Anal. Chem. 1986, 58, 2611-2614 Source Intensity (Volts)
Absorbance
Absorbance O8
r 04[
I
0.0
- 1 0 1 2 3 4 5
C
0.0
Time(s) A
-O: 5 Time@) B
00
2611
for a shot-noise-limited system. Thus, the noise peak in Figure 4 is well within the expected limits. The magnitudes of the systematic errors shown in Figure 4B-E are dependent on all of the factors that influence the shape of the background absorption signal (heating rate, internal gas flow rate, sample size, wavelength, and atomization from wall or platform) and the time interval between the sample and reference measurements. The signal in Figure 4A was acquired using a modest sample size (20 gL), a fairly high internal flow rate (50 mL m i d ) , and determined off the P b line (283.3 nm). The slope of the leading edge was 3.3 A s-l. Shorter time intervals between the sample and reference measurements would reduce the errors in Figure 4B in direct proportion (Le,, a time interval of 4.5 ms would result in half the maximum and minimum errors). The errors in Figure 4C-E would be reduced to a lesser extent. The resulta obtained for 2% NaCl are consistent with the results obtained using computer modeling and simulated background absorbance. The systematic errors grow smaller as more reference points are used. For most background absorbance problems, the advantage of the quadratic 3 and quadratic 4 modes over the bracketing mode would be hidden in the base-line noise. However, the systematic errors would still be present adding a bias to the results. As in most cases, the size of the analytical signal will determine the significance of the error. For a strong background signal, as shown in Figure 2, the difference in errors can be easily seen.
ACKNOWLEDGMENT J.A.H. wishes to acknowledge the National Science Foun-.02 - 1 0 1 2 3 4 5
Time@) E
Figure 4. (A) Recorded lntenstiy for the atomization of 2% NaCI. Background correction errors for the (B) asymmetric, (C) bracketing, (D) nonlinear (three points), and (E) nonlinear (four points) methods of absorbance computation.
is easily seen in Figure,,QC-E and arises from the increased base-line absorbance noise that accompanies the loss of source intensity. The loss of a factor of 9 of source intensity (Figure 4A)would produce a 3-fold (g1I2)increase in the base-line noise
dation's Independent Research Program, which was used for the partial completion of this project.
LITERATURE CITED (1) Harnly, J. M.; Holcombe, J. A. Anal. them. 1985, 57, 1983-1986. (2) Savksky, A,; Golay, M. J. E. Anal. Chem. 1964, 36, 1627-1639. (3) Harnly, J. M.; O'Haver, T. C.; Wolf, W. R.; Golden, B. M. Anal. Chem. 1979, 51, 2007-2014. (4) Slemer, D. D.; Baldwln, J. M. Anal. Chem. 1980, 52, 295. (5) Qrobenskl, Z.,Lehmann, R.; Radzluk, 6.; Voellkopf, U. At. Specfrosc. 1984, 5, 07.
RECEIVED for review April 9, 1986.
Accepted July 7,1986.
Preatomization Separation of Chromium(I 11) from Chromium(V1) in the Graphite Furnace Sonja Arpadjan' and Viliam Krivan* Sektion Analytik und Hochstreinigung der Universitat Ulm, Oberer Eselsberg, 0-7900 Ulm, Federal Republic of Germany
By means of ''Cr as a radiotracer, the behavior of Cr(II1) and Cr(V1) in a graphite furnace was investigated. After the addltlon of a mixture of trlfluoroacetylacetone,tetramethylammonium hydroxide, and 0.5 M sodium acetate (1:2:2) to a water or urine sample, Cr( I I I ) can quantitatively be removed from a graphtte tube combined with an L'vov platform ar fnnn a tungsterrlmpregnatedgraphite tube without an L'vov platform at 400 OC whlle Cr(V1) quantitatlvely remalns in the furnace up to a temperature of 1200 OC. Permanent address: Clement Okhridsky University of Sofia, Faculty of Chemistry, 1, A. Ivanov, Blvd., Sofia 1126, Bulgaria. 0003-2700/86/0358-2611$01.50/0
In certain environmental samples, chromium can occur as chromium(II1) and chromium(V1). While there is little conclusive evidence on toxic effects of trivalent chromium and it is considered to be essential in nutrition and for the maintenance of normal glucose tolerance, many toxic effects including carcinogenicity have been reported for hexavalent chromium (1-4). For this reason, the separate determination of chromium(I1) and chromium(V1)in environmental samples has become very important. The concentration of Cr(V1) in urine may be an indicator of human exposure to hexavalent chromium compounds (5). Van Loon et al. (6) and Naranjit et al. (7) described the 0 1986 American Chemical Society
2612
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
determination of Cr(V1) in the presence of Cr(II1) in water samples and airborne particulate matter by ion exchange chromatography. Batley and Matousek (8) achieved the separation of the two chromium species by electrodeposition of Cr(V1) in the graphite furnace followed by its determination by electrothermal atomic absorption spectrometry. Another method separates Cr(II1) as volatile trifluoroacetylacetonate from Cr(V1). Chromium(V1) is then determined by electrothermal atomic absorption spectrometry and inductively coupled plasma emission spectrometry (9,10). A mixture of trifluoroacetylacetone, urea, and sodium acetate was proposed by Dungs et al. (11,12) to determine Cr(II1) and Cr(V1) directly in the graphite furnace based on the complete volatilization of chromium(II1) trifluoroacetylacetonate at 220 O C and 350 s hold time. The procedures for the separation and determination of the two chromium species are reviewed in ref 13-15. Useful data on the stability of Cr(II1) and Cr(V1) have recently been reported by Pave1 et al. (16). This paper reports the results of radiotracer experiments on the separation of chromium(II1) from chromium(V1) in different media via volatilization of chromium(II1) trifluoroacetylacetonate in the graphite furnace.
EXPERIMENTAL SECTION Reagents, Radiotracers, and Solutions. All reagents used were of “pro analysi” grade and were supplied by Merck, Darmstadt, FRG. The acids were purified by subboiling distillation. The radiotracer experiments were carried out with a commercially available chromium radioisotope 51Cr in the form of Nat1Cr04and WrC13 (Amersham International plc., Buckinghamshire, England). The radiochemical purity of the sodium chromate with regard to Cr(II1) and of the chromium chloride with regard to Cr(V1) was checked by extraction with diethylammonium diethyldithiocarbamate into methyl isobutyl ketone (13). The sodium chromate tracer contained 98% Cr(V1). The chromium(II1) chloride tracer supplied consisted of 20% Cr(II1) and 80% Cr(V1). Therefore, before being used, Cr(V1) was converted to the desired Cr(II1) form by reduction with ethyl alcohol in 0.5 M HC1 on a boiling water bath (17). The tracers had a specific activity of 9-18 MBq/pg Cr and contained 20-80 ng of Cr/mL. Doubly distilled water and rain water with total chromium amount less than 1 ng/mL were spiked with either the Cr(II1) or the Cr(V1) tracer. The carrier amount introduced by labeling was 0.2-0.8 ng of Cr/mL. Similarly spiked solutions were prepared from Danube River water collected in Ulm (total Cr, 3.8 ng/mL) and with a urine standard (total Cr, 53 ng/mL) obtained from Behring Institute, Marburg, FRG. Instrumentation. A Perkin-Elmer HGA 500 graphite tube furnace with power supply and programmer but without optical system was used for the radiotracer studies. The graphite tube furnace was placed in a closed chamber connected to an exhaust hood of the radiochemical laboratory to prevent contamination of the work place. Uncoated, pyrolytically coated, and tungsten-coated graphite tubes were used. For coating with tungsten (169, the graphite tubes were immersed at room temperature in a solution containing 7.8 g of Na2W04/100mL for 24 h and then dried at 120 OC for 12 h. Before use, the tubes were cycled three times between 120 and 2600 “C at a ramp time of 80 s and a hold time of 10 s. A Perkin-Elmer Model 432 atomic absorption spectrometer in conjunction with a deuterium background corrector and a HGA 500 graphite tube furnace was used for the supplementary absorption measurements. The 320-keV y-ray of 51Crwas counted with a well-type NaI(T1) scintillation detector (2 X 2 in.) coupled to a single-channel analyzer. The radionuclidic purity of the chromium radiotracers was checked with a high-resolution y-ray spectrometer. Performance of the Radiotracer Experiments. Aliquots (5-20 pL) of the spiked samples were injected into the tube. The tube was placed into the well-type scintillation detector and the 51Cr radioactivity counted. The tube was then placed in the furnace and the selected temperature program (Table I) started. After the desired part of the temperature program had been
Table 1. Graphite Furnace Operating Parameters for the Preatomization Separation of Cr(II1) from Cr(V1) and the Determination of Cr(V1) step
drying
volatilization of CrtIII) charring atomization
temp, “C
ramp time, s
hold time, s
130 400
20 1
300
1200 2700
10 1
40 40
10
completed, the 51Cractivity in the tube was determined again. Each measurement was repeated with at least three different tubes of the same type. Preatomization Removal of Cr(II1). Trifluoroacetylacetone (10 mL), 25% tetramethylammonium hydroxide solution in methyl alcohol (20 mL), and a 0.5 M aqueous sodium acetate solution (20 mL) were thoroughly mixed. The homogeneous solution was kept at room temperature for at least 1h before use. The water samples were mixed with equal volumes of the trifluoroacetylacetone solution. The urine samples were diluted with three times their volumes of the trifluoroacetylacetone solution. Aliquots (20 pL) of the resulting solutions were injected into uncoated graphite tubes with L’vov platforms and into tungsten-coated graphite tubes. The temperature program is given in Table I.
RESULTS AND DISCUSSION Dungs et al. used an aqueous solution of trifluoroacetylacetone, urea, and sodium acetate to volatilize Cr(II1) in the graphite furnace and thus separate Cr(II1) from Cr(V1) (11, 12). With the 51Crtracer following the procedure reported by Dungs et al., we were not able to prove that chromium(II1) could be quantitatively removed from the graphite tube; up to 70% of the trivalent chromium remained in the tube. In search of a better reagent for the removal of Cr(III), the reports about the extraction of Cr(II1) by trifluoroacetylacetone into benzene (19), the gas chromatographic determination of Cr(II1) in these extracts (20),the effects of amines on the extractability of chromium(II1) acetylacetonates (21), and the complexes formed from metal ions and ammonium trifluoroacetylacetonate (22) served as guidelines. The 51Cr tracer experiments showed that Cr(II1) cannot be quantitatively volatilized from the graphite tube if only trifluoroacetylacetone is added to the sample. The best results were achieved with a reagent prepared by mixing trifluoroacetylacetone (1 volume), 25% tetramethylammonium hydroxide solution in methyl alcohol (2 volumes), and a 0.5 M aqueous solution of sodium acetate (2 volumes). Preliminary experiments gave the same results for uncoated and pyrolytically coated tubes with the L’vov platform and tungstencoated tubes without the L’vov platform. The yields for the preatomization removal of chromium(II1) obtained with these tubes were higher than yields obtained with uncoated and pyrolytically coated tubes without the L’vov platform. The tungsten-coated tubes were used in all further experiments. The volatilization of Cr(II1) and the retention of Cr(V1) as a function of the pretreatment temperature are shown in Figure 1 for solutions containing either Cr(II1) or Cr(V1) in doubly distilled water. Before injection into the graphite tube, the sample was mixed with an equal volume of the trifluoroacetylacetone solution. Below 400 “C, Cr(II1) cannot be quantitatively volatilized. In the temperature range 400-1000 “C, 90 f 2% Cr(II1) is removed from uncoated and coated tubes without L’vov platform and 96 i 1% from uncoated tubes with the L’vov platform or tungsten-coated tubes without the L’vov platform. The incomplete removal of Cr(II1) from tubes without the L’vov platform is probably caused by the higher porosity of the graphite and the formation of chromium carbides. Tungsten-coated tubes are the cheapest
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
2613
t
60
z z
401
\ Crlllll
200
400
PRETREATMENT TEMPERATURE [OC]
Figure 1. Volatilization of Cr(II1) and retention of Cr(V1) in the temperature range 200-1400 "C (0.8 ng of Cr/mL of either Cr(II1) or Cr(V1) in doubly distilled water, uncoated or tungsten coated tube without L'vov platform, sample/reagent 1:l (v/v), 20 pL injected, hold time 300 s).
600
BOO
1000
1200
1400
PRETREATMENT TEMPERATURE [.C]
Flgure 3. Volatllization of Cr(II1) and retention of Cr(V1) in a urine matrix from a tungsten-coated graphite tube wRhout L'vov platform (spike concentration 0.80 ng of Cr/mL either as Cr(II1) or as Cr(V1); pretreatment time, 300 s).
Table 11. Recovery of Chromium(II1) and Chromium(V1) Determined by Atomic Absorption Spectrometry without and with Addition of the Mixture of Trifluoromethylacetone, Tetramethylammonium Hydroxide, and Sodium Acetate
Cr concn given,
Cr concn
ni3ImL
found, ng/mL
2oL c
Cr(II1)
W
=I-W o
200
400
PRETREATMENT TIME
[SI
Cr(V1) addition of reagent
50 50
0 0
no
49
Yes
0 0 50
50
no
0.5 50
50
Yes
50
no
49 98
50
50
Yes
52
Figure 2. Influence of the pretreatment time on the v o l a t i l i t l o n of Cr(II1) at a graphite tube temperature of 400 "C (doubly distilled water,
20 20 90 90
no Yes no
76
0.8 ng of Cr(III)/mL, tungsten coated tube wRh L'vov platform).
60 60 30 30
and most convenient to use. Chromium(V1) is quantitatively (299%) retained in all types of tubes investigated in the temperature range from 200 to 1200 "C (Figure 1). Above 1200 "C Cr(V1) begins to volatilize. Chromium(II1) can be quantitatively separated from Cr(V1) in the temperature range 400-1200 "C. A convenient temperature is 400 "C, which should be maintained to 300 s (Figure 2). After Cr(II1) has been completely removed under these conditions, only Cr(VI) remains in the tube for atomization a t 2600 "C. The volatilization of Cr(II1) is not detrimentally influenced by a 5-fold excess of &(VI). The results with rain water and river water from the Danube were the same as those obtained with doubly distilled water. A 1%sodium chloride matrix did not adversely affect the removal of Cr(II1) or the retention of Cr(V1). When urine was mixed with an equal volume of the trifluoroacetylacetone solution, only 85% of the Cr(II1) was removed from the tungsten-coated graphite tube. When 3 volumes of the reagent were mixed with 1 volume of urine, 96% of the Cr(II1) was volatilized (Figure 3). The procedure elucidated by the radiotracer technique was applied to the determination of Cr(II1) and Cr(V1) in water solutions by atomic absorption spectrometry. The results obtained without and with addition of the mixture of trifluoroacetylacetone, tetramethylammonium hydroxide solution in methyl alcohol, and sodium acetate are given in Table 11. They are in good agreement with results on separation yields for Cr(II1) and Cr(V1) obtained by the radiotracer technique. The results summarized in Figure 1and 3 and in Table I1 show that the described pretreatment of water and urine samples in the graphite tube prior to the atomization provides a simple principle for the determination of chromium(V1) in
Yes
58 114 29
the presence of chromium(II1).
ACKNOWLEDGMENT The authors thank C. Rolle for assistance with the experiments. Registry No. H20, 7732-18-5;%r, 14392-02-0;Cr, 7440-47-3; trifluoroacetylacetone, 367-57-7.
LITERATURE CITED (1) Browning, E. Toxlclty of Industrial Metals, 2nd ed.;Butterworths: London, 1969; p 119. (2) Chromium : The Medlcal and Blobgical Effects of EnvironmntalPOMUtents; National Academy of Science, Washington, DC, 1974. (3) Report EPA-600/1-77-822, "Toxicology of Metals", Vol. 11; U.S. Department of Commerce, National Technical Information Service: Washlngton. DC, May 1977. p. 164. (4) IARC Monographs on the Evaluatlon of the Carclncgenlc Risk of Chemicals to Humans ; World Health Organization, International Agency for Research on Cancer: Lyon, July 1980; Vol. 23. p 205. (5) Tsalev, D. L.; Zaprlanov, 2. K. Atomlc Absorptlon SpectrometryIn Occupational and EnvlronmentalHealth Practlce; CRC Press: Boca Raton, FL, 1984; Voi. I, p 117. (6) Van Loon, J. C.; Radziuk, B.; Kahn, N.; Lichwa, J.; Fernandez. F. J.; Kerber, J. D. At. Absorpt. Newsl. 1977, 16, 79-83. (7) Naranjlt, D.; Thomassen, Y.; Van Loon, J. C. Anal. Chim. Acta 1979, 110, 307-312. (8) Batley, 0.E.; Matousek, V. P. Anal. Chem. 1980, 52, 1570-1574. (9) Wolf, J. R. J . Chromtcgr. 1977, 734, 159-165. (10) Lloyd, R. J.; Barnes, R. M.; Uden, P. C.; Ellio, W. G. Anal. Chem. 1978, 50,2025-2029. (11) Dungs, K.; Lippman. Ch.; Neldhart, B. I n Trace Nement-Analyflcal Chemistry In Medlclne and Blology; Brltter, P., Schramel, P., Eds.; de Gfuyter: Berlin, 1983; Vol. 2, p 981. (12) Dungs, K.; Fleischhauer, H.; Neidhart, B. Fresenius' 2.Anal. Chem. 1985, 322,280-289. (13) Koch, 0. G.; Koch-Dedic, 0. A. Handbuchder Spurenanalyse; Springer-Verlag: Berlin, Heidelberg. New York, 1974; Band 1, p 623. (14) Cranston, R. E.; Murray, J. W. Anal. Chlm. Acta 1978, 99, 275-282. (15) Schwedt, 0. Fresenius' 2.Anal. Chem. 1979, 295,382-387.
2614
Anal. Chem. 1986, 58, 2614-2617
(16) Pavel, G.; Kliment, J.; Stoerle. S.; Suter, 0. Fresenius' Z . Anal. Chem. 1985, 321, 587-591. (17) Blaslus, J. Lehrbuch der analytbchen und praparativen anorganischen Chemie; Hlrzel Verlag: Stuttgart, 1979; p 282. (18) Ortner. H. M.: Kantouscher. E. Talsnts 1975. 22. 581-556. (19) Mahumdar, S.K.; We, A. K. Anal. Chem. 1980, 32, 1337-1339. (20) Savory, J.; Mushak, P.; Sunderman, F. w.; Estes, R. H.; Roszel, N. 0. Anal. Chern. 1970,4 2 , 294-297.
(21) Beyermann, K. Fresenius' Z . Anal. Chem. 1982, 190, 4-33. (22) Moshier, R.; Sievers, R. Ges Chromatographyof Metal Chelstes; Pergamon Press: Frankfurt/Main, New York, 1965.
RECEIVED for review February 10, 1986. Resubmitted July 2, 1986. Accepted July 11, 1986.
Alternative to Solid Sampling for Trace Metal Determination by Platform Electrothermal Atomic Absorption Spectrometry: Direct Dispensing of Powdered Samples Suspended in Liquid Medium Michel Hoenig* and Paul Van Hoeyweghen Institute for Chemical Research, Ministry of Agriculture, Museumlaan 5, B-1980 Tervuren, Belgium
A possible aiternatlve of solld sampllng Is the dlrect Introductlon of the llquld suspended powdered sample Into the E M S devlce uslng the autosampler. I n thb case, sample dilution Is easy and the use of the platform together with matrlx modlflers Is poe8ible. For cadmium and lead determlnatlonr the dmcrlbed procedure permlts the use of dlrect caHbratlon wlth shnple standard rdutlono: the worklng curve slopes are very clore for all the matrkw studled. Powdered plant US~UO,sodlmmt samples, and Iyophyllredanlmal tissues are suspended In a mixture of glycerlne, mahand, nllrlc acid, and adequate matrix modllkr. After the powdered sample b mlxed, the suspension Is stable for about 1 h. Rellablllty of thls procedure was confirmed by the determlnatlon of cadmlum and lead contents In eight standard reference materlals of the NBS and IAEA.
Compared to the analysis of dissolved samples, solid sampling followed by electrothermal atomic absorption spectrometry (EAAS) offers two advantages. First, it saves time and effort for samples that are difficult to dissolve. Second, the solid sampling may be particularly convenient when only small amounts of sample are available. However, direct analysis of solid samples by EAAS is initially handicapped due to some restrictive factors not present in the analysis of dissolved samples: 1. The greatest problem originates from sample heterogeneity which requires a substantial effort to obtain representative subsamples. 2. Determination of relatively high analyte concentrations is limited by the minimum representative sample weight, obtained with a current analytical balance, that can be introduced into the atomizer. 3. One of the major difficulties associated with solid sampling concerns the availability of appropriate calibration standards. 4. For multielement analysis this technique is particularly time-consuming compared to the work with solutions. 5. The interference effecta observed with the solid sampling are probably greater compared to the dissolved sample whose matrix is simplified as a result of the mineralization. 6. The good contact between the analyte and the graphite surface necessary for reproducible heat transfer from platform
to analyte and also for the possible analyte reduction prior
to the atomization is not ensured as satisfactorily as in the case of solutions. 7. The use of matrix modifiers, often required to achieve the efficiency of platform techniques, is problematic. 8. Sample introduction into the atomizer is less convenient compared to dissolved sample. Finally, for all the reasons mentioned above the precision obtained by solid sampling is generally less than that obtained with solution analysis. In spite of this discouraging situation, many researchers (see the recent review by Vollkopf et al. (1))consider that solid sampling facilitates analysis is some specific cases and may lead to consistent results. Moreover, Perkin-Elmer and Hitachi have elaborated commercially available devices for direct solid sampling using EAAS, which act like the L'vov platform. In 1974, Brady et al. (2,3)proposed an interesting method of solid sampling, the dispensing of water suspended powdered sample into the atomizer, using a micropipet. These authors obtained satisfactory results for lead and zinc determinations in plant samples and marine sediments. In this work, we investigated further this approach and found that it indeed offered advantages as compared to true solid sampling. The recent evolutions of EAAS using platforms, modifiers, autosamplers, and adequate signal processing contribute largely to routine applications of this solid sampling alternative.
EXPERIMENTAL SECTION
All work was performed on a Varian AA-1275 BD spectrometer equipped with a deuterium arc background corrector, GTA-95 graphite furnace, and programmable sample dispenser. Pyrolytically coated tubes and solid pyrolytic graphite platforms (both Le Carbone Lorraine, France) were used. The appropriate amounts of the powdered sample were weighed with an analytical balance (Mettler H-64, precision 0.01 mg) in 2-mL polyethylene microvials for autosampler. After addition of the liquid medium (discussed below and shown in Table 11),the samples were suspended and homogenized with a Mini-Mix stirrer (Vitatron Scientific) prior to the analysis. The suspended samples were dispensed on the preheated platform (130 "C) via the autosampler. Details on the electrothermal programs are given in Table I. A Hewlett-Packard 82905-A printer was used for plotting absorbance-time profiles. Signal processing was in the peak-height mode. Lead and cadmium hollow cathode lamps (Instrumentation Laboratory) were operated at 5 and 2 mA, respectively. All experiments were performed at 283.3 nm for lead and 228.8 nm
0003-2700/86/0358-2814$01.50/00 1986 American Chemical Society
