Anal. Chem. 1980. 52, 453-457
dissolved selenium and for selenite by the methods discussed above. I n general selenite, and selenate (taken as the difference between the result for total dissolved selenium and for selenite) were above the detection limits of 0.06 and 0.05 wg Se L-', respectively. For the quite polluted Scheldt river, its estuary, and the North Sea, the concentrations for selenate were generally lower than those for selenite. In drinking water, swimming water, and spring water, the reverse seemed to be true. The methods are at present being applied in a large scale screening program for selenium in the Belgian environment.
ACKNOWLEDGMENT We acknowledge the help and suggestions of H. A. Van der Sloot, Energy Research Foundation ECN, Petten, The Netherlands, in the initial phase of this work.
LITERATURE CITED I. Rosenfield and 0. A. Beath, "Selenium", Academic Press, New York, 1964. K. Schwarz, Nutr. Rev., 18, 193 (1960). E. Wolf, V. Kolionitsch, and C. H. Kline, J . Agric. Food Chem., 1I.355 (1963). L. G. Sillen, Svensk. Kem. Tidskr., 75, 161 (1963). Y. K. Chau and J. P. Riley. Anal. Chim. Acta, 33,36 (1965). 0. Yoshii, K . Hiraki, Y. Nishikawa, and T. Shigematsu, Bunseki Kagaku, 26, 91 (1977). Y. Sugimura, Y. Suzuki. and Y . Miyake. J . Oceanogr. SOC. Jpn., 32, 235 (1976). Y. Sugimura and Y. Suzuki, J . Oceanogr. SOC.Jpn., 33, 23 (1977). T. Yamatshige, Y. Ohmoto, and Y. Shigetomi, BunsekiKagaku, 27,607 (1978). J. H. Tzeng and H. Zeitlin, Anal. Chim. Acta, 101, 71 (1978). Y. Shimoishi, Anal. Chim. Acta, 64, 465 (1973). Y. Shimoishi and K . Toei, Anal. Chim. Acta. 100, 65 (1978). C. I. Measures and J. D. Burton, Nature (London), 273, 293 (1978). G. A. Cutter, Anal. Chim. Acta, 98, 59 (1978). F. D. Pierce and H. R. Brown, Anal. Chem.. 48, 693 (1976). F. D. Pierce and H. R. Brown, Anal. Chem., 49, 1417 (1977).
453
(17) M. Verlinden and H. Deelstra, Fresenius' Z . Anal. Chem., 296, 253 (1979). (18) A. Meyer, C. Hofer, G. Tolg, S. Raptis, and G. Knapp. Fresenius' Z . Anal. Chem., 296, 337 (1979). (19) R. Masshe, H. A. van der Sloot, and H. A. Das, J . Radioanal. Chem., 35, 157 (1977). (20) A. D. Shendrikar, Sci. Total Environ., 3, 155 (1974). (21) G. Gissel-Nielsen, Risb Report 370, Risb National Laboratory, Roskilde, Denmark, November 1977. (22) B. G. Lewis, C. M. Johnson, and T. C. Broyer. Pbnf Soil, 40, 107 (1974). (23) B. G. Lewis and C. M. Johnson, J . .4gric. Food Chem., 14, 638 (1966). (24) C. S. Evans and C. M. Johnson, J . Chromatogr., 21, 202 (1966). (25) C. S. Evans, C. J. Asher, and C. M. Johnson, Aust. J . Bioi. Sci., 21, 13 (1968). (26) B. M. Vanderborght and R. E. Van Grieken, Anal. Chim. Acta, 89. 399 (1977). (27) P. Van Espen, H. Nullens, and F. Adams. Nucl. Instrum. Methods, 142, 243 (1977). (28) P. J. Van Espen. F. C. Adams, L. M Van't dack, and R. E. Van Grieken, Anal. Chem., 51, 961 (1979). (29) M. L. Hollander and Y. E. Lebedeff, U.S. Patent 2 834652 (13-5-1958) as described in "Selenium", R. A. Zingaro and W. C. Cooper, Eds., Van Nostrand Reinhold Company, New York, 1974. (30) F. P. Treadweil and W. T. Hall, "Analytical Chemistry", Vol. I, 9th ed.. John Wiley, New York. 1948, p 110. (31) G. S. Deshmukh and K. M. Sankaranarayanan, J . Sci. Res. Banaras Hindu Univ., 3, 5 (1952-53). (32) H. Robberecht, R. Van Grieken and H. A. van der Sloot, Proceedings of the International Congress on Analytical Techniques in Environmental Chemistry, Barcelona, November 1978, Pergamon, Elmsford, N.Y., 1979, in press. (33) D. M. Fogg and N. T. Wilkinson, Analyst (London), 81, 525 (1956). (34) J. G. Sherrat and E . C. Conchie, J . Assoc. Off. Anal. Chem., 7 , 109 (1969). (35) H. A. van der Sloot, Netherlands, Energy Research Foundation ECN, Petten, The Netherlands, private communication, 1979
RECEIVED for review August 14, 1979. Accepted November 14, 1979. This work is supported in part by the Belgian Ministry of Health through the "Selenium Impact" research project (Promotor: D. Vanden Berghe, Department of Medicine, University of Antwerp).
Vacuum-Ultraviolet Atomic Absorption Spectrometry of Mercury with Cold Vapor Generation Kiyoshi Tanabe, Jun'ichi Takahashi, Hiroki Haraguchi,
and Keiichiro Fuwa
Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Vacuum-uitraviolet atomic absorption spectrometry of mercury has been investigated at 185.0 nm, using an argon gas-purged monochromator (50-cm focal length). The cold vapor generation technique was employed for atomization of mercury from the solution phase. An absorption cell equipped with T-shaped tubes has been devised to connect the optical path between the light source and entrance slit of the monochromator, and purging of the optical path with nitrogen gas is performed. The sensitivity and detection limit for mercury, 0.06 ng and 0.01 ng, respectively, were dependent on the type of the electrodeless discharge lamp.
Current environmental concern over the danger of mercury pollution has accelerated progress of analytical methods for mercury. Especially in the field of atomic absorption spectrometry, the cold vapor generation technique has made it possible to determine mercury at the sub-ppb (ng/mL) level (1-31, and such methods have been extensively applied to mercury analysis in various samples. However, some natural samples (e.g., seawater) contain mercury only a t a few ppt 0003-2700/80/0352-0453$01.00/0
(pgjmL) level (4-6). Therefore, improved methods are required to directly determine such low-level mercury concentration without resorting to tedious preconcentration procedures. Almost all the studies so far performed have utilized the spin-forbidden resonance line a t 253.7 nm, caused by the transition 6s1So-6p3P,. On the other hand, it has been known that the main resonance line of mercury is a t 185.0 nm in the vacuum-ultraviolet (VUV) region. The VUV atomic line, caused by the transition 6s1So-6p1P,, has a higher oscillator strength cf = 1.18) than the 253.7 nm line (f = 0.026). Consequently, improved atomic absorption sensitivity may be expected a t 185.0 nm compared to 253.7 nm, since the absorption coefficient is proportional to the oscillator strength of each line ( 7 ) . Despite the advantage in the use of the 185.0-nm line, based on the consideration of oscillator strength, only a few investigations have been reported in terms of VUV atomic absorption spectrometry o f mercury (8-11). This may be due to the experimental difficulty in the VUV region because of the oxygen molecular absorption interference and background molecular absorption by molecules produced in the atomization process. Dagnall et al. investigated the senC 1980 American Chemical Society
454
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
Table I . Experimental Apparatus and Components apparatus or components
model or size
atomic absorption spectrophotometer photomultiplier hollow cathode lamp EDL lamp
AA-1 Mark-IIa
power supply for EDL absorption cell (quartz)
185E
a
RlO6UH
100-cm length, 4-mm i.d. 30-cm length, 6-mm i.d.
1;
manu fact urer Nippon Jarrell-Ash Hamamatsu TV Hamamatsu TV West inghouse, Hamamatsu TV West inghouse
L-
laboratoryconstructed
The reciprocal linear dispersion was 0.16 nmimm. Figure 1. Schematic diagram of VUV atomic absorption system for mercury. (A) Mercury EDL lamp, (B) T-shaped tube, (C) absorption cell, (D) monochromator, (E) inlet of purge-nitrogen gas, (F) inlet of carrier-argon gas, (G) gas flow meter, (H) reaction vessel, (I) silicone rubber septum, (J) water trap (ice cold), (K) mercury trap (1 % KMnO, acidic solution),(L) inlet of purge-argon gas, (M) photomultiplier
sitivity of the 185.0-nm line, compared with that of the 253.7-nm line, and obtained about a 32 times improvement in sensitivity with the former line (8). Robinson et al. reported a successful analysis of mercury in air samples a t 185.0 nm, using a sophisticated furnace sampling atomization system (9). However, with respect to solution samples, sensitive methods utilizing the 185.0-nm line have not been reported. Recently, the present authors reported an absorption cell system for VUV atomic absorption spectrometry of mercury (12). In the system, an absorption cell for mercury vapor was set inside a large-diameter glass tube, which was equipped with two lenses. T h e system, however, was not so convenient for practical analysis. Hence, in the present paper, a simple absorption cell system for VUV atomic absorption spectrometry utilizing the cold vapor generation technique is described, where an argon-gas purged monochromator and electrodeless discharge lamps (EDLs) were used. Using the novel instrumental system, very sensitive VUV atomic absorption analysis is realized with the experimental procedure similar to the conventional one at 253.7 nm. The sensitivity and detection limit of the present method will be compared with that a t 253.7 nm.
sample solutions with a syringe. Several vessels with different volumes were examined to improve the sensitivity, as will be discussed later. Procedure. First, carrier gas was passed through the bypass. Next, the tin chloride solution (reducing agent) was taken into the reaction vessel, and the carrier gas was directed to the reaction vessel by switching the 3-way stopcock from the bypass to the vessel. This procedure was necessary for removal of air and mercury contained in the vessel and in the reducing solution. After removal of the air and mercury, the carrier gas flow was switched back to the bypass and the mercury sample solution was injected with a syringe into the reaction vessel through the side wall septum. The vessel was shaken for ca. 20 s. Then, the carrier gas was flowed again through the reaction vessel, and the mercury atomic absorption signal was measured. The silicone rubber septum was available for 50-60 times injections. The measurement time for one sample was about 3 min. The signal was measured as the peak height on the recorder chart.
EXPERIMENTAL Chemicals. A stock solution of mercury (10 kg/mL) was prepared by dissolving mercury(I1) chloride in 3% (w/v) HC1. All the standard solutions were obtained by diluting the mercury stock solution with 1% HCl solution, while the standard solutions with a concentration lower than 10 ng Hg/mL were diluted with sub-boiling water to avoid contamination from the HC1 solution. All the mercury solutions prepared were analyzed within 5 h. A 3% SnCl, in 1% HC1 solution was used as the reducing agent. The mercury vapor which passed through the absorption cell was led to a trap containing the 1% KMnO, acidic solution. Instrumentation. A schematic diagram of the measurement system is shown in Figure 1. The apparatus and the components used are listed in Table I. Since the 185.0 nm is in the VUV region, the optical path from the light source to the detector (photomultiplier) should be oxygen-free. In order to satisfy this condition, a nitrogen/argon gas-purging technique was employed in the present experiment. The optical path was purged out as follows. The monochromator was purged by flowing argon gas continuously during all the measurement. The light path between the light source and the monochromator entrance slit through the absorption cell was purged with nitrogen gas using T-shaped tubes set on both sides of the cell. Nitrogen gas led from the inlets of the T-shaped tubes was bled out to the gap spaces between the EDL lamp cavity and the T-shaped tube, the T-shaped tubes and the absorption cell, and the T-shaped tube and the monochromator entrance slit. Nitrogen gas for purging of the optical path from the EDL lamp to the absorption cell was required to allow operation of the EDL lamp. The reaction vessel was constructed from a gas washing trap, in which a side wall window (10-mm i.d.) was made. The window was equipped with a silicone rubber septum for the injection of
RESULTS AND DISCUSSION Optimization of Instrumentation. According to previous , flow rate of the carrier gas, the amount works ( 2 , 3 , 1 3 , 1 4 )the of the reducing solution, and the volume of the reaction vessel influence the analytical sensitivity in atomic absorption spectrometry of mercury utilizing the cold vapor generation technique. Therefore, the influence of the above parameters on the sensitivity of mercury atomic absorption a t 185.0 nm was first investigated in order to achieve the optimum conditions. Flow Rate of Carrier Gas. T h e dependence of peak absorbance on the flow rate of carrier argon gas was studied in the range 0.5-2.0 L/min. The peak absorbance did not vary significantly with change of the carrier gas flow rate, but the repeatability of the signal was good a t 0.8 L/min. Therefore, the flow rate of 0.8 L/min was selected for the cold vapor generation. A m o u n t of Reducing Agent. Using three different volumes (25, 50, and 100 mL) for the reaction vessel, the dependence of mercury atomic absorption on the volume of the reducing solution was investigated a t 185.0 nm. The results are shown in Figure 2. In the experiment, 1 mL of the sample solution was used, and the volume of the tin(I1) chloride solution added was varied from 3 to 30 mL. As can be seen from Figure 2, the atomic absorption sensitivity of mercury with the 25- and 50-mL reaction vessels was improved when a smaller amount of the reducing solution was used. On the other hand, the mercury absorption with the 100-mL reaction vessel decreased slightly when the amount of the reducing solution decreased.
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
455
Table 111. Analytical Values Obtained by VUV Atomic Absorption of Mercury sample
mercury in wateP fish homogenateb
found, d m L
certified, d m L
1 . 4 1 5 0 04 I 0.01
149 0.57c
0.55
NBS Standard Reference Material, SRM 1641. Obtained from International Atomic Energy Association (IAEA), Monaco; IAEA MA-A-2. Measured at 253.7 nm.
0.1 '
/
I
~
10 20 Solution Volume m l
4/
30
Dependence of absorbance on reducing solution volumes Mercury 20 ng, reductant 3% Sn(II)CI,in 1 YOHCI solution -e-: 25-mL reaction vessel, 4-:50-mL reaction vessel, -A-: 100-mL reaction vessel Flgure 2.
Table 11. Comparison of Sensitivities and Detection Limits f o r Mercury Atomic Absorption at 185.0 and 2 5 3 . 7 n m absorption
lampU HCL
EDL(W) EDL ( H ) EDL (W)
cell length, cm 30 30 30 100
sensitivity, ng 185.0 253.7
detection limit, ng 253.7 nm nm 0.07
1'
185.0
nm
nm
-
0.66
0.15
0.02
0.06
0,017
0.60 0.60
0.1
0.06
0.057
0.24
0.009
0.02
W and H in parentheses indicate the EDL lamps from Westinghouse and Hamamatsu T V Co., respectively.
In that sense, using the smaller volumes of the reducing solution is advantageous for obtaining higher sensitivity. In addition, the use of the smaller volume reaction vessel gives better sensitivity. This may be due to the increased dilution effect of the mercury vapor in the dead space of the larger vessel. Consequently, the reducing solution added and the reaction vessel used were 3 mL and 25 mL, respectively, in further experiments. Similar results have been already reported in previous papers (2, 3). Absorption Cell. As listed in Table I, two different sizes of absorption cells were examined in the present experiment. I n general, the longer and smaller i.d. cell gave the better sensitivity, but it required precise optical alignment and a complicated gas-purging system. Therefore, the cell sizes shown in Table I were chosen for the present work. The sensitivity was improved by use of the longer cell, although sensitivity was not exactly proportional to the path cell length, as seen from Table 11. Most data in the present work were obtained by using a 30-cm cell for experimental convenience. Evaluation of Radiation Sources. First, a hollow cathode lamp (HCL) from Hamamatsu T V operating a t 5 mA was investigated, but this was unsuccessful because of the lack of sufficient radiation at 185.0 nm. Therefore, the HCL was used only for measurements at 253.7 nm to obtain the comparison data. The sensitivity and detection limit obtained a t 253.7 nm by using the HCL with the 30-cm length absorption cell are shown in Table 111. In order to obtain significant radiation of mercury a t 185.0 nm, EDLs were adopted. In the present work, two kinds of EDLs were mainly examined; one was a commercially available EDL from Westinghouse [EDL(W)] and the other was a specially-constructed EDL from Hamamatsu TV [EDL(H)]. T h e difference between EDL(W) and EDL(H) was in the
1.52
3 4 Power ( W )
6
Effect of EDL applied power on sensitivities of mercury at 185.0 and 253.7 nm (EDL from Westinghouse). -e-: 185.0 nm, -x-:
Figure 3.
253.7 n m materials sealed in the tubes, Le., mercury metal in the former and mercury(I1) iodide in the latter. In the following experiments, the sensitivities (1% absorption) of mercury atomic absorption at 185.0 and 253.7 nm were investigated by using the EDLs a t several operating powers, where the absorption cell with the 30 cm long X 6 mm i.d. was used. The dependence of the sensitivity on the EDL applied power a t 185.0 and 253.7 nm for the EDL(W) is shown in Figure 3. As is clearly seen in Figure 3, the sensitivities a t both wavelengths increase with decreasing power, and the tendency is more conspicuous at 185.0 nm. It should be noted here that the sensitivity at 185.0 nm is improved for a smaller applied power relative to that at 253.7 nm, although the former is worse than the latter when the applied power is higher than 3.3 W. The sensitivity ratio for 185.0 and 253.7 nm is about 0.2-5 for EDL(W). I t should be noted that these values are very small, compared to the ratio of the oscillator strengths of the 185.0- and 253.7-nm lines. The dependence of the sensitivities on the EDL applied power a t 185.0 and 253.7 nm for EDL(H) is shown in Figure 4. The results indicate that the sensitivity at 253.7 nm is almost independent of the power applied to EDL(H), while the sensitivity a t 185.0 nm increases a little with decreasing power. The sensitivity ratio between those at 185.0 and 253.7 nm varies from 35 to 15 with increase of applied power. The maximum value of 35 is close to the ratio of the oscillator strengths for the 185.0- and 253.7-nm lines. The sensitivities and detection limits obtained with both EDLs a t 185.0 and 253.7 nm are summarized in Table 11. As can be seen from Table 11, the best sensitivity obtained with EDL(H) was 0.017 ng, when operated a t 1 W. This smsitivity is the highest one so far reported. As can be seen from Table 11, a significant difference of sensitivity a t 185.0 and 253.7 nm was observed with both
456
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3,MARCH 1980
C
v
0.51
6 10 15 Power ( W ) Figure 4. Effect of EDL applied power on sensitivities of mercury at 185.0 and 253.7 nm (EDL from Hamamatsu TV). -0-:185.0 nm, 1 2
-x-:
4
253.7 nm
EDLs. Furthermore, the sensitivities obtained with EDL(W) are dependent on the applied power, while those obtained with EDL(H) have only a slight dependence on the applied power. These facts suggest that the line profiles of atomic mercury are different a t each wavelength and the profiles depend on the type of EDL used and on the power selected. Such a change of line profile is possibly caused by the self-absorption of emission light in the EDL; similar consideration was also reported for HCLs by Robinson et al. (9). In the present case, this interpretation may be supported by the following experimental facts: (i) The dependence of the atomic absorption sensitivity on the EDL applied power occurs more remarkably for the resonance line a t 185.0 nm rather than for the nonresonance line a t 253.7 nm. (ii) The dependence of the sensitivity on the EDL applied power occurs more significantly for EDL(W), which gives the higher vapor pressure in the tube, rather than for EDL(H). (iii) The ratio of the sensitivities a t 185.0 and 253.7 nm obtained with EDL(H) operated a t low power is close to the value predicted from the ratio of the oscillator strengths for both lines. This fact suggests that minimal self-absorption occurs for EDL(H). In practical measurements, fluctuations in the radiation intensity of the light source influence the stability and noise level in the system, and determine the detection limit. From this point of view, EDL(W) is superior to the EDL(H), as can be seen in Table 11. Despite the higher sensitivity obtained, EDL(H) used in this experiment is not so useful because of the higher noise level due to the weak and unstable radiation. According to the optimization of the experimental conditions, the EDL(W) operated a t 1.6 W gave the best detection limit, and was stable enough for practical analysis. C a l i b r a t i o n Curves. The analytical calibration curves obtained by using the two atomic lines, EDL(H) and EDL(W), and various applied powers are shown in Figure 5. As seen in Figure 5, the atomic absorption observed at 185.0 nm gives a significantly better sensitivity than that at 253.7 nm. Consequently, quantities below a few nanograms of mercury can be determined. On the other hand, conventional atomic absorption of mercury at 253.7 nm is still advantageous in that a wider dynamic range for the calibration curve compared to t h a t a t 185.0 nm is obtained. Sensitivity a n d Detection Limits. Sensitivities and detection limits (signal level corresponding to twice the standard deviation of the blank signal) obtained in the present work are summarized in Table 11, where the different lamps, absorption cells, and atomic lines were used. It should be noted here that the sensitivities and detection limits of the
Figure 5. Analytical calibration curves obtained with various light sources. -0-: 185.0 nm, EDL (from Hamamatsu TV) operated at 1 W. -0-: 185.0 nm, EDL (from Westinghouse) operated at 2 W . -A-: 253.7 nm, EDL (from Westinghouse) operated at 2 W. -x-: 253.7 nm HCL (from Hamamatsu TV) operated at 5 mA
atomic absorption at 253.7 nm are almost similar to each other with respect to all the HCLs and EDLs. However, the sensitivity and detection limit obtained at 185.0 nm are dependent on the types of EDLs, as discussed earlier. It is noteworthy to point out that the sensitivity a t 185.0 nm obtained with EDL(H) is better than that with EDL(W), while a poorer detection limit was obtained for EDL(H). The improved sensitivity for EDL(H) is considered to be due mainly to the sharper line profile of the source. The poorer detection limit for EDL(H) results from the increased noise fluctuations of the source relative to EDL(W). The best detection limit was obtained a t 185.0 nm with the 100-cm absorption cell and the EDL(W). The relative standard deviation a t 185.0 nm was ca. 4% for 10 ng mercury, and less than 20% for 0.05 ng mercury, where the absorption cell with the 100-cm length and EDL(W) were used. B l a n k Problems. When analysis is carried out a t extremely low concentrations, the blank from the solution, reagents, and environments influence the analytical data significantly. In VUV atomic absorption spectrometry of mercury, there are two possibilities for blank contributions, Le., contaminated mercury and dissolved oxygen in the solutions. Actually, when 10-mL samples were used for analysis, the calibration curve a t 185.0 nm did not pass through the origin of the coordinate system. This problem was examined in more detail in the following experiment. Mixtures of O2 and N2 a t different ratios were passed through distilled water for 30 min to saturate the oxygen gas a t various vapor pressures. The water samples thus prepared were analyzed by the same system for VUV atomic absorption spectrometry of mercury. The apparent absorption of the water samples bubbled with the mixture increased linearly, corresponding to the oxygen ratio in the gas mixture. The blank absorption in the calibration curve for mercury a t 185.0 nm was almost equal to the apparent absorption for the water sample bubbled by the gaseous mixture with a [02]/[N2]ratio of 1/4. The apparent absorption of the water sample bubbled with only nitrogen gas was equivalent to 0.03 ng Hg. This is similar to the value reported previously ( 2 ) . According to the above experiment, it may be suggested that the detection limit in atomic absorption spectrometry of mercury a t 185.0 nm is limited by the blank signal owing to the dissolved oxygen in the sample solution. In the present system, the lower limit may be ca. 0.006 ng Hg, when 10 mL of the sample solution is used. The blank from the dissolved oxygen and concomitant mercury in the reducing solution would be negligible, because the reducing solution was bubbled with argon gas before the
Anal. C h e m . 1980, 52, 457-459
injection of the sample solution. Application. The VUV atomic absorption spectrometric method developed was applied to the NBS standard reference material, mercury in water (SRM 1641), and the fish homogenate (MA-A-2). Mercury in water was diluted 1000 times, a n d analyzed. T h e fish homogenate sample (ca. 1 g) was digested in a wet digestion system with a acid mixture (3 mL concd HNO,, 1 mL concd H2S04and 1 mL concd HC10J for about 15 h. The digested solution was diluted to 50 mL. The analytical results are summarized in Table 111. The data are almost consistent with the certified value of NBS and the value obtained at 253.7 nm, respectively.
457
J. Olafsson, Anal. Chim. Acta, 68, 207 (1974). K. Matsunaga, M. Nishirnura, and S. Konishi, Nature (London),258, 224 (1975). K. Fuwa, "Spectrochemical Methods of Analysis", J. D. Winefordner, Ed., Wiley-Interscience, New York. 197 1, pp 189-234. R. M. Dagnall, J. M. Mansfield, M. D. Silvester, and T. S. West, Nature (London), Phys. Sci., 235, 156 (1972). J. W. Robinson, P. J. Slevin, G. D. Hindman, and D. K. Wolcott, Anal. Chim. Acta. 81, 431 (1972). R. M. Daanall, J. M. Mansfield. M. C. Silvester, and T. S. West, Spectrosc. L e r t . , 6, 183 (1973). G. F. Kirkbright, T. S . West, and P. J. Wilson, Analyst(London),98, 49 (1973). J. Takahashi, K. Tanabe. H. Haraguchi, and K. Fuwa, Bunseki Kagaku, 27, 738 (1978). M. P. Stainton, Anal. Chem., 43, 625 (1971). T. R. Gilbert and D. N. Hume, Anal. Chim. Acta, 65, 461 (1973).
LITERATURE CITED (1) W. R. Hatch and W. L. Ott, Anal. Chem., 40, 2085 (1968). (2) J. E. Hawley and J. D. Ingle, Jr., Anal. Chem., 47, 719 (1975). (3) S. Yamazaki, Y. Dokiya, T. Hayashi, S. Toda, and K. Fuwa, Nippon Kagaku-kaishi. 8, 1148 (1977). (4) S. H. Omang, Anal. Chim. Acta, 53, 415 (1971).
RECEIVED for review May 29,1979. Accepted August 23,1979. This research has been supported by Grant-in-Aid for Environmental Science under grants No. 303022 and No. 303035 from the Ministry of Education, Science, and Culture, Japan.
Retention of Chromium by Graphite Furnace Tubes Claude Veillon," Barbara E. Guthrie,' and Wayne R. Wolf Human Nutrition Center, Nutrition Institute, United States Department of Agriculture, Building 3 0 7 , Room 2 75, Beltsville, Maryland 2 0 7 0 5
Using "Cr tracer techniques, a considerable amount of chromium was found to be irreversibly retained in graphite furnace tubes upon atomlzatiin. Both atomization temperature and sample matrix were found to be very influential in the amount retained. Pyrolytically-coated tubes retained less Cr than uncoated tubes. Considerable errors are likely if the method of additlons is not used. No Cr loss by volatilization was observed for indicated char temperatures up to 1300 OC.
Table I. Percentage of 51CrRetained in Graphite Tubes tube type and atomization temperature
EXPERIMENTAL Apparatus and Reagents. A commercially available graphite tube furnace and power supply (Model 2100, Perkin-Elmer Corp., Norwalk, Conn.) was used with both pyrolytically-coated and uncoated tubes. Radioactive chromium in the form of carrier-free Present address: Nutrition Department, Otago University, Dunedin, New Zealand. This article not subject to U.S. Copyright
'lCr standard
uncoated 2600 'C 2700 3 C
pyrolytic 2600 'C 2700 'C
During an investigation of human urinary chromium excretion using graphite furnace atomic absorption spectrometry (I),it became evident that some of the analyte was being "lost", and so was not available for measurement. Possible mechanisms for the apparent loss include carbide formation, matrix effects in the complex urine ash matrix (such as volatilization of the Cr as non-atomic species, or the formation of refractory compounds), diffusion losses through the graphite walls, mechanical losses during the dry or atomize modes, and losses which occur in the sample preparation procedure prior t o analysis. These various loss pathways, and the effect of furnace temperatures on these pathways, were investigated using "Cr solutions and rat urine containing endogenous 51Cr. The use of W r as a tracer in these studies permits one to unambigously and quantitatively measure and follow the Cr throughout the sample preparation and atomization processes. Considerable evidence of stable carbide formation was found. The atomization temperature was found to be very important, and large differences were observed depending upon the sample matrix used.
urine ash so 1ut io no 58
45
i i
44
i
28
r
3 (8)c
69 ( 2 )
5 (9) 9 1:s) 5 l(3)
32 ( 2 )
Urine samples from rats which were injected subcutaneously with "CrCI,. Samples were dried (60 "C, 125 Torr), oxygen plasma ashed (400-W rf, 1 Torr 0,)and dissolved in 0.1 M HCI. "CrCI, in 0.1 M HCI. Number in parentheses indicates number of tubes i n each group. 51CrC13(New England Nuclear, Boston, Mass.) was used to spike test solutions,and was incorporated into rat urine by subcutaneous injection of adult male CD Sprague-Dawley rats (Charles River Breeding Laboratories, Boston, Mass.). Samples, standards, graphite tubes, and sample containers were measured by y emission spectrometry (Model 1185, Searle Analytic, Inc., Des Plaines, Ill.). All reagents were the same as those used for previous Cr analyses in urine ( 1 ) . Procedures. Labeled samples of 20-50 FL were injected into the furnace in the normal manner ( 1 ) and processed through various cycles. A t appropriate intervals, the tubes were removed and counted. For the sample preparation procedure studies, the porcelain crucibles were similarly counted. All spiked solutions were monitored, and compared to the initial 51Crstock solution to compensate for isotopic decay over the experimental time frame. By spiking with sufficient label to yield relatively high count rates, excellent counting statistics ( < I % )were obtained with counting times of a few minutes. Total counts at each step were checked to be certain that the label was accounted for, and care was taken in the placement of tubes and crucibles, and the volume of solutions, in the counting tubes to minimize any geometric effects on the measured counts. Counting rates were tens of thousands of counts per minute, corresponding to something on the order of lo-" g of jlCr, depending on the specific activity of the isotope. This is comparable
Published 1980 by the American Chemical Society
