946
Anal. Chem. 1983, 55, 946-950
(9) Edmonds, J. W.; Henslee, W. W.; Guerra, R. E. Anal. Cbem. 1977, 4 9 , 2196-2203. (10) National Institute for Occupational Safety and Health, US. Department of Health, Education, and Welfare "Technical Report-Preparation and Characterization of Analytical Reference Minerals"; DHEW (NIOSH) Publication No. 79-139. (11) Kirkbright, G. F.; Smith, A,; West, T. S. Anakst (London) 1967, 92,
411-416.
(12) Mavrodineanu, R. "Analytical Flame Spectroscopy"; Springer-Verlag: New York, 1970. (13) Fuller, C. W. "Electrothermal-Atomization for Atomic Absorption Spectrometry"; The Chemical Society: London, 1977.
for review November 5, 1982. Accepted February
1, 1983.
Oxygen Ashing and Matrix Modifiers in Graphite Furnace Atomic Absorption Spectrometric Determination of Lead in Whole Blood David K. Eaton and James A. Holcombe" Department of Chemlstry, University of Texas at Austin, Austin, Texas 78712
An evaluation of the chemlcal and/or physical Involvement of a number of matrix modifiers and procedures Is presented for the determlnatlon of Pb In blood by graphlte furnace atomlc absorption. I n partlcular, the roles of HNO,, NH,H,PO,, 0, ashlng, and surfactants (such as Trlton-X 100) have been studled. Appearance time shlfts and changes In peak shape are also presented. A rapld method for the determlnatlon of Pb In human whole blood by graphlte furnace atomic absorption spectrometry 1s also described. Sample preparatlon Involves dllutlon of whole red cells 1:4 wlth 1% Triton-X 100 solutlon, followed by an air ash. Oxygen ashlng was used at 900 O C wlthout any loss of Pb, no reductlon In furnace llfetime, and negllglble background scatter.
The use of graphite furnace atomic absorption spectrometry (GFAAS) for the determination of trace metals in biological matrices has increased significantly in the past decade. This method occasionally exhibits interference effects with certain elements in complex matrices. Various matrix modifiers have been used to eliminate interference with varying degrees of success. The usual procedure often involves a specific modifier for a particular sample type. The fundamental physical and chemical involvement by the modifier which allows for the successful analysis is generally unknown or unexplained. One of the more recent developments in matrix modification of biological based samples is the addition of oxygen during the ashing step to facilitate removal of carbonaceous residues. The addition of oxygen to the sheath gas has been shown previously to have varied effects on the behavior of lead and some other volatile metals in GFAAS (I). To establish a better understanding of the interactions between oxygen, lead, and matrix modifiers, a study of lead in whole blood and red blood cells was initiated. The main objectives were to develop a successful method of analysis, while examining the physical and chemical effects of several common matrix modifiers used in this type of analysis. These analysis modifications included oxygen ashing, and the addition of nitric acid, surfactants (such as Triton-X 100))and phosphate salts to the sample. The effects of these additives varied considerably and will be discussed further. Each modification exhibited individual chemical and physical behavior, and utilizing these effects to the maximum may assist in the development of analytical procedures for many other metals in various matrices.
EXPERIMENTAL SECTION Apparatus. The optical system and furnace were designed to facilitate time-resolved absorbance studies and to ensure that the system was as free as possible of entrained oxygen unless it was intentionally introduced. A modified Varian carbon rod atomizer furnace configuration is used for all the studies and is shown in Figure 1. The furnace is designed to eliminate problems encountered when using the commercial furnace design, such as unwanted entrainment of atmospheric oxygen in the sheath gas (2-4). Standard Varian support electrodes are secured in two water-cooledbrass blocks. These blocks are attached to a flexible Teflon plate, which, in conjunction with an aluminum base plate, is secured to the optical rail. A micrometer-driven spring-loaded plunger ensured reproducible pressure of the support electrodes on the furnace and eliminated arcing and poor contact. The support electrode blocks are grooved to accept a pair of Teflon plates, which contained quartz windows for the optical path. The window facing the hollow cathode lamp is tilted 20° from vertical to eliminate reflections of the incandescent radiation of the heated furnace from entering the optical path. An aluminum top plate encloses the furnace to prevent entrainment of unwanted gases in the sheath gas and is electrically isolated from the brass blocks with a paper gasket. The sheath gas enters the bottom of the enclosure and passes through a Teflon block, which is drilled t o allow laminar flow. The sheath gas then exits the top of the enclosure through a 0.5 m length of Tygon tubing. The extended length of the tubing prevents back-diffusion of air into the enclosure. The internal volume of the enclosure is approximately 80 mL, facilitating flushing of the system of oxygen, when using an oxygen ash. Flushing of the system consisted of a 30-9 flush of N2 at 2.0 L/min flow rate, before atomization. The optical system has been described previously (5) and consists of a pair of spherical mirrors arranged in an over-andunder symmetrical arm configuration. The sagittal image of the source is focused on the monochromator entrance slit, and a 500-pm slit is placed in the tangential image plane to eliminate emission from the furnace, while allowing maximum spectral throughput. The furnace is located in the object plane of the two-mirror system. Illumination is provided by a hollow cathode lamp focused on the furnace by a 12 cm focal length, biconvex quartz lens. A 0.35-m Czerny-Turner monochromator (GCA McPherson EU-700, Acton, MA) with a 0.6 nm band-pass was used for all studies. The output of the photomultiplier tube was voltage scaled, digitized by an eight-bit analog to digital converter, and stored as 2000 eight-bit words in a 48K byte microcomputer (Vector Graphics,Westlake Village, CA) for further data reduction. The sampling rate was normally in the range of 1 to 2 kHz.
0003-2700/83/0355-0946$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
947
HNO, and rinsed with distilled and deionized water hefore use. Pyrolytically coated furnaces were used for all determinations. Procedures. Comparative studies evaluating various matrix modifiers and oxygen ashing were conducted. Whole blood was employed in most studies after dilution with various matrix modifiers under investigation. For selected analysis of whole red cells, the cells, after removal of the plasma, were diluted with varying concentrations of 1.0% Triton-X 100 and other matrix modifier solutions and mixed thoroughly. A 2-wL sample volume was used for all analyses, using a positive displacement pipet (Scientific Manufacturing Industries, Berkely, CAI.
Flgure 1. Enclosed graphite furnace atomizer: (A) aluminum base plate, (E) Teflon insulator plate, (C) Teflon laminar flow gas block, (D) electrode support blocks, (E) Teflon window support plates, (F) paper Insulator gasket. (G)aluminum top plate with sample intrcductiin port, (H)
micrometer driven spring tensioning device. ~~~
Table 1. Typical instrumental Parameters for Lead in Blood setting time, s temp, "C drv 43 100 asi;
45
900 (air)
m..n.(N.) ash I1 30 350 '- " atomize 3-5 2000 cool-down 20 25 wavelength, 283.4 nm sheath gas flow rate. 2.0 L/min
sheath gas air or N. air or N,
N, N, N,
The basic furnace power supply waa designed in this laboratory ( I ) , and several modifications have been made to permit oxygen ashing. The power supply has four variable-temperature, variable-time steps, with the option of oxygen introduction during the first two steps, viz., dry and ash. The atomize step can he used in a constant applied voltage mode or with photodiode controlled temperature feedback. The fifth step is a cool-down period to allow the fumace to reach room temperature before the reintroduction of oxygen (see Table I). The sheath gas introduction system incorporates automatic solenoid valve switching to allow oxygen ashing with an inert gas flush hefore and during atomization. Two rotameters in conjunction with two, three-way stopcocks control the sheath gas flow rates, as well as allowing gas mixing if desired The term 0, ashing is used throughout this text to broadly specify the use of an oxidizing sheath gas mixture during one or more of the heating stages. Unless specified within the text, nitrogen at 2.0 L/min flow rate was used as the inert sheath gas, and air (approximately 20% 03,at 2.0 L/min, was used during the dry and first ash step as the oxidizing sheath gas. Reagents. Certified ACS reagent grade Pb(NO,), was used to prepare a stock solution that contains 1000 mg/L of Ph. Working standards of 0.1 to 0.5 mg/L of Ph were prepared in Nalgene containers and were used within 8 h of preparation to prevent analyte los. due to adsorption onto the container. Whole human blood was obtained though the local blood hank in the form of outdated units. Whole cells were prepared by centrifugation for 5 min at 1500 rpm and aspriration of the plasma. Stock solutions of matrix modifier dilutents were prepared and stored in polyethylene bottles. Deionized and doubly distilled water was used to prepare all solutions. All glassware was soaked in dilute
RESULTS AND DISCUSSION The determination of lead in whole blood with no matrix modification presented varied problems. The sample was difficult to pipet accurately with air displacement pipets due to wetting of the pipet tips, and a positive displacement pipet was used to ensure precise sample aliquots. Drying conditions were selected to give gradual evaporation of the liquid portion of the sample; however, without the addition of a surfactant, poor contact of the sample with the furnace walls during the subsequent ash often resulted in physical loss of a majority of the sample out of the ends of the furnace. This loss was very prevalent when using the small furnaces and may not present a severe problem when using the larger graphite furnaces. Even when the sample was contained within the furnace, molecular absorption and scatter often produced background signals in excess of 0.5 absorbance units and often obscured the analytical signal. Continuum background correction (Le., D, lamp) was not used in this study. Thus, the reduction of this type of spectral interference was a primary concern. Background measurements were made with the 280.3-nmnonresonance line of Pb. Ash buildup was also noted after several firings which was difficult to remove even with a high temperature burn-out in inert sheath gas. This residue was thought to be carbonaceous in nature, as it could be subsequently removed with a n air ash. A capsule summary of the impact of various analysis schemes is presented in Table 11. Matrix Modification. Triton-X ZOO. This has heen used frequently as a matrix modifier for the determination of P b and other metals in blood (GZ3),either alone or in conjunction with other matrix modifiers such as phosphate salts and/or HNO,. Concentrations of this surfactant in the analytical solutions ranged from 0.1 to 5.0% (v/v). Procedures for analysis involved in the dilution of the samples from 1:l to 1:20 with aqueous solutions of varying concentrations of Triton-X 100. The surfactant was found to exert little or no chemical effect on the standards or samples (6). Brodie and Stevens (2) and Garnys and Smythe (10) noted ash build-up within the furnace after several firings when using blood diluted with Triton-X 100. This type of ash was found to be easily removed with an air ash and is discussed in the next section. This study confirmed the utility of Triton-X 100 addition in facilitating blood analysis. The surfactant reduced the sample/graphite interfacial tension and allowed improved contact between the sample and the furnace walls. Without the addition of Triton-X 100, the majority of the sample was physically lost from the furnace volume due to frothing during the dry, and the subsequent ash was inefficient due to p w r contact with the graphite tube. Several concentrations were used, ranging from 0.1 to 5.0% (v/v). At higher concentrations, interfacial tension was reduced to the point where the sample would flow out the ends of the furnace, resulting in signal loss, as noted by Nise and Vesterherg (12). Reduction of final sample concentration of Triton-X 100 to approximately 1.0% eliminated this problem, and concentrations between 0.5 and 2.0% gave similiar results. The surfactant also produces complete lysis of the cellular elements and
948
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
Table I1 condition no modification
aqueous standards somewhat broad absorbance peak with relatively early release; Pb loss from working solution on glassware; must use plastic containers, e.g., Nalgene
Triton-X 100 0.1-1.0%
difficult to pipet; severe wetting of graphite surface (sample loss from furnace during dry is likely)
> 5% HNO
< 1% > 1%
stabilizes Pb standards and minimizes adsorption on glassware; peak light varies with concentration (generally an enhancement over samples with no HNO,) late shifted peak (often with double peaks); sample can be ashed at elevated temperatures; stabilizes standards
0, ash
late shifted peak; sample can be ashed at elevated temperatures
NH,H,PO, 0.1-1.0%
late shifted peak; sample can be ashed at elevated temperatures
stabilizes the solution by assisting in protein solubilization. The resulting solutions were conveniently pipetted. The addition of Triton-X 100 to aqueous standards was not used. Extreme wetting of the graphite by these solutions produced depressed absorbance signals as a result of the solutions creeping out of the furnace volume before and during the dry cycle. Oxygen Ashing. The successful use of O2 (e.g., air) during the ash cycle has been reported elsewhere (14-16) and serves two primary functions. First, it allowed for a more complete removal of the organic-based matrix by converting a pyrolysis process to a combustion process. This eliminated the build-up of a carbonaceous residue which is characteristically found when ashing in an inert gas. This residue blocked the optical beam, increased the nonspecific absorbance signal during the atomize cycle, and produced an erratic release of the analyte metal which affected the analytical accuracy. Additionally, incorporating air during the ash cycle allowed the use of elevated ashing temperatures for P b without analyte loss. This point is illustrated in Figure 2 where ash curves with and without the use of an air ash for aqueous P b solutions are presented. In the absence of air a maximum ash temperature of 500 "C could be employed without a significant loss of Pb, while in the presence of air, ash temperatures up to 950 "C were used. The mechanism involved has been discussed in greater detail by Salmon et al. (17). From their results, it is suggested that similiar advantages of O2 ashing should also be realized for the determination of Zn, Cd, and, to a lesser extent, Ag. The proper selection of ashing conditions was dictated by a temperature that is sufficiently high to ensure combustion of the matrix without analyte loss. It was found that this temperature was easily arrived a t by constructing an ashing curve similar to Figure 2 and using an ash temperature 50-100 "C lower than the roll-off point where P b was lost. In these studies 900 "C worked extremely well. There was no measurable decrease in the furnace lifetime as a result of air ashing
whole blood pipetting very difficult; large nonspecific absorbance signal (scatter); incomplete removal of residue after ash; erratic peak shapes; frequent physical loss of analyte from furnace during dry or ash provides better solution contact with graphite; minimizes physical loss of sample during char severe spreading of sample solution in furnace and poor reproducibility (with Triton-X 100) little difference compared to samples without HNO, at high concentrations, allows quantitation against aqueous standards; increased analysis time if predigestion used; can reduce lifetime of furnace (with Triton-X 1 0 0 ) sample can be ashed at elevated (-900 "C) temperatures; removes carbonaceous residue; improved reproducibility over inert atmosphere ash; direct comparison with aqueous standards not possible (with Triton-X 100) sample may be ashed at elevated temperatures; inhibits removal of carbonaceous residue (with air ash); peak late-shifted; direct comparison with aqueous standards has been reported
D
1
0. 200 0
488
6m
am
1ME
1200
RSH TEMPERRTURE ( C 1 Flgure 2. Effect of ashing temperature on the absorbance peak height for lead (100.0 pgIL) with and without air ashing: (A) N, ash, (B) air ash.
as long as the furnace was not exposed to air as temperatures in excess of 1000 OC. Furnace lifetimes in excess of 200 firings using air ashing were routinely achieved. Again, this is reasonable based on the mechanism of graphite combustion, Le., O2 being adsorbed as a surface oxide on the graphite and desorbed as CO. Only at elevated temperatures (>lo00 "C) is the CO released at any appreciable rate. Thus, only a few monolayers are removed on every firing as long as an inert atomsphere exists within the furnace at temperatures greater than 1000 "C. Whole blood did not behave identically to the aqueous standards. While the elevated air ashing temperature (900 OC) was successfully used, the appearance time of the P b absorbance peak was not shifted late in time as had been observed with the standards (Figure 3). Our studies also showed that the appearance times of P b standards, with an air ash, were also early time shifted when atomized with a carbonaceous residue produced by ashing a Pb-free protein solution. It is suspected that this residual carbon from the
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
W
‘949
I
0.0
a
I
1.5
1.0
1.5
‘T I ME [ SECONDS1
T I ME (SECONDS I
Figure 3. Absorbance-time profiles for lead standards and blood samples with 900 O C air ash: (A) blood, (B) standard lead solution.
Flgure 4. Absorbance-time profiles for lead (100.0 pg/L) with varying concentrations of HN03 in N, sheath gas: (A) 0.001 %, (6)0.01 %, (C) 0.1%, and (D) 1.0%.
organic matrix rnay provide the needed reducing sites for low temperature reduction to form elemental Pb. With air ashing, the nonatomic absorbance signal from the blood was reduced to a level such that it did not interfere with the magnitude of the peak height. With simple background correction, the s,mall scatter signal (C0.05 absorbance units) near the trailing edge of the peak could be compensated for easily and also permit the accurate use of peak areas. For the determination of Pb in blood, there is little advantage to having the plasma present in the sample since Hursch et al. (18,19)have clearly shown that Pb is adsorbed on the surface of the red cells. Farrely and Pybus (20) also recommended that the whole red cells be used for the determination of Pb due to the error introduced by variations in individual hematocrits. Fernandez (21) reported better precision for the determination of Pb in blood using red cells rather than whole blood. Everson and Pendergast (6) accurately pointed out that the concentration of Pb relative to red cell volume and not whole blood volume should be the primary value of interest for toxicological studies. Also, removal of the plasma eliminated many of the salts present in the sample. No attempt was made to determine P b in the plasma for the obvious reason. For other metals, it is expected that only a slight advantage might be gained for plasma analysis by employment of 0,ashing. While combustion of the proteins is likely, the 0, would be expected to have minimal impact in reducing the interference due to the high salt content of the plasma. Nitric Acid. A large number of studies involving the determination of lead and other metals in blood and plasma used HN03 as an additive (2, 6, 10, 12, 21-25). For discussion and purposes an arbitrary division between dilute (6%) concentrated ( > 5 % ) acid concentrations will be made. The addition of dilute HN03 to both standards and samples was found to be very common; however, we found it unnecessary for the successful determination of Pb in blood. Additionally, significant variations in peak heights and areas caused by minor changes in acid concentrations outweighed the benefits of its use. Other studies have also shown that addition of low concentrations of HNOBstabilized aqueous solutions of Pb and prevented absorption of the metal onto the container walls (26). The use of Nalgene containers in this study eliminated the need for such solution stabilization, and standard working solutions remained reliable for up to 8 h. Evenson and Pendergast (6) found that the Pb atomic signal was severely attenuated if the pH of the solutions was above 4.0, and recommended that all samples and standards be prepared with “strongly acidic solutions”. They also found that the relationship between lead absorbance and pH was the same for three different acids. Similar variations of Pb
absorbance signal magnitude related to changes in HN03 concentrations were also found in this study (Figure 4). However, the absorbance maxima only existed over a small concentration range (k1pH unit). Figure 4 shows that the variations in acid concentrations also affected the shape of the absorbance-time profiles and complicated the use of simple peak height as a measure of Pb concentration. Regan and Warren (27)also noted peak splitting for Pb when using comparable concentrations of HN03. In contrast, variations in HN03 concentrations did not affect the absorbance signal of P b in blood, further indicating the difference in release mechanisms between samples and standards. A similar effect was also noted by Everson and Pendergast (6). Yasuda (28) observed through the use of molecular absorption that with solutions of Pb and Cd halides, the presence of 0.5 M HN03 or 0.01 M HzS04prevented the loss of the metal halides during heating. This behavior was believed to be due to the conversion of the metal halides to the more refractory nitrates and sulfates during the preheating of the solutions. The addition of acid may also promote the volatization of the corresponding acid halides. At higher concentrations of HN03 (1.0%) a slightly higher appearance temperature was observed for the Pb standards. The HN03, or thermal decomposition products of the adsorbed acid may render an oxygenated graphite surface similar to that which resulted from O2pretreatment. The oxidized surface would then have fewer active reducing sites available for the reduction of Pb compounds to elemental Pb. It is also possible that this type of surface is capable of more strongly adsorbing the metal to the surface than would be otherwise observed. The ability of HN03 to oxidize the graphite surface has been reported (29) and is a plausible explanation for the later release times observed for Pb with the higher concentrations of HN03. The enhancement of the Pb absorbance signal at lower acid concentrations shown in Figure 4 is not clearly understood. The very pronounced changes in peak height with relatively minor changes in HN03 concentration was alluded to previously. Unless the sample matrix is relatively uncomplicated and reasonably well matched by the standards, it seems advisable to avoid HNOBaddition in this concentration range. The addition of high concentration of HN03 to blood in a sample pretreatment step has been used by several workers (2, 10, 22, 23, 25). This approach constitutes a wet ashing procedure and probably destroys the majority of the organic matrix, thereby converting the former “complex biological matrix” to a simpler system consisting of short chain hydrocarbons and dissolved salts. A similar effect appears to have been realized by Posma et al. (23) and Garnys and Smytbe (10) by adding concentrated acid directly to the sample within the atomizer.
950
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
0.4
-
0.3
-
w
U
z
U
B
m
0
m
m U
TIME
(SECONDS)
Flgure 5. Absorbance-time profiles for lead (100.0 pg/L) standards with NH4H,P04 addition in N2 sheath gas: (A) standard lead solution, (B) addition of 1.0% NH,H,P04.
The utilization of HNOBoxidation of the sample may offer the unique advantage of allowing the use of aqueous standards for calibration. However, the approach is more time-consuming in the case of sample predigestion, and high concentrations of salts have been shown to catalyze the oxidation of carbon (30) and may promote erosion of the furnace interior. Brodie and Stevens (2)noted severe corrosion of the interior pyrolytic surface after 200 firings when using high acid concentrations in their samples. Also, high reagent blanks may be encountered unless high purity acids are used, or the reagent blank is accounted for in computing the analytical concentrations. Phosphate Addition. Several researchers have added NH4HzP04 to blood for the determination of Pb and Cd. Subramanian and Meranger (11)added 0.1% NH4H2PO4and 0.1% Triton-X 100 to facilitate the determination of P b in whole blood. They also reported that the addition of phosphate salts to whole blood allowed them to use aqueous standards for quantitative analysis. Delves and Woodward (24) used NH4H2P04in addition to oxygen ashing for the determination of Cd in whole blood, and although they were able to ash successfully up to 900 "C, the background absorbance at that temperature was four times higher for the sample with the added phosphate. In our study, the addition of phosphate salts to whole blood and whole red cells resulted in considerably higher background absorbances (0.2 absorbance units) after air ashing than samples without phosphate. Earp and Hill (30) reported that phosphate salts inhibited the oxidation of graphite, and this may have the effect of reducing the efficiency of the air ash in removal of the carbonaceous residue. If O2 ashing is employed, the utility of adding phosphate salts may be questionable; however if O2 ashing capabilities are unavailable, the addition of some phosphate species may be beneficial in the determination of some metals in complex matrices. Czobik and Matousek (31) also reported increased appearance temperatures for Pb and other metals with the addition of H8P04. They postulated a heterogeneous gas phase reaction of PbO and P4010to form the refractory pyrophosphate. Our studies showed that the addition of NH4HzP04 to Pb standards and blood shifted the absorbance peak to higher temperatures (Figure 5 ) . The time (temperature) shift was also concentration dependent, i.e., the higher con-
centrations of phosphate produced the larger time (temperature) shifts. Over a 3 order of magnitude change in NH4H2P04 concentration (0.001-1.0% (w/v)) the appearance temperature increased approximately 400 "C. Unlike the late-shifted peaks for Pb produced by air ashing, the absorbance peaks for Pb in blood, with the addition of phosphate, were also late shifted to approximately the same location as the Pb standards. This behavior is as yet unexplained, but further studies are continuing in this area. Ammonium phosphate salts have also been shown to be effective at the removal of interference from halide salts by the prevolatization of the ammonium halides (32). The high salt concentration of blood may be effectively reduced in the determination of Pb in blood by the use of red cells rather than whole blood. Phosphate salts, however, may be of some utilty in reducing salt interference when using samples such as plasma or serum. Registry No. Pb, 7439-92-1; HN03, 7697-37-2; NH,H,PO,, 7722-76-1; 02, 7782-44-7; Triton-X 100, 9002-93-1.
LITERATURE CITED (1) Salmon, S. G. Ph.D. Dissertation, University of Texas at Austin, Austin, TX, 1981. (2) Brodie, K. G.; Stevens, B. J. J . Anal. Toxicol. 1977, 1 , 282-2135, (3) Salmon. S. G.: Davis. R. H.. Jr.: Holcombe. J. A. Anal. Chem. 1981. 5 3 , 324-330. Rayson, G. D.; Holcombe, J. A. Anal. Chim. Acta 1982, 136, 249-260. Salmon, S. G.; Holcombe, J. A. Anal. Chem. 1978, 5 0 , 1714-1716. Everson, M. A.; Pendergast, D. D. Clin. Chem. (Winston-Salem, N . C . ) 1974, 2 0 , 163-171. Lagesson, V.; Andrasko, L. Clin. Chem. (Winston-Salem, N . C . ) 1979, 2 5 , 1948-1953. Kubaslk, N. P.; Volosin, M. T. Clin. Chem. (Winston-Salem, N . C . ) 1974, 2 0 , 300-301. Paschal, D. C.;Bell, C. J. At. Spectrosc. 1981, 2 , 146-150. Garnys, V. P.; Smythe, L. E. Talanta 1973, 2 2 , 881-887. Subramanian, K. S.; Meranger, J. S. Clin. Chem. ( Winston-Salem, N . C . ) 1981, 2 7 , 1866-1871. Nise, G.; Vesterberg, 0. Clln. Chim. Acta 1975, 8 4 , 129-136. Bailey, P.; Kilroe-Smith, T. A. Anal. Chim. Acta 1975, 7 7 , 29-36. Beaty, R. D.; Cooksey, M. M. A t . Absorpt. Newsl. 1978. 17, 53-58. Beaty, M.; Barnett, W. At. Spectrosc. 1980, 1 , 72-77. Nichols, J. A.; Jones, R. D.: Woodriff, R . Anal. Chem. 1978, 5 0 , 2071-2076. Salmon, S. G.; Holcombe, J. A. Anal. Chem. 1982, 5 4 , 630-634. Hursch, J. B.; Schaub, A.; Sattler, E. L.; Hofmann, H. P. Health Phys. 1989. 16. 257. Hursch, J: B.; Suomela, J. Acta Radiol. 1988, 7 , 108. Farrely, R. 0.; Fybus, J. Clin. Chem. (Winston-Salem, N . C . ) 1069, 15, 566. Fernandez, F. J. Clin. Chem. (Winston-Salem, N . C . ) 1975, 21, 556-561. Hinderberger, E. J.; Kaiser, M. L.; Koirtyohann, S. R. A t . Spectrosc. i.n- -a .i, -2 , 1-7 . .. Posma, F. D.; Balke, J.; Herber, R. F. M.; Stuik, E. J. Anal. Chem. 1975, 4 7 , 834-638. Delves, H. T.; Woodward, J. A t . Spectrosc. 1981, 2 , 65-67. Garnys, V. P.; Matousek, J. P. Clin. Chem. (Winston-Salem, N . C . ) 1975, 2 1 , 691-893. Issaq, H. J.; Zielinski, W. L., Jr. Anal. Chem. 1974, 4 6 , 1328-1329. Regan, J. G. T.; Warren, J. At. Absorpt. Newsl. 1978, 17, 89-90. Yasuda, S.; Kakiyama, H. Anal. Chim. Acta 1977, 8 9 , 369-376. Boehm, H. P. "Advances in Catalysis", Eley, D. D., Pines, H., Weisz, P. B.. Eds.: Academic Press: New York. 1966: Voi. 16. D 179. Earp, F. K.; ~Hili, M . ~ W .I n "Industrial' Carbon 'and Graphite Conference"; Soclety of Chemical Industry: London, 1956; DD 326-333. Czobik, E. J.; Matousek, J. P. Talanta 1977, 2 4 , 573-577. Ediger, R. D.; A t . Absorpt. Newsl. 1975, 14, 127-130.
RECENEDfor review November 17,1982. Accepted February 9,1983. This work was supported in part by a grant from the National Science Foundation CH-07632.
