Anal. Chem. 1980, 52, 1582-1585
1582
Boumans, P. W. J. M.; Bastings, L. C.; deBoer, F. J.; van Kollenburg, L. W. J. Fresenius' 2. Anal. Chem. 1978, 291, 10. (6) Boumans, P. W. J. M.; deBoer. F. J. Specfrochim. Acta. Part51977, (5)
32,365. (7) Ajhar. R. M.; Dalager. P. D.; Davlson, A. L. Am. Lab. 1976, 8 ( 3 ) ,71. (8) Winge, R. K.; Peterson, V . J.; Fassel. V. A. Appl. Specfrosc. 1979, 33, 206. (9) Boumans, P. W. J. M.; Bosveld, M. Specfrochim. Acta, Part 5 1979, 3 4 , 59.
(10) Horllck. Gary Appl. Spectrosc. 1978, 30, 113. (11) Greenfield,S. ICP Inform. New/. 1978, 4 , 199. (12) Watters, R. L.; Norris, J. A. FACSS V 1978, Paper No. 85. (13) Edmonds, T. E.; Horlick, Gary Appl. Specfrosc. 1977, 31, 5 3 6 . (14) Ohis, K. ICP Inform. New/. 1978, 4 , 83.
RECEIVED for review August 14,1979. Accepted May 28,1980.
Determination of Fluorine in Urine and Blood Serum by Aluminum Monofluoride Molecular Absorption Spectrometry and with a Fluoride Ion Selective Electrode Koichi Chiba, Kin-ichi Tsunoda, Hiroki Haraguchi, and Keiichiro Fuwa * Department of Chemistry, Faculty of Science, University of Tokyo, Bunkyo-Ku, Tokyo 7 73, Japan
Fluorine in blood serum and urine samples was determined by both AIF (alumlnum monofluoride) molecular absorption and an ion-selective electrode. In the AIF molecular absorption method, fluorine in the samples was determined by measuring at 227.45 nm the molecular absorption of AIF, which was produced in a graphtte furnace by mhtlng 5 pL of pure or dlluted sample solution and 20 pL of AI(N0,)9 aqueous solution (0.01 M). The ion-selective electrode was used to estlmate free fluoride ion. For urine samples, values for fluorine determlned by the AIF molecular absorption and Ion-selective electrode methods were consistent wlth each other. However, the values in blood serum obtafned by the AIF molecular absorption method were larger by about 2-10 times than those obtained by the ion-selective method. These results suggest the existence of some protein-bound fluorine in blood serum.
I n recent years, the determination of fluorine in various biological samples has been extensively investigated because of clinical and environmental interest. Among the studies, increasing attention has been paid to the analysis of blood serum and urine since the study by Singer and Armstrong (1). In 1968, Taves reported the possibility that two forms of fluorine exist in human blood serum (2, 3); one is the free fluoride ion and the other, the protein-bound fluorine. The Taves study stimulated much work aimed at characterizing fluorine in serum (4-14). Recently, Ekstrand et al. (13)used gel chromatography and 18Fradioisotope analysis to investigate the binding of fluoride to macromolecules in human plasma. They found that fluoride was eluted together with low molecular weight fractions but that there was no indication of fluoride binding to macromolecules. They suggested that the discrepancy between their results and those obtained by Taves might be due to the methodological differences in sample pretreatment and fluorine analysis. Hence, more extensive study on the analysis of fluorine in blood samples as well as other biological samples is desirable to characterize chemical forms of fluorine. An ion-selective electrode (ISE) for the fluoride ion has been extensively used in recent papers (4,8, 10-12), while colorimetric analysis is also still applied ( 5 , 6). As is well-known, the ISE is sensitive only to free fluoride ion. T o estimate total fluorine, some sample pretreatment such as dry ashing or oxygen bomb digestion should usually be carried out. In 0003-2700/80/0352-1582$01 .OO/O
general, the difference between total fluorine and free fluoride ion is assigned to protein-bound fluorine. However, the procedures for sample pretreatments are tedious, and loss or contamination of fluorine may occur during the procedures. Therefore, more simple and convenient methods for sample treatment and fluorine determination are required to characterize fluorine in biological samples. Recently, the authors developed a new spectrochemical method for fluorine analysis (15, 16), where molecular absorption of aluminum monofluoride (AlF) produced in a high-temperature graphite furnace was measured a t 227.45 nm using a deuterium lamp (15) or a platinum hollow cathode lamp (16) as the light source. It has been shown that AlF molecular absorption spectrometry (MAS) can determine organic fluorine as well as the free fluoride ion. Therefore, this method may be suitable for the determination of total fluorine without any sample pretreatment, since biological samples can be digested in the graphite furnace during the analytical procedures for AIF molecular absorption measurement, as will be mentioned later. Hence, the AlF molecular absorption method will be applied to the determination of fluorine in blood serum and urine samples. In addition, blood serum and urine samples have been also analyzed by the ISE method for fluorine, and the results are compared to those obtained by the A1F molecular absorption method.
EXPERIMENTAL Apparatus. An atomic absorption spectrophotometer with a simultaneous background correction system (Model AA 170-50) from Hitachi Co., Ltd., was used for the measurement of A1F molecular absorption. A carbon rod furnace FLA-100 from Nippon Jarrell Ash Co., Ltd., was used as a high temperature cuvette. Argon gas (1.0 L/min) was used to purge the carbon rod furnace of air. A platinum hollow cathode lamp and a deuterium hollow cathode lamp from Hamamatsu T V Co., Ltd., were adopted as the light sources for molecular absorption and background absorption measurements, respectively. The spectral band-pass of the monochromator was usually set at 1.1nm. A fluoride ISE from Denki Kagaku Keiki Co., Ltd., was also used for the determination of fluoride ion in urine and blood serum samples, The electrode potential was measured via a potassium chloride reference electrode (Model HS305DP) from Towa Dempa Co., Ltd., together with a system for high precision on-line A/D conversion (17). Chemicals. All the reagents used, except for Na2CO3,were of analytical reagent grade purchased from Wako Pure Chemical Co., Ltd. Na2C03used for alkaline fusion of serum samples was of guaranteed reagent grade from Merck Co., Ltd. The fluoride 0 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
Table I. Experimental Procedures and Conditions for Fluorine Determination by AlF Molecular Absorption Spectrometry with the Carbon Rod Furnacea Application of aluminum solutionb (0.01 M, 20 pL)
.1 Drying I ( 2 5 A, 20 s)
3. Ashing I ( 6 0 A, 1 5 s)
4 Cooling of furnace
4 Application of sample solution ( 5 CtL)
3.
Drying I1 ( 2 5 A, 20 s) 4
Ashing I1 (60 A, 30 s)
4 Atomization and measurement (280 A, 7 s) a Experimental conditions at each stage are shown in parentheses. b Aluminum nitrate solution including iron(II1) nitrate (0.01 M) and strontium nitrate (0.01 M). ion standard solution was prepared by dissolving sodium fluoride in distilled water. All metal ions were in the form of nitrates. Total ion strength adjustment buffer (TISAB) solution for fluoride ion analysis with the fluoride ISE was prepared as follows: 57 mL of glacial acid, 58 g of NaCl and 300 mg of sodium citrate were dissolved in 500 mL of distilled water. The pH of the solution was adjusted to 5-5.5 with 5 N NaOH aqueous solution in a cold water bath. The buffer solution was then diluted to 1 L with distilled water. Preparation of Urine Samples. The standard reference material of freeze-dried urine (SRM 2671) certified for fluoride supplied from the National Bureau of Standards (NBS) was chosen as a reference sample for urine analysis. The standard consists of two bottles of freeze-dried human urine, one containing fluoride at a low level (0.835 f 0.082 mg F/L) and the other at an elevated level (7.14 f 0.48 mg F/L). These standards can be reconstituted by adding 50 mL of pure water to each bottle, according to the instructions of NBS. The elevated-level sample was diluted to 20, 40, and 100 times, respectively, in order to eliminate the influence of co-existing salts in the urine, and used as the test solutions. The low-level sample was also diluted to 5 and 10 times for a similar purpose. Then, an aliquot (5 pL) of the test solution was analyzed by AlF MAS. These diluted samples were also used as test solutions for the determination of fluoride ion with the fluoride ISE method. Fresh human urine samples which were collected from adult males were also analyzed as unknown urine samples. These samples were diluted to 10 times, and an aliquot (5 wL) of the diluted urine solution was used for the measurement of fluorine by the AlF MAS method. For the measurement of fluoride with the fluoride ISE, however, the nondiluted samples were used because of the low-level of fluoride in human urine samples. Measurement Procedure. The procedure and the experimental conditions for the measurements of A1F molecular absorption are summarized in Table I. In practical measurement, the appropriate amounts of Fe3+and Sr2+were added t o aluminum solution in order to enhance the sensitivity and to improve the precision of analysis. More details have already been described in previous papers (15, 16). In the measurement using the fluoride ISE, 10 mL of TISAB solution was added to 10 mL of each sample, and both fluoride ion and reference electrodes were set in the test solution, which was mixed by a magnetic stirrer. After 50 min, the potential of the fluoride ISE was measured and plotted vs. the fluoride ion concentration. All measurements were carried out at 20.0 f 0.5 "C. These procedures for fluoride ion measurements were almost similar to the previous work ( 1 7 ) . Preparation of S e r u m Samples. Direct Determination of Fluorine i n Serum. Serum samples were taken from patients in
1583
the hospital of Tokyo University and measured within 48 h after collection. For measurement using AlF MAS, serum samples were diluted to 10 times and then 10 pL of the diluted samples were used as a test solution. This dilution procedure was necessary because the high viscosity of serum caused sampling errors when the micropipets were used. For the measurement using the fluoride ISE, the experimental procedures were the same as those for urine samples. With respect to protein in serum, Singer et al. (8) reported that protein in biological fluids caused difficulty in obtaining reproducible potential readings with the fluoride ISE. However, we observed no influence of protein (albumin) on the fluoride ISE measurement with the artificial serum samples including 6%, 8%, and 12% albumin, respectively. Furthermore, there was no evidence that added fluoride ion newly bonded to added albumin. Then, ionic fluorine in serum was determined by the fluoride ISE without any pretreatment. Digestion Procedure for Serum Samples. For the measurement of total fluorine concentration, serum samples were digested as follows; 5 mL of 1 N NaOH was added to 25 mL of serum, and the serum solution was evaporated to 5-7 mL at 80 "C. Then, 2.0 g of Na2C03was added to this concentrated solution. After evaporating to dryness, this sample was ashed at 600 "C for 12 h. The ashed sample was added to 60 mL of 70% H2S04and distilled with a steam distillation system. Fluoride in serum was distilled as a form of HzSiFs at about 140 "C. Fluoride in this test solution (the collected solution volume: 200 mL) was measured with both the AlF MAS and fluoride ISE methods. RESULTS AND DISCUSSION As has been described earlier ( 1 5 , 16), the molecular absorption spectrum of AlF, which is produced in high temperature media such as flames and carbon rod furnaces, provides a sharp line-like absorption band near 227.45 nm. Therefore, the molecular absorption of A1F can be measured easily in a manner similar to standard atomic absorption spectrometry. In the previous paper (16),it was reported that the platinum atomic lines at 227.438 and 227.485 nm were the good absorbers for the A1F molecular band. Consequently, the experimental system using a platinum hollow cathode lamp and a deuterium lamp as the light sources for the A1F molecular absorption and the background correction, respectively, allowed the determination of fluorine with high sensitivity. The analytical working range for the AlF molecular absorption spectrometric method was ca. 10-5-10-6 M (0.19-0.019 pg F/mL), and the sample volume required was only 5 FL. Furthermore, the background absorption due to NaCl up to 0.05 M could be corrected by the present system. In addition to sensitive detection of fluorine, the present system is suitable for the determination of total fluorine, because the digestion of the samples can be performed in the carbon rod furnace a t the ashing I1 and atomization stages as shown in Table I. Therefore, in the following experiment, AlF molecular absorption spectrometry was applied to the determination of total fluorine in urine and blood serum. Determination of F l u o r i n e in Urine. Fluorine in urine samples was determined by A1F MAS and fluoride ISE methods. (The normal fluorine concentration level in human urine is from 0.2 to 2 pg/mL (18).) First, a n artificial urine sample, which contained 5 g / L of NaCl, 3 g / L of KCl, 3 g / L of NH4H2P04,and 20 g / L of urea, was prepared in order to investigate the influence of coexisting constituents in urine, and the experimental conditions for the determination of fluorine by A1F MAS were examined, using the artificial sample. It was found that 10 times dilution (or at least 5 times dilution) of the urine samples allowed the determination of fluorine in the urine sample directly without any interference due to coexisting constituents. Then, fluorine was determined in the freeze-dried urine from NBS (SRM 2671) to check the reliability (accuracy and precision) of A1F MAS. The results obtained by AlF MAS are summarized in Table 11, which also includes the fluoride content in the same urine samples as determined by the fluoride ISE method. T h e
1584
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
Table 11. Determination of Fluorine in Freeze-Dried Urine from NBS (SRM 2671) certified found, pg/mL values, dilution samples factor ~ 1 ~ 4ISEb d m L elevated- 100 times 7.1 t 0.1 7.6 t 0.3 level 40 times 6.90 f 0.18 7.00i 0.24 7.14 ~t0.48 sample 20 times 7.10t 0.18 7.00t 0.24 low-level 1 0 times 0.89 t 0.03 0.80 t 0.03 sample 5 times 0.81 r 0.05 0.80 t 0.03 0.84f 0.08 AlF molecular absorption method. selective electrode method.
Fluoride ion
AlF ISE
0.85 0.84
0.77 0.72
sample I1 M A 0.46 0.49
samPle
found
I I1 I11 IV
0.13 t 0.02 0.12t 0.02 0.21 I 0.03 0.16 i 0.02
recovery,
addedb
ISE found
%
0.34 t 0.04 105 0.31 i 0.04 95 0.40i 0.04 95 0.35 i 0.04 95
0.076 t 0.002 0.025 I 0.001 0.020 I 0.001 0.024f 0.001
Table V. Experimental Results for Determination of Fluorine in Human Blood Serum
fluorine content,b pg/mL
methoda
AlF
a A1F = AlF molecular absorption, ISE = fluoride ion selective electrode. b 0.2 pg/mL fluoride ion was added.
Table 111. Determination of Fluorine in Fresh Human Urine Samples sample I M A
Table IV. Determination of Fluorine in Blood Serum fluorine content: pg/mL
0.24 0.23
sample I11 A 0.44 0.37
A1F = A1F molecular absorption, ISE = fluoride ion selective electrode. M and A mean that urine was sampled in the morning and afternoon, respectively. analytical values obtained by the two methods are almost consistent with each other and also with the certified values of NBS, as can be seen in Table 11. Furthermore, it is clear that the results for elevated and low urine standard samples are not affected by the dilution factor. Fluorine concentrations in unknown human urine samples were also determined by both the A1F MAS and the fluoride ISE method, and the results are presented in Table 111. The analyzed values indicate that both AlF MAS and fluoride ISE are very reliable methods for the determination of fluoride in urine samples. According to Reference 18, the fluorine concentration level in normal human urine is in the range of 0.2-2 yg/mL. The results shown in Table I11 suggest that the fluoride concentrations in the urine samples investigated are a t the normal level. Furthermore, it may be concluded from the consistent fluorine contents obtained by the AlF MAS and the fluoride ISE methods that most of the fluorine in urine is in the form of free fluoride ion. A similar conclusion has been drawn in the previous work (18). Determination of Fluorine in Blood Serum. Fluorine in blood serum was also determined by both the A1F MAS bnd the fluoride ISE methods. In blood serum, the main inorganic constituents, which provide the background absorptions in A1F MAS, are contained a t lower concentration levels than those in urine. However, this caused a large experimental error due to the incomplete background correction, when blood serum was analyzed directly by AIF MAS. Therefore, sample dilution of more than 5 times was necessary for accurate determination of fluorine in blood serum by A1F MAS. The other experimental difficulty in the analysis of blood serum arises from the large viscosity due to the coexisting protein at about 8%. The high viscosity of the sample resulted in a large sampling error, when a few microliters of blood serum were taken with a micropipet. Actually, when 5 yL of 5 times diluted-blood serum was pipetted, the relative standard deviation (rsd) of pipetting was about 15% (at maximum). Pipetting 10 pL of 10 times-diluted serum, the rsd was less than 4%. Although more dilution of blood serum was desirable for more precise pipetting, the concentration of fluorine in the diluted serum became too low to be analyzed by A1F MAS. Therefore, 10 pL of 10 times diluted blood
total fluorine, @/mL 0.12 i 0.020 0.13 t 0.030 0.15 t 0.018
___
___
0.082t- 0.0052 0.25 i 0.009 0.24 t 0.020 0.13 t 0.02 0.12t 0.02 0.21 t 0.03
ionic fluorine, clg/mL
___
-_0.015 i 0.025 i 0.023 t 0.026 t
___ ___
0.001 0.0012 0.055 0.0012
0.076 t 0.002 0.025 2 0.001 0.020 t 0.001
ref. 1 1 8
9 10 11 12 12
this work
Table VI. Fluorine Contents in Blood Serum with and without Sample Pretreatment method AlF MAS4
without sample no. treatment, pg/mL
serum I 0.13 t 0.02 serum I1 0.12t 0.02 serum I11 0.21 i 0.03 F-ISE~ serum I 0.076 i 0.002 serum I1 0.025t 0.001 serum I11 0.020 t 0.001 AlF molecular absorption spectrometry. ion selective electrode.
with ashing, pg/mL 0.16 i 0.21 t 0.27 i 0.24i 0.18 t 0.29 i b
0.03 0.03 0.03 0.001 0.001 0.001
Fluoride
serum was provided for A1F MAS. The analytical results obtained by A1F MAS are summarized, along with those obtained by the fluoride ISE method, in Table IV. In Table IV, the recovery values obtained by A1F MAS are also shown. The recovery values from 95% to 105% may indicate that AlF MAS is a reliable method for the determination of fluorine in blood serum. I t should be stressed that the analytical values obtained by A1F MAS are larger by about 2-10 times than those obtained by the ISE. These facts suggest the existence of protein-bound fluorine in blood serum, as mentioned previously (21, because A1F MAS and ISE may measure total and ionic fluorine, respectively. Several reported values of total and ionic fluorine in blood serum are summarized in Table V, and the values obtained in the present work are also included. According to the data shown in Table V, the values of total and ionic fluorine obtained in the present work appear to the reasonable. Therefore, it may be considered that the fluorine content determined by A1F MAS corresponds to the total fluorine in blood serum. To examine total fluorine content, blood serum was digested by the dry ashing method with Na2C03. The digested sample was dissolved in 70% H2S04,followed by steam distillation. The serum samples treated above were provided for the determination of fluorine by A1F MAS and the ISE. As can be
Anal. Chem. 1980, 52,
seen from Table VI, the values for the digested sera obtained by A1F MAS and the ISE show fairly good agreement, while the values for the digested sera are slightly larger than those for the nondigested sera obtained by AlF MAS. There may be a possibility t h a t the differences of analytical results for nondigested and digested sera originate from contamination during the complicated procedures of dry ashing or from the existence of another form of fluorine; e.g., volatile organic fluorine compounds which may not be detected by AlF MAS. However, since A1F MAS has less chance of contamination because of its simplicity and rapidity, the present results may strongly support the existence of fluorine bound to protein, which was suggested by Taves (2, 3 ) .
ACKNOWLEDGMENT T h e authors thank t o S. Kamei for providing the blood serum s a m d e s . We also exmess our thanks to S. Fuiiwara and Y. Umwawa for the kind permission to use their electrochemical measurement instrument with an on-line A/D conversion system.
LITERATURE CITED (1) Singer, L.; Armstrong, W. D. Anal. Chem. 1959, 3 7 , 105-108. (2) Taves, D. R. Nature (London) 1968, 217, 1050-1051.
1585-1588
1585
Taves, D. R. Nature (London) 1968, 220, 582-583. Taves, D. R. Talanta 1966, 75, 969-974. Cox, F. H.; Backer Dirks, 0. Caries Res. 1968, 2 , 69-78. Venkateswarlu, P.; SRa, P. Anal. Chem. 1971, 4 3 , 758-760. Bock, R.; Semmler, H. J. Fresenius' 2. Anal. Chem. 1967, 230, 161- 184. Singer, L.; Armstrong, W. D. Biochem. Med. 1973, 8 , 415-422. Venkateswarlu, P. Anal. Chem. 1974, 4 6 , 878-862. Venkateswarlu, P. Clln. Chlm. Acta 1975, 59, 277-282. Venkateswarlu, P. Anal. Biochem. 1975, 66,512-521. Singer, L.; Ophaug, R. H. Anal. Chem. 1977, 49, 38-40. Eksband, J.; Ericsson, Y.; Rosell, S . Arch. OralBid. 1977, 22, 229-232. Belisle, J.; Hagen, D. F. Anal. Biochem. 1978, 8 7 , 545-555. Tsunoda, K.: Fujiwara, K.; Fuwa, K. Anal. Chem. 1977, 49, 2035-2039. Tsuncda, K.; Chlba, K.; Haraguchi, H.; Fuwa, K. Anal. Chem. 1979, 51, 2059-2061. Sawatari, K.; Imanishi, Y.; Umezawa, Y.; Fujlwara, S.Bunsekl Kagaku 1978, 27, 180-183. Tusl, J. Clin. Chem. 1970, 27, 216-218.
RECEIVEDfor review March 19,1980. Accepted June 5, 1980. This research has been supported partly by a Grant-in-Aid for Environmental Science under grant No. 403029, and also partly by a Grant-in-Aid for Special Project Research under grant No. 421704 from the Ministry of Education, Science, and Culture, Japan.
Comparison of Methods for the Determination of Trace Elements in Seawater R. E. Sturgeon,* S. S. Berman, J. A. H. Desaulniers, A.
P. Mykytiuk, J. W.
McLaren, and D. S. Russell
Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada, K I A OR9
Five different analytical methods comprising three instrumental techniques were utilized in a study to determine the trace metal content of coastal seawater. Results obtained using isotope dilution spark source mass spectrometry, graphite furnace atomic absorption spectrometry (GFAAS), and inductively coupled plasma emission spectrometry foilowing trace metal separation-preconcentration (using ion-exchange and cheiation-solvent extraction), or direct analysis (by GFAAS) were collated. Comparison of data between suitably different analytical methods is a practical way of testing the validity of those methods, gives increased confidence in the results obtained, and is especially valuable when standard reference materials are not available.
T h e determination of trace elements in seawater is pursued with great difficulty (I). Quantitation of extremely low concentrations of analyte (0.02-10 pg L-I) accompanied by a matrix consisting of 3.5% dissolved solids imposes great demands on instrumental techniques. Sample preparation schemes designed to both preconcentrate the trace elements and separate them from major interfering components prior to analysis are numerous (e.g., 1-7). All such methods invariably increase sample manipulation and the relatively large amounts of reagents and container surfaces brought into contact with the sample often give rise to unacceptably high and/or random procedural blanks. These problems are exacerbated by a lack of standard reference materials which would permit detection of systematic errors such as contamination or analyte losses introduced during sample manipulation and the presence of matrix or spectral interferences 0003-2700/80/0352-1585$01 .OO/O
perturbing instrument response. A logical approach which serves to minimize such uncertainties is the use of a number of distinctly different analytical methods for the determination of each analyte wherein none of the methods would be expected to suffer identical interferences. In this manner, any correspondence observed between the results of different methods implies that a reliable estimate of the true value for the analyte concentration in the sample has been obtained. T o this end the analysis of coastal seawater for Cd, Zn, Pb, Fe, Mn, Cu, Ni, Co, and Cr was carried out using isotope dilution spark source mass spectrometry (IDSSMS), graphite. furnace atomic absorption spectrometry (GFAAS), and inductively coupled plasma emission spectrometry (ICPES) following trace metal separation-preconcentration (using ion-exchange and chelationsolvent extraction), and direct analysis (by GFAAS). T h e analytical advantages inherent in such an approach are discussed in the present paper.
EXPERIMENTAL Instrumentation. A Varian lechtron atomic absorption spectrometer, model AA-5, fitted w i h a Perjkin-Elmer HGA-2200 graphite furnace and temperature ramp accessory was used for all atomic absorption measurements. Simultaneous background corrections were made using a deuterium lamp. Sample solutions were delivered to the furnace in either 10- or 20-pL volumes using a Perkin-Elmer AS-1 autosampler, and absorbance peaks were simultaneously recorded on a fast response Speed Servo I1 strip-chart recorder (Esterline Corp.) and a digital storage oscilloscope (Gould Advance). Samples were held in acid washed polypropylene cups prior to injection. Pyrolytic graphite coated furnace tubes were used exclusively. A modified Perkin-Elmer graphite tube was used for direct GFAAS determinations of iron and zinc (8).
0 1980 American Chemical Society
