS) is a highly specific means of elemental analysis based on the selective absorption of line radiation by atomic species in the vapor phase. To observe AA signals it is necessary to generate a population of free, neutral atoms of the element of interest. Although combustion flames are the most widely used media for the atomization of samples (conversion of the sample into atomic vapor) in AAS, the demand for better sensitivity and limits of detection, the necessity for more economical use of the sample, and the fundamental limitations of flame techniques have led to the development of a variety of electrothermal (flameless) atomizers as an alternative atomization medium (1-3). The advantages inherent in the application of electrothermal (ET) atomization to AAS are well recognized: the possibility of determining a large number of elements with high sensitivity, selectivity, accuracy, and speed, coupled with the simplicity and relatively low cost of apparatus, makes AAS with ET atomization potentially ideal for trace and ultratrace analysis. It is therefore not surprising that the technique has been shown to be of considerable value for the detection and quantitative determination of trace amounts of metals and metalloids in a variety of matrices (1-5). Despite widespread use, however, the basic principles of operation of ET atomizers remain largely unclear. The purpose of this article is to focus attention on some fundamental aspects of electrothermal atomization and methods of recording AA signals generated by these devices. Discussion 1 Present address, Division of Chemistry, National Research Council of Canada, Montreal Road, Ottawa, Ont., Canada KIA 0R6.
Graphite Furnace Atomic Absorption Spectrometry will be limited to tube-type graphite furnaces of the Massmann design (6), shown in Figure 1, since these are currently the most popular models commercially available. In most cases, however, the principles covered will also apply to ET atomizers of less familiar design. An understanding of their fundamental operation will identify those factors important in the design of more effective ET atomizers as well as provide a useful theoretical framework from which interpretation of analytical data may be drawn. Principles of Operation Commercial ET atomizers are resistively heated devices that generally employ a three-stage heating program to dry, char, and atomize the sample. Power is supplied from a low-voltage (~ 10 V), high-current source capable of delivering up to 500 A. The temperature of the atomizer is a complicated function of time and various other factors such as the input power, the mass of the atomizer, and its heat losses by convection, conduction, and radiation. Figure 2 shows the time-dependent central temperature of the interior surface of a Perkin-Elmer HGA-2100 atomizer (a popular commercial atomizer of the Massmann design) at various levels of input power. The time axis commences with the initiation of the atomization stage of the heating program. The 500 K intercept common to each heating curve is the temperature of the atomizer at the completion of the charring stage. In all cases, the temperature asymptotically approaches an upper steady-state value that reflects the balance between the input power and the heat losses. The initial rate of heating of the atomizer as well as its steady-state temperature is proportional to the input power. The general shape of these heating curves applies to all
Steel Holder
Graphite Washers
C3. Electrode with " Sample
Figure 1 . Massmann furnace for (a) A A S ; (b) L'vov graphite cuvette
commercial atomizers employing a three-stage heating program. Figure 3 shows the relationship between the temperature of the surface of the atomizer and the absorbancetime characteristics of the signals from a number of elements atomized in the HGA-2100 (the magnitudes of the signal peaks are not drawn to scale as the relative peak heights for the elements vary approximately in the ratio 250:5:3:1 for Cd:Cu:Cr:Mo). In contrast to the steady-state signals obtained with the flame AA technique, analytical signals generated by ET atomizers are transient pulses of, at most, a few seconds duration, their exact shape for a given element being determined by the physical and chem-
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977 • 1255 A
2400-
3000-
2300-
*
Ga
2200 —
2500-
2100 — ^
2000-
2000-
1
1900-
1
1800-
E 1700-
Time, s Figure 2. Temperature-time characteristics of interior surface of Perkin-Elmer HGA-2100 at various power outputs of atomization stage • 3.2 kW O 2.6 kW A 1.8 kW A 1.3 kW
omization medium and the atomic vapor is rapidly changing. Figure 4 shows the spectroscopically measured excitation temperature (two-line absorption method) of In, Ga, Fe, and Sn vapor produced in the HGA-2100 (11). The measurement of vapor temperature by spectroscopic methods is equivalent to the measurement of the population ratio of the absorbing species included within the viewing field of the spectrometer, and to the determination of an effective temperature that corresponds to this ratio according to a Boltzmann distribution:
3000-
i
10
11
Figure 3. Absorbance-time profiles of a number of elements superimposed on temperature-time profile of surface of HGA-2100. Input power = 3.2 kW
ical properties of the sample matrix, by the properties of the atomizer (geometry, construction material), its rate of heating, and by the distortion of the signal caused by the finite response time of the amplifier-recorder system. The signal shapes are similar to those observed for consecutive firstorder chemical reactions, i.e., the concentration of free atoms (product) is kinetically controlled by the rates of two opposing processes: the rate of introduction of atoms into the analytical volume of the atomizer (the analytical volume being defined by the geometry of the beam of source radiation that is within the viewing field of the spectrometer) and their rate of loss from
Figure 4. Excitation temperature-time profiles of atomic vapor in HGA-2100 with heating rate of 1.23 K m s " ' of atomizer
it, the signal maximum marking the point of balance between the two rates. The temperature at which rapid atomization commences (appearance temperature) is uniquely determined by the physicochemical properties of each element and is a constant, independent of the rate of heating of the atomizer, its geometry, and surface characteristics (7-10). Unlike the flame and the L'vov graphite cuvette technique (to be discussed), sample atoms produced by commercial ET atomizers are released at specific minimum temperatures into a non isothermal environment, and the signal is recorded during a time period over which the temperature of both the at-
1256 A • ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
£2 exp \-(E2 - £i)AT e d = J ^ n2(x)dx/ £
ni(x)dx
(1)
where n\(x) and ri2(x) are the densities of the thermometric species in the lower and upper quantum states of the transition, and I is the optical path length. Consequently, an average or weighted average temperature is not measured (12); rather, the effective temperature is applicable only to the population ratio of states 1 and 2. Although the steady-state temperature of the atomizer was 3000 K, the measured excitation temperature varied with the element atomized. Maximum excitation temperatures attained by the vapor are lower than the temperatures of the atomizer surface and do not coincide with the peaks of the absorption pulses of the elements. The gradient in the surface temperature along the optical axis of the atomizer (spatial nonisothermality) is of primary importance in determining the temperature-time characteristics of
the atomic vapor. T h e magnitude of the thermal gradient is dependent on the geometry of the atomizer, its material of construction, and the arrangement by which the electrical contacts supply power to the atomizer. T h e thermal gradient is also time dependent since it is proportional to the instantaneous temperature of the center of the atomizer [thermal gradients in excess of 1000 K c m - 1 exist in the HGA-2100 when the central temperature is 3000 K ( / / ) ] . During atomization, atomic vapor is distributed throughout the volume of the atomizer by a process of accelerated diffusion of vapor from the center to t h e cooler extremities of the atomizer (13). T h e rate of diffusion across any imaginary plane within t h e atomizer is affected by the instantaneous gradient in the temperature of the atomizer across the plane. T h e atomic vapor is in thermal equilibrium with the wall of the atomizer at each point throughout its length; hence, the flux of atoms across the thermal gradient quenches t h e population of the excited electronic state, lowering the measured excitation temperature. As a result, the excitation temperature-time characteristics of the atomic vapor are dependent on the physicochemical properties of the thermometric species (atomic weight a n d size, reactivity, appearance temperature) and reflect the temporal and spatial nonisothermality of the atomizer.
Signal Measurement T h e most widely used method of measuring signals given by E T atomizers is the measurement of the maximum or peak absorbance attained. An alternative method is to record the integrated absorbance, obtained by measuring the area under the signal pulse over the period of time during which free atoms reside within the analytical volume of the atomizer. L'vov
Table I. Integrated Absorbance by 2.5 X 10 1 0 g Tl for Various Rates of Sample Atomization a Peak absorbance
Integrated absorbance
0.72 0.66 0.51 0.32 0.24 0.16 0.13 0.09
1.7 1.7 1.8 2.0 1.7 1.7 1.9 1.7
* Reproduced from ref. 15; cuvette: length 40 mm. diameter 2.5 mm. T = 1670 K. pressure = 4 atm Ar.
Table II. Determination of Copper in Solid Materials3 Copper, % Certified Found 6
Sample
MgO Fe 2 0 3 Cr 2 0 3 Al 2 0 3 ZnO Al-Mg ferrite Ni-Zn ferrite BaC0 3 BaC0 3
0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.01
0.0027 0.0028 0.0027 0.0028 0.0027 0.0034 0.0030 0.0029 0.0088
* Reproducedfromref. 17. b Calibration carried out using aqueous standards and measured by integration.
and coworkers (14-16) have assessed the relative merits of both methods of signal measurement from a theoretical viewpoint. By considering only the kinetics of mass transfer of sample vapor through the analytical volume of an isothermal cuvette (Figure 1), with the assumptions t h a t the element to be determined is completely atomized and t h a t removal of atomic vapor from the analytical volume is determined solely by diffusion under a concentration gradient, expressions were derived relating t h e peak and integrated absorbance to analyte mass. T h e peak absorbance is, as expected, a complex function of both the time taken to atomize the sample and the residence time of the atomic vapor within the analytical volume, i.e., Ap =
kA2N0^
n
fc-
1 + exp ( - n / r 2 )
(2)
whereas the integrated absorbance is dependent solely on the residence time, i.e.,
X'
Atdt
= kAN(>T2 = A0T2
(3)
where Ap is the peak absorbance, kA is a constant relating absorbance to atom concentration, N„ is the number of atoms of the element to be determined in the sample, T\ is the atomization time—the time duration of transfer of atoms into the analytical volume, T2 is the residence time of the atomic vapor—the mean length of time spent by an atom within the analytical volume, and A0 is the absorbance t h a t would result if all of the analyte atoms were confined within the analytical volume a t a single instant of time. If a sample can be completely atomized within a time « r 2 , then from Equation 2 kAN„ = An
1258 A • ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER
1977
(4)
in which case t h e peak absorbance corresponds to a measure of the total number of atoms in the sample. Provided T I / T 2 « 1, small variations in the value of T\ and/or T 2 t h a t may take place during measurements have only a minor effect on the relationship expressed by Equation 4. If, however, T\ > r 2 the peak absorbance depends on both TI and T 2 SO t h a t a simple relationship between Ap and N„ is possible only if TI/T 2 = constant. T h e integrated absorbance is neither depend e n t on the rate nor on the time, TJ, of entry of sample vapor into the analytical volume. This is extremely significant since measurement of J o Atdt permits one to prevent varying sample composition from affecting the analytical results. T h e avoidance of peak height measurement also allows for less demanding power requirements for atomization and consequent lower operating temperatures of the atomizer. Additionally, the possibility of measuring the absorption given by low volatility metals arises, such as tungsten, whose p t a k sensitivity is extremely poor. L'vov and coworkers (5, 15, 17) have successfully used the integration method of measurement for the analysis of trace metals in a variety of matrices. With the use of an isothermal graphite cuvette employing separate heating to vaporize and atomize the sample, as shown in Figure 1, the integrated absorbance is constant, independent of the rate of sample atomization (15) and also of the sample composition (17). Table I presents results obtained by L'vov et al. (15) for the atomization of 2.5 X 1 0 - 1 0 g T l in a graphite cuvette at various levels of additional heating of the electrode and without additional heating (to vary the rate of sample atomization). T h e integrated absorbance remains practically constant despite the considerable differences in rates of atomization t h a t are reflected, indirectly, in the magnitude of the peak absorbance. T h u s , with a properly designed cell, the integration method of measurement is not influenced by the kinetics of sample entry into the analytical volume. Table II presents the results obtained for the determination of copper in various powdered materials (17). T h e copper content was calculated using a calibration curve prepared from an aqueous solution of pure copper. Agreement between the certified and measured contents is confirmation of the advantages of the integration method of measurement because the analytical results were independ e n t of the sample composition. T h e principles of operation of commercial E T atomizers are similar to those of the graphite cuvette; however, the environment into which the atom-
Table III. Effect of Atomization Power on Peak and Integrated Absorbancea Element
Cd
Zn
Cu
Al
Sn
Power, W
AD
3150 2580 1800 1320 1000 600 460 3150 2580 1440 830 680 3150 2800 2370 3150 2580 1950 3150 2580 1950
0.144 0.155 0.171 0.142 0.108 0.048 0.022 0.257 0.298 0.273 0.141 0.135 0.220 0.215 0.140 0.236 0.196 0.040 0.312 0.214 0.128
fAtdt
0.066 0.091 0.137 0.178 0.195 0.220 0.198 0.145 0.214 0.263 0.332 0.273 0.430 0.450 0.420 0.436 0.380 0.186 0.482 0.442 0.404
* HGA-2100; Ar sheath gas, internal purge gas interrupt.
ic vapor is passed is distinctly different. In view of this, it remains to compare the analytical performance of commercial ET atomizers with that of the graphite cuvette, as well as to consider the applicability of signal integration to these devices.
Analytical Results L'vov's model for signal measurement is based on the pulsed atomization of samples, the atomic vapor subsequently passing into an isothermal cuvette. It is evident from earlier discussion that, with commercial ET atomizers, introduction of the sample into the analytical volume takes place over a comparatively long period of time (Figure 3) and into an environment of rapidly rising temperature, complicated by the presence of temperature-dependent thermal gradients. Additionally, loss of atomic vapor from the analytical volume is not by a process of simple diffusion. The primary factors that influence the signal-time characteristics of all elements are the rate of heating of the atomizer and its geometry. Both parameters play a significant role in determining the sensitivity of a measurement and will be briefly considered in assessing the analytical performance of commercial ET atomizers. A third parameter that can also alter the kinetics of atom formation is the physicochemical nature of the graphite surface (porosity, reactivity, density, permeability). This aspect will not be considered in this discussion since its
effects are most pronounced with refractory analytes, there being little effect on medium and high volatility analytes (10). The effect of the atomization power on the peak and integrated absorbance by a number of elements in the HGA-2100 is shown in Table III. The peak absorbance is highly dependent on the kinetics of formation and loss of atoms, as predicted by Equation 2. Numerous authors have studied the effects of the rate of heating of the atomizer (atomization power) on the peak absorbance, the reported relationships varying from linear-to-exponential increases in the signal peak to a decrease in the signal peak as the rate of heating is increased. Because the rate at which a sample evaporates (and also the rate of molecular dissociation) depends on the rate of increase of temperature, at temperatures at which the analyte has a significant vapor pressure, it is important that the atomizer attain a sufficiently high temperature before a sizable fraction of the element is lost from the analytical volume. On the other hand, the temperature of the atomizer should not be increased more than is necessary to completely vaporize (and dissociate) the analyte, since the rate of loss of atomic vapor becomes too large. Table IV shows the effect of the atomization power on the measured atomization and residence times for a number of elements in the HGA-2100. Although the measured TI and T 2 values only approximate the true magnitudes of these parameters, because of the difficulty in experimentally measuring these variables (18), the data are useful for the trends which
Table IV. Effect of Atomization Power on Ratio T-,/T2 with HGA-2100 Element
Power, W T I , ms
T2, rns T J / T 2
3150 220 2580 240 2200 270 1800 320 1440 390 3150 230 2580 250 2200 290 1440 430 1240 500 3150 930 2580 1060 2200 1190 3150 760 2580 1010 2200 1470 3150 820 2580 1190 2200 1450
220 1.0 260 0.9 270 1.0 310 1.0 360 1.1 260 0.9 280 0.9 380 0.8 640 0.7 880 0.6 1140 0.8 1360 0.8 1520 0.8 1180 0.7 1690 0.6 3420 0.4 700 1.2 1110 1.1 1360 1.1
1260 A • ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
they show. In all cases, the r\ and T 2 values decrease with increasing atomization power: r\ because the rate of atomization increases, T 2 because the upper temperature of the atomizer increases; hence, the rate of loss of atomic vapor increases. It is the inseparability of the rate of heating of the atomizer and the maximum temperature which it attains that presents a major problem with commercial ET atomizers employing three-stage heating programs. The advantages of using the maximum rate of heating (minimum TI) cannot be fully realized because of this effect. The ratio TI/T 2 for most elements, measured under optimum conditions of atomization, is close to unity (18). The low rates of heating (large TI) and unfavorable rate of loss of atomic vapor (small T 2 ) result in a peak absorbance less than the maximum potentially attainable (cf. Equation 4). In contrast to the predictions of Equation 3, the integrated absorbance is also highly dependent on the kinetics of atom formation and loss in commercial ET atomizers (7, 8, 18). This is largely the result of the variation of T2 with atomization power, as shown in Table IV. In addition to its thermal characteristics, the geometry of the atomizer also exerts a large effect on the peak and integrated absorbance. Absorbance is proportional not to the total number of atoms present within the atomizer, but to the effective length of the absorbing volume, f0 N(x)dx, where N(x) is the concentration of atoms at point x in an absorbing volume of total length /. L'vov (5) has shown that the effective length of the absorbing volume at the peak of the absorbance pulse is given by:
I C'N(x)dx\ L »/0
=NJS
(5)
J peak
where S is the cross-sectional area of the atomizer. The integral value of the effective length of the absorbing volume, QNI, is given by: QM=T2N0/S
= IW0/8DS
(6)
where D is the diffusion coefficient characterizing the loss of atomic vapor. Equations 5 and 6 have been derived with the assumptions that (a) atomic vapor is uniformly distributed over the cross section of the atomizer; (b) TI/T2 « 1, hence, the number of
atoms within the atomizer at the peak of the absorbance pulse is Na (Equation 4); (c) Equation 3 is valid; and (d) the loss of atomic vapor is by diffusion from the center to the extremities of the atomizer. For comparison of the performance of different atomizers the ratio of the effective length of the absorbing volume to the total number of atoms of an element is important (19).
Table V. Experimental Sensitivity Ratios Between HGA-2100 and CRA-63* Element
Cd Zn Sn Cu Al Mo
Peak method, g HGA CRA
5.4 X
9.5 X
10-13 2.2 X 10~ 1 3 2.9 X 10~ 11 8.7 X 10~ 12 1.1 X
10-14 8.4 X 10-14 9.1 X 10~ 12 2.7 X 10~ 12 1.3 X 7.5 X
itr 1 1
V
io - 1 1
1.1 X 1 0
HGA/CRA ratio
- n
10
4.6 X 10-n
5.7
3.3 X
5.2 X
0.9
10-13 4.1 X 10-13 1.9 X 10~ 11 1.1 X 1CT11 2.7 X
10-13 6.3 X 10-13 4.8 X 10"11 6.7 X 10~ 12 4.8 X
1.5
6.3 X
7.7 X
2.6 3.2 3.2
-12
3.1 X -n
Integration method, g HGA CRA
1.5
10
io - 1 1
CRA/HGA ratio
1.6 1.5 2.5 0.6 1.8
io~11
10-12
10
2.7 X -n
5.5 X 10-n
10
delay the release of atomic vapor from the sample (20, 21), thereby varying the thermal characteristics of the environment into which the analyte a t o m s are released (Figure 3). Such an effect will alter the kinetics of formation and loss of atomic vapor (since both processes are t e m p e r a t u r e dep e n d e n t ) and hence the measured peak height a n d integrated absorbance. In addition to the kinetics of atomization and sample matrix composition affecting the integrated absorbance, signal integration with commercial E T atomizers offers little improvement over the peak height m e t h od of m e a s u r e m e n t with respect to sensitivity (Table V), limit of detection, precision, and linear dynamic range of working curves (7, 8). T h e advantages of signal integration predicted by theory cannot be realized with existing commercial E T atomizers. It is evident t h a t these atomizers do not meet t h e requirements of correct use of the integration method of measurement, primarily because of their temporal and spatial nonisothermality as well as t h e inseparability of the rate of heating from the maximum temperature attained by these devices. T a b l e VI summarizes the assumptions made in applying the peak height and integration m e t h o d s of signal measurement, and the suitability of commercial E T atomizers in meeting t h e m .
1.2
-12
2.0
* Obtained using pyrolytic-graphite-coated tubes. Sensitivity defined as the mass of element in grams which gives a peak absorbance of 0.0044 or an integrated absorbance of 0.0044 absorbance X second.
W i t h the peak method of measurem e n t one gets:
| f N(x)dx] L«/0
/N„= \/S (7)
J peak/
and for the integration method of measurement: QNI/N„
= /2/8
DS
(8)
Differences in sensitivity between the peak and integration m e t h o d s of meas u r e m e n t for various electrothermal atomizers may be compared with the use of E q u a t i o n s 7 and 8. Table V compares the ratio of t h e peak and integrated sensitivity for a n u m b e r of ele m e n t s using the HGA-2100 and a Varian Techtron CRA-63. A diffusion coefficient of 5 cm 2 s _ 1 was selected as a reasonable value of D in an argon atmosphere at 2000 K (5, page 288). From the geometry of the two atomizers, Equations 7 and 8 predict t h a t the HGA-2100 will give a 2.5-fold increase in the integration m e t h o d of measurem e n t over t h a t given by the CRA-63, and t h a t the peak height method of m e a s u r e m e n t with the CRA-63 offers a fourfold increase in sensitivity over t h a t given by the HGA-2100. T a b l e V shows t h a t the differences in sensitivities between the atomizers are roughly as predicted for Cd, Zn, a n d Sn. Agreement between theoretical and experimental values for other medium and low volatility elements is poor because of the nonisothermality of the HGA-2100 atomizer along its length as compared to the uniformly heated CRA-63 (11). With high volatility elements this problem is not severe since with constant heating the subsequent t e m p e r a t u r e increases as the extremities of the atomizer reevaporate the condensed analyte into the analytical volume. W i t h low volatility elements
the final t e m p e r a t u r e at the extremities of the atomizer is not sufficient for rapid reevaporation of the condensate. As a result, the effective residence time of atomic vapor is decreased, i.e., t h e characteristic value of / for the HGA-2100 is not the geometric length of the atomizer b u t the length of its hot "working zone". An additional factor t h a t can influence the peak height and-integrated absorbance is the n a t u r e of the analyte matrix. Depending on its composition, the matrix may accelerate or
Despite numerous shortcomings, commercial E T atomizers remain excellent analytical tools for trace metal
Table VI. Applicability of Peak Height and Integration Methods of Signal Measurement with Commercial ET Atomizers Assumption All sample a t o m s enter analytical volume
Diffusional loss of atomic vapor under a concentration gradient
A,dt=
e.
s;
Ntdt =
A0T2
At = kAN0
1262 A • ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
N0
T2 = constant
r \ « 4)
''peak = A0
s:
Applicability to commercial ET atomizers
Condition
T
2 (Equation
Many elements f o r m stable carbides and are not vaporized f r o m surface of atomizer Temporal and spatial nonisothermality of atomizer causes T 2 to be sensitive to atomization conditions and sample matrix Generally, T-\ ~ T2 with tube-type atomizers; hence, m a x i m u m peak sensitivity is not realized
Assumptions a and b above
Integrated a b s o r b a n c e is highly sensitive to atomization conditions
Suitable r RC for detector system
Response t i m e of read-out system may be too slow to record signals without distortion
analyses. Although many analyses are routinely accomplished under nonoptimal conditions, the peak height method of recording has proved to be a reliable and valuable measurement technique, provided its limitations are realized and the proper precautions are taken during its use. It is becoming increasingly evident, however, that ET atomization is subject to severe matrix interference effects as a result of variations in matrix composition influencing the kinetics of formation and loss of atomic vapor. In such circumstances, peak height measurement is not practicable. Superior analyses would be obtained if the integration method of measurement could be successfully applied. Conclusions
The unsuitable thermal characteristics of present atomizers are not conducive to the correct use of the integration method of signal measurement. In this respect, a number of factors are considered important in the design and operation of more efficient ET atomizers. • The atomizer should be tubular in design to contain the atomic and molecular vapor. This improves vapor-tube wall thermal equilibration and increases the vapor temperature
(as opposed to atomizers of the open design, i.e., filament devices), thus promoting molecular dissociation. • The atomizer should be constructed of pyrolytic graphite. This material has the desirable properties of low permeability to gases, low porosity, high purity, a high sublimation temperature, a high resistance to oxidation, and a high thermal conductivity. Additionally, incandescent graphite provides a highly reducing environment that aids in the reduction of thermally stable oxides to metals. • The atomizer should be capable of achieving high rates of heating. This maximizes the rate of analyte evaporation and the rate of molecular dissociation, thereby increasing the instantaneous atom population within the analytical volume, minimizing the amount of analyte lost by diffusion both as atoms and molecules, reducing the extent of penetration of the analyte into the pores of the surface of the graphite tube, and minimizing the extent of fractional atomization of elements from samples with different volatilities. • The maximum temperature attained by the atomizer must be made to be independent of its rate of heating. Separation of the rate of heating from the maximum temperature to be
attained by the atomizer allows an optimum steady-state temperature to be set for each element which should be high enough to prevent vapor condensation but low enough to prevent excessive diffusional loss from the analytical volume. • Temporal and spatial nonisothermality of the atomizer must be minimized, both to generate higher atomic vapor temperatures (promoting molecular dissociation) and to reduce the concentration gradient within the atomizer (hence, loss of vapor). Nonisothermality may be eliminated or reduced through proper design of the atomizer, use of suitable construction material, and by the use of high rates of heating. • The atomizer should be as long as possible, its upper length being limited by the requirements of higher power consumption, of achieving rapid heating, of maintaining a condition of isothermality along the length of the tube and a reasonable throughput of the emission source radiation. The integral value of the effective length of the absorbing volume is proportional to the square of the length of the hot working zone of the atomizer. • The atomizer tube should be of minimum cross-sectional area to maximize both the effective length of the
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1264 A • ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
absorbing volume a t t h e peak of t h e absorbance pulse a n d t h e integral value of t h e effective length of t h e a b sorbing volume. T h e m i n i m u m crosssectional area will be determined by t h e t h r o u g h p u t of t h e emission source radiation. • T h e atomizer t u b e should be of m i n i m u m mass t o maximize its rate of heating. References (1) G. F. Kirkbright, Analyst (London), 96,609 (1971). (2) A. Syty, Crit. Rev. Anal. Chem., 4,155 (1974). (3) R. Woodriff, Appl. Spectrosc, 28,413 (1974). (4) G. M. Hieftje, T. R. Copeland, and D. R. deOlivares, Anal. Chem., 48,142R (1976). (5) B. V. L'vov, "Atomic Absorption Spectrochemical Methods of Analysis", Adam Hilger, London, England, 1970. (6) If. Massmann, Spectrochim. Acta, 23B, 215 (1968). (7) R. E. Sturgeon, C. L. Chakrabarti, I. S. Maines, and P. C. Bertels, Anal. Chem., 47,1240 (1975). (8) R. E. Sturgeon, C. L. Chakrabarti, and P. C. Bertels, ibid., p 1250. (9) R. E. Sturgeon, C. L. Chakrabarti, and C. H. Langford, ibid., 48,1792 (1976). (10) R. E. Sturgeon and C. L. Chakrabarti, ibid., 49, 90 (1977). (11) R. E. Sturgeon and C. L. Chakrabarti, Spectrochim. Acta, 32B, 231 (1977). (12) I. Reif, V. A. Fassel, and R. N. Kniseley, ibid., 31B, 377 (1976). (13) R. E. Sturgeon and C. L. Chakrabarti, Anal. Chem., 49,1100 (1977). (14) B. V. L'vov, Zh. Prikl. Spektrosk., 8, 517 (1968). (15) B. V. L'vov, D. A. Katskov, and G. G. Lebedev, ibid., 9,558 (1968). (16) D. A. Katskov and B. V. L'vov, ibid., 15,783(1971). (17) V. P. Borzov, B. V. L'vov, and G. V. Plyushch, ibid., 11, 217 (1969). (18) R. E. Sturgeon, PhD thesis, Carleton University, Ottawa, Ont., Canada, 1977. (19) B. V. L'vov, "Electrothermal Atomization—The Way Towards Absolute Methods of Atomic Absorption Analysis", presented at the 3rd Federation of Analytical Chemistry and Spectroscopy Societies Conference and the 6th International Conference on Atomic Spectroscopy, Philadelphia, Pa., Nov. 15-19, 1976. (20) K. Ohta and M. Suzuki, Talanta, 23, 560 (1976). (21) J.G.T. Regan and J. Warren, Analyst (London), 101, 220 (1976).
Ralph Sturgeon is a research associate with the Division of Chemistry of the National Research Council of Canada (Ottawa). His current research interests focus on the development of suitable standards to be used in the analysis of marine samples for a variety of trace metals.
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