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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
It is anticipated that high precision analyses via the ICP can be expanded to a wide range of elements and can be applied t o a large variety of samples which are currently analyzed by other methods.
Thompson, K. C.; Thompson, D. R. Analyst (London) 1974, 9 9 , 595. Fiorino, J. A.; Jones, J. W.; Capari, S. G. Anal. Chem. 1976, 48, 120. Terashima, S. Anal. Chim. Acta 1976, 86, 43. Dickinson, G. W.: Fassel. V. A. Anal. Chem. 1969, 4 7 , 1021. Kirkbright, G. F.: Ward, A. F.; West, T. S. Anal. Chlm. Acta 1973, 6 4 , 353. Kirkbright. G. F.; Ward, A. F.; West, T. S. Anal. Chim. Acta 1972, 62, 241. Fassel, V. A.; Kniseley, R. N. Anal. Chem. 1974, 46, 1110A. Boumans. P. W. J. M.; DeBoer. F. J., Spectrochim. Acta, Part B 1975, 30, 309. Schrenk, W. G. in "Flame Emission and Atomic Absorption Spectrometry-Vol. 2"; Dean and Rains, Ed.; Dekker: New York, 1971: p 314 Kalnicky, D. J.; Kniseley, R. M.; Fassel, V. Spectrochim. Acta, Part 6 1975, 30, 511. Kornblum, G. R.; DeGabn, L. Spectrochim. Acta, Part 6 1977, 32, 455. Skogerboe, R. J.; Olson, K. W., Appl. Spectrosc. 1978, 32, 181. Meggers, W. F.; Corliss, C. H.; Scribner, B. F. "Tables of Spectral-Line Intensities" Natl. Bur. Stand. ( U . S . ) .Monogr. 1975, No. 145. Schrenk, W. G., "Analytical Atomic Spectroscopy": Plenum Press: New York. 1975: p 223. Gaydon, A. G.; Wolfard, H. G. "Flames, Their Structure, Radiation and Temperature"; Chapman and Hall: London, 1970. Gaydon, A. G. "Dissociation Energies and Spectra of Diatomic Molecules", 3rd ed.: Chapman and Hall: London, 1968.
LITERATURE CITED Cotton, F. A,; Wilkinson, G. "Advanced Inorganic Chemistry", 2nd ed.. Wiley-Interscience: New York, 1966; pp 938-946. Rajkovic-Blazer, L. M. PhD. Thesis, Georgetown University. Washington, D.C., 1978; p 22. Kwak, W.; Rajkovic, L. M.; Stalick, J. M.; Pope, M. T.: Quicksall, C. 0. J . A m . Chem. SOC. 1976, 75,2778. Barkigia, K. M.; Rajkovic, L. M.: Pope, M. T.; Quicksall, C. 0. J . A m . Chem. SOC. 1975, 9 7 , 4146. Kwak, W.; Rajkovic, L. M.; Pope, M. T.; Quicksall, C. 0.: Matsumoto, K ; Sasaki, Y. J . A m . Chem. SOC.1977, 9 9 . 6463. Wasfi, S.; Kwak, W.; Pope, M. T.; Barkigia, K. M.; Butcher, R. J.; Quicksall, C. 0. J . A m . Chem. SOC. 1978. 100, 7786. Haywood, M. G.; Riley, J. P. Anal. C h m . Acta 1976, 85, 219. Fishman, M.: Spencer, R . Anal. Chem. 1977, 4 9 , 1599. Sutcliffe, P. Analyst(London) 1976, 101, 949. Halmann, M. "Analytical Chemistry of Phosphorus Compounds ' : Wiley-Interscience: New York. 1972. Kolthoff, I. M.; Elving, P. S. "Treatise on Analytical Chemistry". Part 11, Vol. 5; Interscience: New York. 1961: pp 341-354 Wilson, C. L.; Wilson, D. W. "Comprehensive Analytical Chemistry", Vol IC; Elsevier Publishing Co.: New York. 1962; p 220. Smith, D. P. FhD. Thesis, Georgetown University. Washington, D.C , 1975: p 40. Landis, A. M. Ph.D. Thesis. Georgetown University, Washington, D.C.. 1977; p 56. Lichte, F. E.; Skogerboe. R. K. Anal. Chem. 1972, 44. 1480. Skogerboe, R . K.; Bejmuk, A. P. Anal. Chim. Acta 1977. 94. 297.
RECEIIFDfor review February 14,1979. Accepted May 1,1979. This project was supported by National Science Foundation Grant No. CHE75-22848. Presented in part a t the Fifth Annual Federation of Analytical Chemistry and Spectroscopy Societies Conference.
Comparative Interference Study for Atomic Absorption Lead Determinations Using a Constant Temperature vs. a PuIsed-Type Atomizer Lynn Robert Hageman, John A. Nichols, Puligandla Viswanadham, and Ray Woodriff Department of Chemistry, Montana State University, Bozeman, Montana 59717
belong all commercial atomizers, such as the Massman and the mini-Massman designs, the sample is vaporized and atomized under conditions of rapidly changing temperature. Their interference tendencies are similar. In the second type, exemplified by the atomizers developed by L'vov ( 2 ) in Russia and by Woodriff and his associates (2, 3 ) a t Montana State Llniversity in the United States, the sample is vaporized into a tube which has already achieved a preset optimal atomization temperature (2-7). At least a few workers in both Russia and the United States have recognized from the early days of electrothermal atomizers that constant temperature was a vital feature for interference-free analyses. L'vov (8) in particular has justified this belief on both theoretical and experimental grounds. Lead was chosen as the element of interest in this study for several reasons: (1) the importance of lead analyses in many different matrices for environmental and health studies; ( 2 ) the availability of extensive literature on lead determinations and interferences in pulsed-type atomizers; and ( 3 ) the desire t o extend the type of comparative interference study, previously implemented with a constant temperature furnace for manganese (91, to a more volatile element-for which interferences are very common in pulsed type atomizers because of the limited ashing temperature. Many simple matrix interferences observed in pulsed-typed atomizers have been documented in the literature. Regan and iyarren (I0, I I ) reported about 40% enhancement of lead AAS signals from a 1% ascorbic acid solution, using a Perkin-Elmer
During the one to three seconds necessary to heat commercial electrothermal atomizers to the desired atomization temperature, many reactions take place, and analyte compounds may be lost from the rapidly heating furnace at varying temperatures with varying matrices-often at sub-optimal temperatures with inadequate atomization, since residence times are short. Thus, matrix interferences are common in these pulsed-type atomizers. However, the same solutions, when atomized in a constant temperature furnace (CTF), show no significant matrix interferences. Lack of ruggedness of analytical procedures using pulsed-type atomizers seems to be an inherent limitation, whereas equipment ruggedness limitations of the CTF are amenable to elimination by appropriate attention to engineering aspects of fabrication. Difficult samples representing common matrices reveal the ease of obtaining interference-free results directly with the CTF-and the difficulty, even with pretreatments,of correcting for interferences on a routine basis in pulsed-type atomizers.
T h e various electrothermal atomizers available commercially for atomic absorption spectrometry (AAS) provide a lei-!: sensitive, as well as relatively inexpensive, means of analyzing for lead. However, effects such as matrix interferences have disenchanted many users. Electrothermal atomizers current11 in use can be classified into two categories, pulsed-type and constant temperature atomizers. In the former type. to which 0003-2700/79/0351-1406$01 O O i O
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1979 American Chemical Societ)
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
HGA 7 2 . Tartaric acid and sucrose also caused enhancement. T h e same authors also found a suppression of the lead signal due t o various salts of calcium, magnesium, strontium, and barium-up t o 88% suppression for 100 pg/mL MgCl,; ascorbic acid was used to eliminate suppressions. McLaren and Wheeler (12) observed a n interference in the form of double peaks for lead absorption in the presence of ascorbic acid, HF, or digested NBS orchard leaves, using a Perkin-Elmer HGA 2O00. Czobik and Matousek (13) observed that the absorption vs. time profile is influenced by the presence of phosphate ions when using a Varian Model 63 carbon rod atomizer (CRA). Significant suppression of the lead signal due t o various salts in urine was noted by Hodges (14) with a n Instrumentation Laboratory 445 carbon furnace. T h e interferences were overcome, however, using a furnace precoated with molybdenum in the presence of phosphoric acid. Thompson et al. (15) noted up t o a 70% suppression of lead while analyzing natural waters using a Varian CRA-90. A lanthanum precoated carbon tube was employed to overcome the interference and improve precision. T h e mechanisms of complex-matrix interferences are generally not known; however, a number of simple matrix interference mechanisms have been described ( 8 ) ,especially for the common chloride interference. Frech and Cedergren (16) used high temperature equilibrium calculations to show that undissociated volatile PbCl could be lost during the early moments of heating toward atomization temperature. They also showed how hydrogen and a high ash temperature could eliminate the chloride from dissolved steel samples. Czobik and Matousek (1 7) studied chloride interference effects in pulsed-type atomizers using both conventional and fast oscilloscopic detection. They conclude, from comparisons of analyte atomization times to time-resolved interferent populations, that atom populations are depleted by chloride formation in the vapor phase. Another type of interference has been attributed to solid phase interactions prior to volatilization, causing more or less thermally stable analyte species to shift their appearance time. McLaren and Wheeler (12) attributed double peaks for lead to the presence of dimorphic forms of lead(I1) oxide; by studying X-ray powder diffraction patterns, they have shown t h a t under conditions similar to those during ashing in a graphite furnace, lead nitrate decomposes to a less stable form of lead oxide when organic matter is present. Instead of forming only the more stable red litharge, 10% ascorbic acid in the lead nitrate resulted in the thermally unstable yellow massicot's composing as much as 40% of the lead oxide formed. The differing thermal stability of the yellow massicot caused it to vaporize and atomize a t a n earlier time. Phosphate interference has also been attributed to a reaction in the solid phase which caused a change in thermal stability (13). Lead hydrogen phosphate decomposes at 623 K to produce t h e relatively stable lead pyrophosphate, which is stable t o 1200 K. An interference occurs owing to the later peak time, relative to standards. Previous reports (8, 18) show that little or no interference is observed in the analysis of manganese and several other elements when utilizing the constant temperature furnace (CTF). Data showing reduction of background and minimal interferences for direct analysis of biological samples have also appeared (19). Thus, a comparative study of the C T F and CRA, for the volatile element lead, was undertaken. T h e emphasis in this study will be on difficult sample materials, such as digested whole fish, livestock feeds, and coal slurry waters. Synthetic interferent solutions are also studied.
EXPERIMENTAL Two different electrothermal atomizers were employed in collecting data: A constant temperature furnace (CTF) developed
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at Montana State University (MSU) used in conjunction with a Varian AA-6 spectrometer, and a Varian model 63 carbon rod atomizer (CRA) used in conjunction with a Varian AA-5 spectrometer. The Varian AA-5 was equipped with an AA-6 conversion module, BC-6 simultaneous background corrector, model 63 carbon rod atomizer, and a Beckman model 1005 strip chart recorder. The Varian AA-6 spectrometer used with the CTF is similarly equipped with a BC-6 and a Beckman model 1005. An integrator constructed with an operational amplifier, voltageto-frequency converter, and a digital counter was used for integrating the absorption signals for both atomizers. In addition to the strip chart recorder, a Hewlett-Packard model 1220 A oscilloscope was used to observe the absorption signals, especially truncated or double peaks that might otherwise be missed because of slower response of the chart recorder. Similar operating conditions were used for both the instruments. The lead resonance line at 217.0 nm and a spectral band pass of 0.33 nm were employed for all absorption measurements. The Varian P b lamp was operated at 10-mA current for the CRA-AA-5 and 15 mA for CTF-AA-6. The Varian H2 lamp for background correction was operated at 2 mA for the CRA-AA-5 and at 5 mA for the CTF-AA-6. Simultaneous background correction was used for all absorption measurements. When using the CRA, nitrogen was used as the inert gas a t a flow rate of 5 L/min. Methane was introduced at 0.5 L/min by means of a solenoid valve during the ashing and atomization cycles to provide a reducing atmosphere and to retain the pyrolytic coating on the carbon tubes. The presence of methane causes only a slight increase in background, which is adequately compensated for by the simultaneous background correction. The thermal program with the carbon rod was comprised of drying a t 3.5 for 28 s (380 K ) , ashing a t 4 for 15 s (750 K), and atomization at 7 for 2.5 s (2100 K). Temperatures cited are maximum temperatures attained during each cycle; >ZOO0 K is attained after about 1.5 s of the atomize cycle has elapsed. Argon was flushed through the CTF at a flow rate of 0.3 L/min. The temperature of the CTF was set at 1900 K for maximum sensitivity, with a current throughput of 100 A. The CRA was operated according to the manufacturer's specifications and directions; ash and atomization temperatures were optimized on real samples to minimize interferences. The description and operation of the CTF has been discussed adequately in the literature (2-7). A carbon tube of length 30 cm and diameter 1cm in the optical path is maintained at the selected temperature by resistance heating. Samples pipetted into small graphite crucibles and dried under a heat lamp are introduced into the furnace by means of a pedestal and push rod assembly in the side tube, which is at right angles to the optical path. The crucible is flushed with argon as it enters the furnace. Owing to its relatively small mass, the crucible is heated by conduction and radiation rapidly enough that the evaporation time is small in comparison to the residence time. The pedestal makes a seal with the constriction in the side tube and the sample vapors diffuse out through the heater tube. Graphite felt insulation surrounding the heater tube reduces heat losses, facilitating the use of a small power supply. Simple matrices with one or several possible interferents were prepared from analytical reagent grade interferent salts and deionized water. The 1000 pg/mL lead reference standard was obtained from the Fisher Scientific Company. All the acids used in the study were reagent grade chemicals. Working standards were prepared by appropriate dilution of 1000 gg/mL lead and interfering solutions. Working standards were analyzed within a few hours of preparation and no preservatives were added. Biological materials that were used for analysis and spiking experiments were digested by weighing 3-8 g into a 125-mL Erlenmeyer flask, adding 25 mL of H N 0 3 and heating in a microwave oven to achieve lead solubilization and removal of about half of the acid (20). The digested solution was then transferred to a 50-mL volumetric flask and made up to volume with deionized water. Interference data were obtained by addition of 2.5 pL of standard and 2.5 p L of sample or interferent solution, both pipetted into the CRA or CTF crucible with an adjustable 10-gL Unimetrics Teflon tipped syringe. Data for standard curves were obtained in the same manner using 2.5 pL of standard and 2.5 pL of 20% HN03. Total volume and acid concentration is kept constant because CRA absorptions are somewhat dependent on
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
1408 0 3
~
'
Carbon Rod Atomizer
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~
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< 7
"
e
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-
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/ I?
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./;
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Figure 1. Lead analyses with carbon rod atomizer: Standard curve ---0---; Standard addition curves for livestock feed -0(A), fish -@(B), and coal slurry water -8(C)
4
00)
~- A
~
~
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Figure 2. Lead analysis with constant temperature furnace: Standard curve ---0---, Standard addition curves for feed -0(B), and coal slurry water -8(C)
-a-
(A), fish
Table I. Pb Results from Various Samples ( p g / g ) atomizer
feed
fish
coal
CRA CTF
0.094 0.088
0.060 0.050
0.010 0.010
Table 11. Comparison of Lead Results for Fish Analysis with CRA and Constant Temperature Furnace CRA
_ _ _
CTF
%
these factors. Each absorbance value reported is a mean of at least three replicate measurements on each solution. In order to examine if the suppressions are occurring because of volatilization in the drying and ashing cycles, a loaded sample crucible was placed between the CRA electrodes for drying and ashing in the usual manner. The crucible was then removed and introduced into the CTF for atomization and measurement of absorbance.
RESULTS Data obtained on three typical samples, each representing a common matrix type (plant, animal, or coal material), are represented graphically in Figures 1 and 2. The method of standard additions was used in all cases. Figure 1 depicts standard addition plots for three samples when run on the CRA; the standard curve is also shown as a dashed line, to allow visualization of the extent of suppression for each sample. Slopes ranged from 25 to 83% of that of the calibration curve; the plots are seen to be linear with minimal scatter of the points, thus verifying that the extent of suppression does not vary significantly with concentration of lead over this range. Figure 2 depicts analogous data for the same three sample solutions atomized in the CTF; the standard addition slopes are parallel to the calibration curve within 5% and no suppressions are observed in the CTF. Reading the standard addition plots and comparing the CRA and C T F results shows agreement within 20% as shown in Table I. Five fish samples, all northern pike from the Tongue River reservoir in Montana, were analyzed after microwave digestion with nitric acid. T h e results of analyses with the CRA and CTF are compared in Table 11. The data obtained with the CTF did not require use of the method of standard additions, as is shown for one fish in Figure 2, and hence the direct result from the calibration curve is presented. Data obtained with the CRA, on the other hand, had to be calculated using the method of standard additions; both the result and the per cent suppression observed (from the relative slopes of the sample plot and the standard curve) are tabulated. It should be noted t h a t these samples contained about 20% "OB, which has been demonstrated to be effective for removal of certain simple interferences. T o test whether lead was being lost in the dry or ash cycles in the CRA, samples were subjected to these treatments on the CRA and then introduced into the CTF for atomization.
sample 16 17 18
19 20
live weight of fish, g 4.8 16.0 8.4 18.0 12.3
std. addn. suppression std. curve results, from std. calculation, pgig curve P gig 0.007 58 0.010 0.110 72 0.125 0.060 83 0.050 0.050 75 0.060 0.050 75 0.030
___-
-
The CTF results showed no suppression and no lead loss due to the CRA dry and ash steps. Tables I11 and IV summarize the results of our interference studies with synthetic samples. Several results from published literature are also included and are identified by a reference number. Peak height measurements were used to construct this table; an analogous table based on our peak area data, collected simultaneously, is very similar. Varying interferences occur in the pulsed-type atomizers, as seen in Table 111-often 50% or greater for common chlorides and sulfates. The identical solutions showed less than 7 % interference when atomized in the CTF. (A typical standard deviation of 5-8% is generally encountered using an electrothermal atomizer @I).) Data in Table I11 concerning chlorides show a significant suppression, except KCl and HC1. Most can be eliminated by the addition of nitric acid; however, the interference from CaC12 cannot be eliminated by addition of H K 0 3 . Sulfates are also shown to cause suppression of the lead atomic absorption, and addition of H N 0 3 does not effectively decrease the amount of interference. Phosphates in general cause an enhancement of lead absorption which can be decreased with nitric acid, except in the case of calcium phosphate which causes a suppression that is not significantly affected by nitric acid addition. The results with nitrates generally show no significant interference except for the calcium salt, which again causes a suppression of lead signal. The 1% H F or ascorbic acid solutions caused an enhancement in the CRA-63, as has been previously reported using a Perkin-Elmer HGA 72 ( 2 1 ) . In the present study, the degree of enhancement relative to standards run before or after ascorbic acid test solutions varied widely, ranging as high as 150% when new furnace tubes were used. The amount of difference between the standard and test solution signals diminished progressively as the tube aged and the pyrolytic coating on the furnace deteriorated, allowing
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
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Table 111. Matrix Salt Interferences in Electrothermal Atomization AAS during Determination of Lead (expressed in %) constant temp. furnace, 1000 pg/mL
interfering salt KC 1 KC1- HNO, NaCl NaCl + HNO, ZnC1, ZnC1, HNO, CaCI, CaC1, HNO, MgCl, MgCI, + HNO, CUCI, Ca,(PO,), Ca,(PO,), + HNO, Na,HPO, Na,HPO- + HNO, K,HPO, K,HPO, HNO, M g( N 0 3 )? Mg(NO,), HNO, NaNO, NaNO, + HNO, KNO, KNO, + HNO, WNO,), Ca(NO,), t HNO, Na,SO, Na,SO, t HNO,
+2 -5 -30 -2 -20 1-3 -33
+2 -4 -50
-72
-36 ( I l ) , 4 2 (14)
-3
-21 -40
-60 -75
-88 (11),-76 ( 1 4 )
-1
-2
-1
+l 0 +2 -5 -3
-3 0
-2 t 3 L3 -1
72 tl
-50 (13)
--20 -18
+ 15 +5 + 22
-10 -68 t4
+2 -100 ( 1 3 ) -50 -57 1- 19
t5
+ 28
+4 +2
+6 +3
0
74
+7
+3 +6
-5
-1 0
-1 0
t 6 t7 -2
-4 -4 -4 -3
-6 -10 -22 -22
+2 +3
T
Massman and IL furnace, 100 pg/mL
Mini-Massman atomizer 100 pg/mL 1000 Gg/mL
T
+3
+ 3 3 (11)
-4
-22 --20 -60
-17 ( 1 1 )
-12 ( 1 4 )
-50
Table IV. Acid Interferences in Electrothermal Atomization AAS during Determination of Lead (expressed in 5%)
interfering acid 1%ascorbic acid 1%H F
20% HNO, 20% HC1 20% H,SO,
constant temp. furnace
MiniMassman atomizer
+l
+ 45
0 0 0
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-2
+ 33
'
0.05 P P I P b
Massman furnace
0, I A b s o r b a r c e 1
-
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1
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greater penetration of the solutions into the graphite. T h e 45% enhancement observed for old furnace tubes agrees with the literature value reported for an uncoated tube ( 1 1 ) . Typical traces are shown in Figure 3 for lead and leadascorbic acid solutions in the CRA (top) and C T F (bottom). T h e shift to an earlier peak with ascorbic acid in the CRA can be discerned to be about 0.5 s. The recorder trace for the CTF shows neither the enhancement effect nor double peaks, and any peak shift is insignificant relative to the longer residence time in the CTF. All four traces are a t a chart speed of 25 cm/min, and the pen's time constant was about 0.25 s for 95% of full scale response. Although a fast response detection system is generally unnecessary with the CTF, an oscilloscopic trace of the C T F signal was also observed; the traces appeared identical whether ascorbic acid was present or not.
DISCUSSlON Accuracy a n d Ruggedness. Several types of interferences which have been described in the literature have been examined in this comparative study using both simple and complex matrices. T h e constant temperature atomizer has proved to be resistant to interferences which occur when the same solutions are atomized in pulsed-type atomizers. T h e pulsed-type atomizer, when used in accepted procedures for the analysis of fish samples, not only showed severe interferences, but also gave variable suppressions on different samples with essentially the same matrix. Even for simple synthetic solutions, e.g., ascorbic acid, varying degrees of
1 0
1
1 6
I2
I8 Tilie
1 24
, 33
5
Figure 3. High chart speed (25 cm/min) traces of lead absorption with and without presence of 1 % ascorbic acid. Top two curves, carbon rod atomizer; bottom two curves, constant temperature furnace
interference could be obtained with slight variations in operating conditions. A procedure in which the results are sensitive to apparently minor input variables is deemed to be not rugged (22);it appears that a number of CRA procedures for analyzing real samples fit into this category. In all fairness, it should be pointed out that ruggedness is normally tested by variations in procedural steps applied to replicate aliquots of a single sample. However, it seems highly relevant to analytical needs to extend the concept of ruggedness to cover a nonprocedural input variable when the said variable would not be expected t o affect the dependent variable of interest. For example, in Table 11, it would be reasonable to expect that different fish from the same species and environment would exhibit the same per cent suppression, just as their analyses should show the same precision (RSD). When the digested solutions were applied t o the C T F instead of the CRA, consistently accurate results were obtained. T h e C T F
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
procedure was, therefore, not sensitive to input variables other than the actual analyte concentration, and thus meets both the normal and the extended criteria for ruggedness of the procedure. Variability of per cent suppression among similar samples on the CRA, such as that seen in Table 11, has also been cited by other authors using the Massman or mini-Massman pulsed-type atomizers. Thompson et al. (15)reported from 35 to 81% suppression of lead for four river water samples. Furnace condition was held constant by using only relatively new tubes. Regan and Warren (10) reported 22-84% suppression of lead for nine drinking water samples. In both publications, hardness and other constituents were included in their data, but no correlations with per cent suppression were evident. One of the present authors (23),using a CRA-63, has measured from two- to tenfold suppression of lead in filtered river water collected a t the same location in different months throughout the year. Chloride interference was a significant part of the problem, and use of hydrogen (16)and 850 K ash temperature reduced a typical threefold suppression. The worst interferences occurred in spring and summer, when snowmelt increased stream flow and diluted the total solids by 20 to 40%. In the case of variable suppressions when digested fish samples were run on the CRA, we may speculate that the lack of ruggedness is due to varying amounts of organic matter remaining after digestion, or that it is due to variations in the minor inorganic constituents of the fish. The first is plausible because some samples may have digested more rapidly or completely than others; a carbonaceous residue may provide enough of an enhancement effect to partially offset the more severe suppression due to major inorganic salts in the matrix. T h e second makes sense, a t least theoretically, in the light of recent thermodynamic studies (16,24) showing a significant role of various matrix constituents (including Fe and Mg) in determing the volatility of the lead compounds just above normal ash temperatures; such constituents would, therefore, affect the degree of chloride interference on lead during the early stages of atomization. The first could be solved by more exhaustive digestion procedures, but these are more time consuming and may even risk loss of some volatile elements. T h e second type of problem is more fundamental to the atomizer. I t could conceivably be dealt with in a manner analogous to the interelement corrections used in X-ray techniques, provided all relevant interferent concentrations were known. Many workers have also tried various reagents to alleviate interferences, but these seem to lack general applicability. Pretreatments. In order to improve accuracy and ruggedness obtainable with pulsed-type atomizers, many pretreatments, including extraction or coprecipitation, have been tried (25-27). Our data with simple matrices show that HNO, is generally effective in minimizing simple chloride interferences, as are other methods that remove chloride before the atomization cycle begins. However, CaC12 interference remains, and other complex matrix interferences probably remain also. Thus, the failure of 20% HNO, to eliminate the interferences seen in Table I1 does not rule out vapor phase interference due to chloride. A related problem is that, if NaCl is a major matrix component, H N 0 3 has the disadvantage of producing lead volatility losses a t ash temperatures which would otherwise be satisfactory ( I 7). For interferences other than chlorides, no simple method has been found for reducing interferences. The difficulty of using pretreatments takes on a new dimension once we realize the practical importance of being able to do multielement analysis. While use of H 3 P 0 4 is a recommended method (17, 28) to eliminate the interference in
the Cu-NaC1 system, H 3 P 0 4 causes enhancement in the P b N a C 1 system (13,17). This is but one of the many possible examples indicating why the implementation of practical multielement analysis with electrothermal AAS system is further delayed by the analyte-specific approach to interference reduction-which seems to be necessary (29) when consideration is limited to pulsed-type atomizers. Mechanism of Interference. I t is impossible to find one or two well-defined mechanisms that can explain the wide variety of lead interferences observed in pulsed-type atomizers, especially when we go beyond the studies of mechanisms in synthetic solutions to consider complex matrices. Nonetheless, it may be helpful to classify interference mechanisms into two broad and easily expandable groups which are evident from this and other work. The first is a vapor phase type of interference, caused by inadequate heat delivery to the analyte as it moves out of the furnace. Here the term “vapor phase interference” is used to include either a recombination of analyte atoms with interferent species, or a lack of dissociation of volatile analyte containing compounds before atomization temperature is achieved. This type of interference is generally a suppression, since it involves a depletion of the observable analyte atomic population; the interference cannot be alleviated by switching from peak height to peak area measurement. Note that many authors consider it a true vapor phase interference only when the analyte atom combines with interferent after entering the vapor phase; this mode of interference could presumably be distinguished by vaporizing analyte and interferent from separate sites (30) within the atomizer, though this test is limited to synthetic solutions. The second type of interference is what we might term a solid phase interference, in which differing volatilities of analyte occur, depending upon the form(s) and thermal stability of the analyte in the furnace before volatilization begins. Less stable forms of the analyte may come off a t an earlier time. In the simplest model case, the analyte would presumably be dissociated promptly or a t the same rate as the more stable forms. This results in an enhancement effect-or a double peak, if relatively stable and unstable forms co-existed in the sample before atomization. Prompt dissociation is an unlikely assumption in the most general case; it may be more likely to be valid for lead, with an appearance temperature of 1040 K (31),than for most elements. This type of interference has been documented (11)and confirmed in this study for ascorbic acid and H F interferences on lead. The exact reasons why an earlier peak is a larger one have not been fully explored; it may also be difficult to isolate this interference from the vapor phase interference. Rate of atom introduction is one factor, and lower temperature is another with its slower diffusion rate and lesser convective disturbances resulting in a larger number of analyte atoms accumulating in the absorption cell before they move out of the optical path. Thus, although peak area might reduce the extent of interference slightly, peak area and peak height would both suffer interference relative to standards due to longer residence time at lower temperatures-because each atom absorbs more light during the longer interval. This work supports the vapor phase mechanism as a primary mechanism for chloride, and probably other interferences in pulsed-type atomizers. The fact that the amount of chloride interference is dependent upon the cation with which the chloride is associated suggests that the chloride formation is occurring in the vapor state; a similar situation has been demonstrated for C1 interference on manganese (9). In each case, those metal chlorides which vaporize and dissociate to provide chlorine at the temperature when the analyte normally atomizes show the greatest interferences. Analysis of the
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
varying cation effects in Table 111indicates general agreement with Czobik and Matousek‘s excellent work using time-resolved studies (17),showing that the degree of suppression depends upon the availability of atomic chlorine for recombination in the vapor phase at the time of P b atomization. Frech and Cedergren (16) also indicate the thermodynamic stability of gaseous PbCl during early atomization. L’vov (8) has theoretically and experimentally illustrated the effects of C1 as a vapor phase interferent and its suppression by the addition of excess LiN03, thereby binding excess C1 through formation of thermally stable LiC1. An important additional clarification of the vapor phase mechanism can be inferred from the absence of chloride interferences in the CTF. The same reactions are most likely occurring, as the sample crucible heats up immediately after insertion into the CTF, but the lead chloride has a much greater chance to decompose before leaving the optical path (or, perhaps, a lesser chance of forming a t all due to the close proximity of the heater tube). Because the sample is volatilized into a graphite tube which is already a t optimum atomization temperature, there is no pathway available for a molecule to escape from the furnace without attaining atomization temperature. Thus, we come to an important point of disagreement with Czobik and Matousek ( I 7 ) ,concerning the role of residence time. Although our work generally supports their data, their conclusion that increased residence time in a longer furnace causes a greater probability of interference (due to concurrent presence of analyte and interferent in the vapor phase) may be not valid beyond a limited set of apparatus design parameters. The lack of interferences cited in the present paper and in Reference 9, using a 30-cm CTF, can lay to rest the notion that one must effect a physical separation of analyte from potential interferents-which is what time-resolution of analyte and chloride peaks in a short residence-time furnace accomplishes. If long residence time does not cause interferences, what does? The data from the CTF reveals that the fundamental variable is temperature-specifically, the temperature seen by the sample as it vaporizes and before it leaves the furnace; this is generally not even close to the maximum atomization temperature obtained late in the atomization cycle. In either the Massman or mini-Massman furnace, lead has a short residence time compared to the time required to heat the furnace tube; only 1YO of the lead remains in the CRA furnace after 0.1 s ( l a ,indicating that the lead peak is observed when the temperature is, a t most, a few hundred kelvins above the appearance temperature of lead. Also, the vapor temperature lags behind the furnace wall temperature in pulsed-type atomizers, and the rate of heat transfer to the vapor a t various times during atomization is expected to depend on matrix salts present. The extent of the temperature lag is currently in dispute, with estimates ranging from 15 to 1300 K (32-34). In the constant temperature furnace, vapor phase interferences do not occur because the sample compounds are heated sufficiently to avoid interferences before leaving the furnace. Thus, the results of this study support those who believe on theoretical grounds that vapor phase interferences should be unlikely, prouided that an optimally high atomization temperature is maintained. It has been the failure to realize the need for improved temperature control-when and where it counts-that has caused many difficulties for analytical chemists using pulsed type atomizers. Attempts to reduce interferences in pulsed-type atomizers with rapid heating ( 8 ) , with a L’vov Platform (8, 35, 36) to delay atomization of analyte, or with a high gas temperature furnace (371, have tended to confirm that temperature is the key variable-but not the only variable (36). Provided a minimum
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temperature is maintained along the furnace, long path length appears to minimize the impact of most of these variables, as seen below. The advantage of a long residence time in minimizing the effect of differing volatilities may be seen by considering Figure 3 in the light of McLaren and Wheeler’s proposed mechanism (12) for the formation of a less stable dimorphic form of P b in the presence of ascorbic acid. No interference is seen in the CTF, where the difference in vaporization times during the rapid heating-up of the crucible becomes negligible compared to the longer residence time a t the pre-set heater tube temperature. In the CRA a pronounced interference is observed because the difference in time of volatilization is larger than the residence time. I t has been noted that a solid-phase interference, of the general type described by McLaren and Wheeler (12),can be induced in the CTF when operated a t very low furnace temperatures-roughly as interferences can be exaggerated in the CRA by using lower than optimal atomization voltages. Work on zinc in this laboratory (38), using X = 307.6 nm and the three-post CTF described in Reference 19 operated at 1180 K, shows a 30% lower peak height when 1 pg of zinc is run as ZII(NO~)~ rather than as ZnClp. This is because of differing volatilities and appearance temperatures; ZnO, to which Zn(N03)zdecomposes, has an appearance temperature of 1140 K, vs. 940 K for ZnClz (31). By increasing the temperature from 1180 to 1350 K (still well below the 1900 K temperature a t which optimal sensitivity for zinc is obtained), the peak heights agree within 5%. Peak area is less prone to this induced interference a t 1180 K, although 10% less peak area was measured for the nitrate. The diffusion rate and residence time are scarcely affected by the original form of the zinc, therefore; but the rate a t which atoms enter the optical path differs. This rate differs greatly when the furnace temperature is only 40 K above the appearance temperature of the less volatile species, but only insignificantly when the furnace temperature is raised. Considerations for Practical Analyses. Now that the several interrelationships of temperature and residence time have been explored, it is appropriate to summarize several practical advantages of the relatively interference-free atomizer over the currently available, interference-prone atomizers. (1) It facilitates determination of many elements simultaneously in the same sample solution (39), by avoiding the necessity of different treatments for different analytes; in other words, accurate element analysis is not highly sensitive to matrix composition or operating conditions such as temperature. (2) Pretreatments of a sample solution, such as addition of interference inhibitors, are generally unnecessary; thus, the associated reagent contamination, as well as time-consuming extraction or other separation procedures, etc., can be eliminated. (3) Direct solid sampling of complex matrices is feasible ( I 9 ) ,or brief heating with nitric acid can be used to solubilize samples. (4) Sampling errors can be reduced by using relatively large sample sizes, since the large furnace can easily analyze 50 mg of solids. (5) Furnace maintenance can be reduced because the graphite is not subjected to frequent thermal stresses, and the atmosphere is more readily controlled to exclude air and water. (6) Most importantly, one can realize great time savings: standard additions are generally unnecessary; batch processing avoids tying up the atomizer during dry and ash cycles; and, finally, less uncertainty and fewer reruns are associated with data interpretation. These advantages must be weighed against several disadvantages which have been cited for the current CTFs. (1) Sample introduction has been cumbersome in pressurized systems such as L’vov’s ( I ) ; however, well over a hundred samples per hour can be run easily when sample crucibles are
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
introduced against a flushing stream of argon from atmospheric pressure ( 2 ) . (2) Most current atomic absorption spectrometers are not designed with adequate space to accommodate a long-path CTF. Serious light losses may also occur due to the long furnace, resulting in poorer signalto-noise ratio, unless the spectrometer has an appropriately collimated light beam to match the furnace dimensions. I t should be noted t h a t a shorter, 15-cm C T F has been built which possesses essentially the same resistance to interferences as the larger ones (40);however, for practical solid sampling, a reasonable compromise between convenient furnace size and sufficient sample size to minimize sampling errors may dictate a furnace length of a t least 20-25 cm. (3) Heater tubes tend to burn out after several months' steady use, and a replacement tube may require careful realignment to optimize light throughput. (4) High temperature operation of CTFs, for elements which form highly refractory carbides, has not yet been described as extensively as in the case of pulsed-type atomizers, although enough work has been done to demonstrate its feasibility. Vanadium, boron, silicon, and others have been determined in a long path CTF; but tungsten and tantalum gave no absorption signals, even when the CTF was operated above 3200 K (41). L'vov reports good sensitivities for Be, Ge, Mo, and Ti, among others, using his constant temperature furnace a t temperatures up to 3073 K (42). The sublimation of graphite, of course, determines the upper temperature limit for both CTFs and pulsed type atomizers. ( 5 ) General maintanance, such as refurbishing electrical contacts, occasionally takes up an operator's time. Thus, it is fair to say that the current home-made models of the C T F do not yet have "ruggedness" built into the equipment. Such problems are normally worked out of equipment during the commercialization process. From the standpoint of the analytical chemist, considering analytical procedures as a whole, the lack of ruggedness and versatility of analytical procedures using the pulsed-type atomizers may now appear to be a more fundamental difficulty. As Smeyers-Verbeke e t al. ( 2 9 ) have cogently expressed, pulsed-type atomizers' interferences for each kind of matrix on each atomized species should probably be regarded as a separate phenomenon. The constant temperature furnace, engineered for routine use, would provide a viable alternative to the extensive cataloging of analyte- and matrix-specific pretreatments now coming into vogue for pulsed-type atomizers.
LITERATURE CITED L'vov, B. V. "Atomic Absorption Spectroscopy"; Israel Program for Scientific Translations: Jerusalem, 1969. Woodriff, R.; Ramelow, G. Spectrochim Acta, Part B 1968, 2 3 , 665. Woodriff, R. Appl. Spectrosc. 1968, 2 2 , 207. Woodriff. R.; Lech, J. Anal. Chem. 1972, 4 4 , 1323. Woodriff, R. Appl. Spectrosc. 1974, 9 8 , 413. Marinkovic, M.; Woodriff, R. Appl. Spectrosc. 1976, 30, 458. Woodriff, R.; Marinkovic, M.; Howald, R. A.; Eliezer, I . Anal. Chem. 1977, 4 9 , 2008. L'vov, B. V. Spectrochim. Acta., Part B 1978, 33, 153. Hageman, L.; Mubarak, A.; Woodriff, R. Appl. Spectrosc. 1979, 33(3). Regan, J.; Warren, J. Analyst(London), 1978, 103, 447. Regan, J.; Warren, J. Analyst(London) 1976, 101, 220. McLaren, J.; Wheeler, R. Analyst (London) 1977, 102, 542. Czobik, E.; Matousek, J. Talanta 1977, 2 4 , 573. Hodges, D. J. Analyst (London) 1977, 102, 66. Thompson. K.; Wagstaff, K.; Wheatstone, K. Analyst(London) 1977, 102, 310. Frech, W.; Cedergren, A. Anal. Chim. Acta. 1976, 8 2 , 83. Czobik. E.; Matousek, J. Anal. Chem. 1978, 50, 2. Mubarak, A.; Hageman, L.; Howaid, R.; Woodriff, R. SoilScl. SOC.Am. J . 1978 4 2 , 889. Nichols, J. A.; Jones, R. D.; Woodriff, R. Anal. Chem. 1978, 50, 2071. Abu-Samra, J. Morris; Koirtyohann. S. R. Anal. Chem. 1975, 4 7 , 1475. Smeyers-Verbeke, J.; Michotte, Y.; Van den Winkel, P.; Massart, D. L. Anal. Chem. 1976, 4 8 , 125. Youden, W. J. J . Assoc. Off. Agric. Chem. 1963, 4 6 , 55. Nichols, J. A,; Ferguson, W. S.;unpublished data, Colorado State University, Ft. Collins, Col. 1973-75. Johansson, K.; Frech, W.; Cedergren, A. Anal. Chim. Acta. 1977, 9 4 , 245. Okuno, I.; Whitehead, J.; White, R. J . Assoc. Off. Anal. Chem. 1978, 6 1 , 664. Boyie, E. A.; Edmond, J. M. I n "Analytical Methods in Oceanography", Adv. Chem. Ser. 147, 1975. Nichols, J. A. Trace Analysis Research Centre International Conference 111, Dalhousie University, Halifax, N.S., Canada; August 4, 1976, Paper No. 11. Churella, D. J.; Copeland, T. C. Anal. Chem. 1978, 50, 309. Smeyers-Verbeke, J.; Michotte, Y.; Massart, D. L. Anal. Chem. 1976, 50, 10. Ohta, K.; Suzuki, M. Talanta 1976, 23, 560. Sturgeon, R. E.; Chakrabarti, C. L.; Langford, C. H. Anal. Chem. 1976, 4 8 , 1792. van den Broek, W. M. G. T.; de Galan, L.; Matousek, J. P.; Czobik. E. J. Anal. Chim. Acta 1978, 100, 121. Adams, M. J.; Kirkbright, G. F. Anal. Chim. Acta 1976, 8 4 , 79. Sturgeon, R. E.; Chakrabarti, C. L. Spectrochim. Acta, Par?B 1977, 3 2 , 231. GrBgoire, D. C.; Chakrabarti, C. L. Anal. Chem. 1977, 4 9 , 2018. Slavin, W.; Manning, D. C. Anal. Chem. 1979, 5 1 , 261. Koizumi, H.; McLaughlin, R. D.; Hadeishi, T. Anal. Chem. 1979, 51, 387. Nichols, J. A.; DewaR, F. G.; Woodriff, R. ACS/CSJ Chemical Congress, Honolulu, Hawaii, April 2, 1979. Anal. Chem. Session, Paper No. 27. Woodriff, R.; Shrader, D. Appl. Spectrosc. 1973, 2 7 , 181. Howell, H. Masters' Thesis, Montana State University, Bozeman, Mont., 1978. Stone, R. W. Ph.D. Thesis, Montana State University, Bozeman, Mont., 1974. L'vov, B. V. "Atomic Absorption Spectrochemical Analysis", American Elsevier Publishing Co.: New York, 1970, p 228.
ACKNOWLEDGMENT The Varian AA-6 and AA-5 spectrometers were provided through the courtesy of the MSU Fisheries Bioassay Laboratory and the Laboratory Bureau of the Montana Department of Agriculture, respectively.
RECEIVED for review March 5 , 1979. Accepted May 2, 1979. This work was partially supported by the National Science Foundation through grants no. CHE-74-15060 and OCE74-24317.
