ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
incident radiant power, measurements could be made in the presence of several units of interferent absorbance without loss in precision. Here the results may at first appear artificial; radiant power was attenuated at low @ and D rather than being increased a t high D or @ > 0.0. However, it must be remembered that P M T fatigue at excessive light level and electronic over-range at high input current present real limits to the dynamic range. Furthermore, the detector signal can be integrated over longer periods of time by using integrating detectors or by using an analog-to-frequency converter and counter. Although the range of allowable molecular interferent may be very great, when the interferent is a scattering substance, one must be much more cautious. As has been discussed previously (9),the effect of a scattering interferent cannot ever be completely suppressed by application of Equation 1. This is d u e to the fact that scattering of radiation monotonically decreases with wavelength so that no wavelength pair exists where the scattering would exactly cancel. T h e instrumental work in this investigation illustrates the types of adaptations and corrections which may be made when an instrument is under complete computer control. For example, the addition of attenuator and encoder was a relatively simple task entailing the construction of only very simple additional circuits and the addition of a few statements in BASIC. However, although the methods of programming the attenuator and of modulating wavelength were imminently suitable for an investigation as this, they might not be preferred for routine work. A servo motor could probably be used to bring the attenuator to balance more rapidly, and other techniques for obtaining dual wavelength modulation have been discussed previously (10) and may be preferred for particular applications.
261
We should point out that in addition to extending the range of the DWS technique, a significant application of the work described herein should be the weighting of points used for a calibration curve. I t is now clear that the precision of a measurement may vary widely with AI, p , and possibly D, and low precision data points should not be valued as strongly in a calibration curve. Consequently, when executing a linear or polynomial regression analysis of calibration curve data, the data should be weighted according to their reliability ( 4 ) . This could be readily written into a program which would either store both absorbance and precision as a function of concentration or use relative weighting factors computed from equations presented here.
LITERATURE CITED K. L. Ratzlaff and D. F. S. Natusch, Anal. Chem., 49, 2170 (1977). L. D. Rothman, S.R. Crouch, and J. D. Ingle, Jr.. Anal. Cbem.,47, 1226 (1975). J. D. Ingle, Anal. Chim. Acta, 88, 131 (1977). Philip R. Bevington, "Data Reduction and Error Analysis for the Physical Sciences", McGraw-Hill, New York, 1969. Kenneth L. Ratzlaff, Am. Lab., 10, (2). 17 (1978). Gary Horlick, Anal. Chem.. 47, 352 (1975). J. D. Defreese, K. M. Walczak, and H. V. Malmstadt. Anal. Chem., 50. 2042 (1978). RCA Photomultiplier Manual, RCA Electronic Components, Harrison, N.J., 1970. K. L. Ratzlaff and D. F. S.Natusch, "Theoretical Assessment of Accuracy in Dual Wavelength SpectrophotometricMeasurement", submitted to Anal.
_ _
Chem
K. L Ratzlaff, F. S. Chuang, D. F. S Natusch, and K R. O'Keefe. Anal. Chem , 50, 1799 (1978).
RECEIVED for review August 14, 1978. Accepted November 20,1978. Paper presented in part at the Great Lakes Regional American Chemical Society Meeting, Stevens Point, Wis., 1977. Partial financial support provided by the Research Corporation.
Reduction of Matrix Interferences for Lead Determination with the L'vov Platform and the Graphite Furnace Walter Slavin" and D. C. Manning The Perkin-Elmer Corporation, Main A venue, Norwalk, Connecticut 06856
The addition to the graphite furnace of a thin pyrolytic graphite plate (L'vov Platform) on which the sample is deposited, makes it possible to atomize the sample at more nearly constant temperature conditions. This reduces analytical interferences that arise from a variation in the appearance temperature for Pb when it is present in different matrices. I n addition, this platform makes it possible to volatilize the sample into a gas that is hotter than the surface from which the sample is volatilized. This reduces the interference resulting from the volatility of Pb halides. Using the platform, we can determine Pb in matrices which contain chloride, sulfate, and phosphate without resorting to matrix modifications. It remains necessary to carbide-coat the platform surface to reduce its reactivity with Pb.
We have studied ( I ) the potential interference effects that occur in atomic absorption graphite furnace analyses. Our 0003-2700/79/0351-0261$01.00/0
initial work used P b as a test element because it appears to be the most widely determined in the furnace. We used a chloride matrix because the literature indicated that the chloride matrix introduced the greatest problems in the determination of Pb. We showed that P b can be determined in a chloride matrix using ",NO3 as a matrix modifier additive to permit removal of a large proportion of the chloride in the charring step prior to atomization of the Pb. In addition, it was necessary to control the surface of the graphite tubes (we used molybdenum coating on pyrolytic tubes), to use signal integration to avoid errors due to changes in peak shape, and to use an atomization ramp which separated in time the residual background signal from the P b signal. With these precautions, we were able to detect less than 20 pg of P b in solutions containing 1% NaCl or MgC12. While the method of additions has been widely used to quantitate analyses where interferences are present, this procedure involves extrapolation from the observed results. It would be distinctly preferable to use simple standards with 1979 American Chemical Society
262
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
a minimum of matrix matching to interpolate the analysis of unknown samples from conventional analytical curves. It would also be preferable to reduce or omit the matrix modifiers previously required because materials added in large quantity t o samples or standards carry the risk of contamination. L'vov's original furnace design (2)used an electrically heated tube into which the sample was volatilized from a graphite electrode t h a t was independently heated. T h u s the sample was introduced into t h e furnace which had already reached constant thermal conditions. T h e Massmann (3)design (on which most of the commercial furnaces are based) was simplified by depositing the sample directly on the wall of the furnace tube. T h u s the sample is volatilized into the furnace tube as the wall passes through a temperature appropriate for the particular sample. An important criterion for using the signal integration method of quantitating furnace analyses is that the residence times of all analyte atoms in the furnace are equal during the measurement period ( 4 ) . However the residence time depends strongly upon the temperature of the furnace, because both the diffusion coefficient and the vapor density are temperature dependent. Since the metal is volatilized while the furnace temperature is changing, changes in the matrix which alter the time when the metal vapor is atomized will cause the metal to be atomized a t a different temperature. This in turn will alter the residence time and thus the analytical results even when signal integration is used for quantitation. I t would clearly be advantageous to volatilize the sample into an environment that is not changing in temperature. It is also important to reduce to a minimum the time taken to establish constant temperature conditions. Previous Massmann-type furnace power supplies achieve the final temperature by applying a specific voltage across the graphite furnace so that a t equilibrium the desired temperature will be produced. I n this situation, the rate of temperature increase depends upon the difference between the starting and final temperature for the atomization step. In the Model HGA-2200 (and HGA-76B and HGA-500), a silicon diode detector is used ( 5 ) to determine when the temperature has reached the desired value. This permits the final temperature to be selected independently of the heating rate. When the diode detector observes that the final desired temperature has been reached, the electrical conditions of the furnaces are automatically reset to the conditions required to maintain that temperature. In this study, we use the maximum heating rate to heat the furnace as rapidly as possible to its final temperature. L'vov et al. (6) have recently described the use of a thin pyrolytic graphite plate (L'vov Platform, trademark of the Perkin-Elmer Corporation) added inside the furnace tube. T h e sample is deposited upon this platform which rests on the inner wall of the furnace. T h e platform is largely heated by radiation from the furnace tube. This arrangement provides two theoretical advantages. First, the time when the platform reaches the appearance temperatures for a particular metal will be delayed relative to the time-temperature relationship of the furnace wall. T h u s the metal will be atomized when the furnace has more nearly reached constant temperature conditions. T h e second advantage concerns the volatilization of molecular halides. Segar and Gonzalez (7) reported that copper was co-volatilized with sodium salts in the graphite furnace producing an attenuated signal for copper in the presence of NaCl. L'vov (8)and others have shown that such interferences arise for many metals in a halide matrix because the metal halide is volatilized as a molecule at a temperature lower than required to decompose the compound on the graphite surface. Since the gas temperature follows the wall temperature quite
Table I. Experimental Conditions for Pb with L'vov Platform h 283 nm, slit 5 ( 2 . 0 nm) Pb EDL D, background correction used Furnace: step 1 (dry), 270 "C for 30 s step 2 (char), ramp 1 5 s, 550 " C for 1 5 s step 3 (atomize), max power, 2000 "C for 1 6 s step 4 (auto max temp) argon flow 12 mL/min 15-s integration closely ( 9 ) ,the L'vov Platform makes it possible to volatilize the sample into the inert gas which is at a higher temperature than the sample itself. Thus there is more likelihood that the metal halide will be decomposed a t the higher temperature. In their publication, L'vov and his colleagues (6) reported gains in peak amplitude sensitivity using the platform for the highly volatile elements (Cd and P b ) and moderately volatile elements (Cu and Ni), and no improvement for relatively involatile metals like Mo. More importantly, they found a significant reduction of interference effects for reasons that L'vov defends on theoretical grounds (8). In addition, the platform extends the life of the furnace tube. T h e platform can be made of solid pyrolytic graphite, a material which is less chemically active than ordinary graphite tubes. It should make the furnace more easily adapted to corrosive materials such as perchloric acid. Gregoire and Chakrabarti ( I O ) investigated the platform with an aim to increasing the sensitivity of furnace methods, but they were not successful. In this paper, we will show that the interference effects of chloride, sulfate, and phosphate upon the P b determination are greatly reduced using the L'vov Platform.
EXPERIMENTAL The Perkin-Elmer Model 603 atomic absorption spectrophotometer was used with the Model HGA-2200 graphite furnace and the Temperature Ramp Accessory. Pyrolytically coated graphite tubes were used in the furnace. The Model AS-1 Auto Sampler was used to improve the precision. In all cases, 20-pL aliquots were used. We used disposable polystyrene cups in the AS-1, throwing them away after a single usage. The cups were rinsed with deionized water and dried prior to use. Eppendorf Micro Pipets with polyethylene tips were used for all the dilutions reported. Throughout this study, the background signal was recorded simultaneously with the analytical signal on a Perkin-Elmer Model 56 two-pen recorder using the deuterium arc as the light source. To correct automatically for absorption base-line drift, we used a circuit described by Epstein ( 1 2 ) . Most measurements were repeated in triplicate. Integrated absorbance readings were printed using the Model PRS-10 Printer Sequencer. The experimental conditions are summarized in Table I. It should be noted that the Platform does not reach the wall temperature, which explains the 270 "C wall temperature for drying. The fourth step on the temperature program of the HGA-2200 is an automatic 5-s interval at the maximum temperature. To be sure that the tube is purged, we lengthened that step to 8 s. This was accomplished by changing a resistor in the Temperature Ramp Accessory. We also lengthened the cool-down interval prior t o the start of the next cycle to permit the platform t o cool sufficiently. This was done by taking the trigger signal for the next cycle from the circuit that indicates that cool down is complete. These capabilities are routinely available on the new HGA-500 graphite furnaces. Choice of Analytical Wavelength. Our early experimental work on the furnace wall had utilized the 217.0-nm line, the most sensitive absorbance line for Pb. Since the highest matrix concentration that can be used experimentally was limited by the amount of background that could be corrected, we tested the several matrices for background absorption at the 217.0-nm and the 283.3-nm Pb lines. These background signals may be due to scattering by solid particles or to molecular absorption. We did
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979 Table 11. Peak Background Absorbance of Several Salts peak absorbance 217.0 n m 283.3 n m 0.36 0.1% NaCl 0.26 0.2% NaCl 0.49 0.60 0.74 1.10 0.5% NaCl 0.07 0.57 0.1% NaH,PO, 1.16 0.17 0.2% NaHiPO; 0.40 0.5% NaH,PO, 0.08 0.66 0.1% Na,SO, 1.05 0.17 0.2% Na,SO, ._ 0.30 0.5% Na, SO, 0.46 0.9% Na,SO, 0.70 2.0% Na,SO, ~
12
263
I
~~
__
Figure 1. Design of the L'vov Platform used in these studies, not to scale not attempt to distinguish between the two effects in this study. The data are summarized in Table I1 for solutions of sodium phosphate, chloride, and sulfate. The background for phosphate and sulfate materials was much smaller a t 283.3 nm so we used this wavelength. If solutions contain mostly chloride, the 217.0-nm line may be preferable. Platform Design. L'vov used a rectangular plate of pyrolytic graphite, 4 X 5 mm and 1 mm thick. However, in our hands that platform would hold no more than about 10 pL of sample and even in that case there was a tendency for sample to spill over the sides. We have therefore designed a larger platform (Figure 1)of somewhat greater mass, about 90 mg. It is considerably easier to use. With this design we can use as much as 50 pL of sample, although in all of the work reported here we have used 20-pL samples, dispensed from the Model AS-1 Auto Sampler. We have observed no problems due to dispensing. In the particular Model 603 we used, the platform vignetted about 109'0 of the beam from the lead electrodeless discharge lamp. The platform is made of solid pyrolytic graphite with the lamellae oriented parallel to the plane of the plate. Thus the thermal conductivity in the plane of the plate is very high, tending to maintain the temperature constant over the surface. We have experimented with platforms of different materials and different designs. Solid pyrolytic graphite was preferable to ordinary graphite, although the observed phenomena were not very different. When the contact edge between the platform and the tube was reduced to three points, the reduced thermal transfer did not materially alter the performance. We saw very little difference in performance between the smaller, lighter platforms as used by L'vov and the larger ones used in most of this work. Heating Rate. In this work and in our earlier work ( I ) ,we have consistently observed that the Pb absorption signal decreases as the heating rate increases. This is true whether peak area or peak height measurements are used to quantitate the P b signal. We studied the effect systematically with data summarized in Figure 2. The heating rate was determined by timing the period between a charring temperature and a temperature about 70% of the final selected atomization temperature. The furnace tube temperature was determined either with an optical pyrometer (Leads & Northrup Model 8632-C) or with an automatic pyrometer (Ircon Model 2000) a t temperatures above 1500 "C. In all cases the sample was deposited on the L'vov Platform, although i t is the heating rates of the tube that are recorded in Figure 2. An experiment with solutions containing 1% NaCl as well as 0.05 pg/mL P b produced a curve with the same slope as shown in Figure 2. The reduction of sensitivity as the heating rate increases
t
O4 O L
iCO
4C3
GOO
830
IOCC
1200
HEATING RATE >/SEC
Figure 2. Heating rate versus integrated absorbance for a solution containing 0.05 pglmL Pb (1 ng). The points represent different initial and final temperatures. The spread of the data about the best fit curve reflects the difficulty of measuring the heating rate accurately can be explained by a reduction of the residence time. Consistent with this explanation is the observation that the P b absorbance signal found is not directly dependent upon the final steady-state temperature. Despite some loss in sensitivity, we used the maximum heating rate for this work because, in conjunction with the platform, this condition mimimized interferences. Optimum Thermal Conditions. Almost all experimental conditions that we have used with the platform remove the interference experienced with a chloride matrix upon the absorption of Pb. When the platform was used with the same experimental conditions that we have used in our previous work ( I ) for the determination of P b in a chloride matrix on the wall of the furnace, no interference from the chloride was found up to concentrations in excess of 4 % NaC1, even though the peak NaCl background absorption was very large. However, these experimental conditions did not remove P b interferences previously reported for a sulfate matrix (12) nor for a phosphate matrix. Probably this is because the phosphate and sulfate interferences result from a shift of the appearance temperature for Pb, as documented below. Theoretically preferable experimental conditions require fast heating of the furnace to the selected steady-state temperature. We compared the recovery of P b from solutions containing chloride, phosphate, and sulfate a t different final temperatures from 1500 to 2500 "C, using the maximum rate of heating to achieve these final temperatures. The test solutions contained 0.05 pg/mL P b added to solutions containing 1% NaCl, 1% NaH2P04,and 1% Na2S0,. While the recovery of P b in the presence of NaCl was reasonably independent of the experimental conditions, we obtained better recovery of P b in the presence of sulfate and phosphate using lower final temperatures. Observation of the recorder tracings indicated that the P b peak a t higher final temperatures extended for a longer time and often resulted in a second smaller peak following the initial peak. Previous experience in our laboratory and in L'vov's (6) indicates that this effect results from condensation of P b and the matrix on the cooler outer ends of the furnace tube and the subsequent revolatilization of the P b as the heat reaches the ends. At lower final temperatures, the portion of the tube above the atomization temperature for P b reaches equilibrium more rapidly. The condensed metal and matrix are then removed in the final high temperature clean-up cycle. One limitation of the method we describe here is that the background and the element absorption signals are not separated in time as was the case in our previous method for P b in a chloride matrix atomized from the wall of the furnace tube (I). Thus the present procedure does not tolerate so large an amount of NaC1. However it accommodates other potential interferences by providing more theoretically optimum thermal conditions.
RESULTS T h e object of these experiments was t o reduce t h e interference of various matrices upon t h e absorbance of Pb. W e believe that some of these interference effects result from a shift of the P b appearance temperature in different matrices. We confirmed this using samples containing 0.1 kg/mL P b and appropriate concentrations of chloride, phosphate, sulfate,
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
Table 111. Pb Platform Studies signal (abs-sec) 76 NO ADDITION
0 400
800
1200
1600
2000
“C
NaCl added
mfg 1 “A” “B,,
0
0.77
0.001 0.76 0.005 0.81 0.01 0.81
20
0.02 0.05
16
0.1
v) W
12
0.2 0.4
v)
0.8
v
m a
1.
04
0 400
B v)
800
1200 “C
1600
2000
nP
2. 3. 4. mean S. D.
0.83 0.82 0.78 0.79 0.84 0.76 0.80 0.83 0.80 0.80 0.025
0.78 0.78 0.82 0.82 0.85 0.84 0.81 0.86 0.87 0.84 0.84 0.84 0.84 0.83 0.029
mfg 1 0.77 0.76 0.79 0.81 0.84 0.85 0.82 0.86 0.87 0.84 0.83 0.84 0.85 0.83 0.034
mfg 2 “B”
“B”
“A”
0.80 0.81 0.85 0.87 0.88 0.87 0.87 0.87 0.84 0.87 0.74 0.77
0.74 0.72 0.77 0.77 0.78 0.78 0.75 0.78 0.80 0.81
‘GA”
0.80
0.85
0.87 0.87 0.83 0.79 0.049 0.048 0.76
0.79 0.79 0.80 0.80 0.80 0.73 0.78 0.80 0.82 0.83 0.83 0.82 0.80 0.027
& m os a
00 4400
800
1200 IN PHOSPHATE 1600 2000
O C
Figure 3. Char and atomization curves for samples deposited on the furnace wall containing 0.1 pg/mL Pb and: (A) no additions, the Pb being added as nitrate; (6)in 0.04% NaCI; and (C) in 0.01 ‘YO NaH,PO,. For the char cycles, the atomization temperatures was always 1400 O C . For the atomization cycle of A and B, the char temperature was 600 OC, for C it was 1000 OC
and nitrate matrices that were charred and atomized on the furnace wall a t different temperatures (13). Some of the resulting curves are shown in Figure 3. The lefthand portion of each curve indicates the absorbance signal found for P b using the charring temperatures indicated. Atomization was performed a t about 1400 “C in each case. In the right hand portion of the curve, charring is performed at the levels specified in the figure caption, close to the maximum found before loss of P b in the charring cycle. Curves for 0.07% NaN03 and 0.06% Na2S04are omitted but both were at about 80 “C higher temperatures t h a n the chloride curve. For each matrix the P b signal begins to appear in the atomization curve a t about the same temperature that the P b signal has begun to decrease in the charring curve. However, it is clear that this temperature is different for the several matrices studied. Obviously, for P b determination with samples atomized from the furnace wall, these variations in appearance temperature will cause the P b to be volatilized into the gas atmosphere at a temperature which will depend upon the matrix, thus producing different signals for different matrices. Deposition of the sample onto the platform should cause these variations to be smaller since for each matrix the gas temperature should be more nearly constant when the P b appearance temperature is reached. Within the limits that we could measure, this was true. Extensive experiments were run with graphite furnace tubes, some of which were pyrolytically coated and some that were not. In some cases the graphite platform was molybdenum coated by our procedure ( 1 ) and in some cases it was not. There were variations in the experimental data as a result of the somewhat different environments. Since Fuller (14) and we ( 1 ) have shown that the degree of interference of chloride in the P b determination depends upon the surface
-c
y
OX’ 5302
coo5
332
301
L
c05
35 32
,“
% NoCI
Figure 4. Interference of NaCl upon the absorbance of 0 05 Fg/mL
Pb atomized from the platform and from the tube wall The wall conditions are the same as those used for the platform in Table I except that drying was done at 110 OC and max power was not used for atomization
history of the graphite tube, the observations were adequately consistent. These comparison experiments involved the removal of the chloride interferences which are discussed in a later section of this paper. Some indications of the consistency of the data is indicated in Table 111. “A“ and “B” are platforms of somewhat different design in that the contact edge of‘ the “B” platforms was reduced. Three different furnace tubes are represented from two manufacturers. Using the experimental conditions of Table I, the effect of chloride, phosphate, and sulfate matrices on the absorbance of P b was determined on the L’vov Platform. These results were compared with measurments made by atomizing samples from the wall of the furnace tube using conditions that differed from Table I in that drying was accomplished a t 110 “C and normal power wa5 used in the atomization cycle. The results are shown in Figures 4-6. In the case of chloride and sulfate, the interferences on the platform are small until about 1% of the interfering material, where the amount of background absorption is already limiting. For particular experimental purposes, the addition of appropriate amounts of ammonium nitrate will reduce the background absorption for the chloride in situations where higher concentrations of chloride are encountered. In the case of the phosphate, the interference even on the platform begins to be significant a t about 0.1%. However, the interference is considerably smaller on the platform t h a n i t is when the sample is determined by atomization from the wall of the furnace tube. Some experiments were run with mixtures of the several interfering species with results that were similar to the results
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
0
A
1
0002
0005
1 001
002
01 005
1
02
35
1
20
3
0
10
20
40
60
80
100
% OF WATER SAMPLE IN TEST SOLUTION
*A N e 2 SO4
Figure 5. Interferences of sulfate on the determination of Pb. Varying amounts of Na2S0, were added to 0.05 kg/mL Pb and these solutions were atomized both from t h e platform and from the tube wall. The conditions are as in Figure 4
265
Figure 7. Recovery experiments from an environmental water sample. The sample was diluted and 0.05 gg/mL Pb was added to each solution and plotted against the absorbance signal. The abscissa indicates the fraction of the original environmental water sample (in percent) remaining after dilution. The conditions for this analysis were as shown in Table I
ported. From many experiments, two typical curves are shown in the lower part of Figure 7 . We Mo treated the Platform by our previous procedure ( I ) , presumably producing a Mo carbide surface. T h e variation in recovery was much smaller and there was little variation with condition of the platform surface. Similar results were obtained by treating the platform with solutions of a T a salt as shown in Figure 7. However, in our hands, the T a carbide tended to flake off after about ten firings, returning the platform to its performance prior to T a coating. Additions of 0.5% La (as the sulfate) to each solution produced results that were very similar to those of the Mo coated platform. The similarity of the results for the several carbide-forming metals suggest that it remains necessary to carbide-coat both the platform and the furnace tube to obtain useful analytical results for P b in complex matrices. Combining the advantages of the determination of P b in complicated matrices with the L'vov Platform with a matrix modifier technique using ammonium nitrate ( I ) , ascorbic acid (15),or lanthanum additions (12)may provide procedures that will produce satisfactory results on real samples with complicated matrices.
ACKNOWLEDGMENT We thank Sabina Slavin, R. D. Ediger, F. J. Fernandez and W. B. Barnett for their suggestions and criticisms.
LITERATURE CITED ( I ) D. C. Manning and W. Slavin, Anal. Chem., 5 0 , 1234 (1978). (2) B. V. L'vov, Spectrochim. Acta, 17, 761 (1961). (3) H. Massmann, Spectrochlm. Acta, Part B , 23, 215 (1968). (4) B. V. L'vov "Atomic Absorption Spectrochemical Analysis", Adam Hilger, Ltd., 31 Carnden Rd., London NW1, 1970 (Distributed in the Unked States by American Eisevier Publishing Co.. New York). (5) F. J. Fernandez and J. Iannarone, At. Absorpt. Newsl., 17, 117 (1978). (6) B. V. L'vov, L. A. Peiieva, and A. 1. Sharnopolskii, Zh. frlkl. Spectrosk., 27, 395 (1977). (7) D. A. Segar and J. G. Gonzalez, Anal. Chlm. Acta, 58, 7 (1972). (8) 8. V. L'vov, Speckochlm. Acta, f a r t B , 33, 153 (1978). (9) W. M. G. T. van den Broek and L. de Galan. Anal. Chem., 40, 2176 (1977). (10) D. C. Gregoire and C. L. Chakrabarti, Anal. Chem., 49, 2018 (1977). (11) M. S. Epstein, A t . Absorp. Newsl., 16, 75 (1977). (12) A . Andersson, At. Absorp. Newsl., 15, 71 (1976). (13) E. Welz, Paper presented at the XX Colioqium Spectroscopicum Internationale, Aug. 1977, Chium, Czechoslovakia. (14) C . W. Fuller, At. Absorp. Newsl., 16, 106 (1977). (15) J. G. T. Regan and J. Warren, Analyst (London), 103, 447 (1978).
RECEILZD for review September 28,1978. Accepted November 13, 1978.
