Anal. Chem. 1984, 56,829-831
829
Graphite Furnace Atomic Absorption Spectrometry with Graphite Cloth Ribbon for Selenium Determination Chan-Huan Chung, Etsuro Iwamoto, Mahabu Yamamoto, and Yuroku Yamamoto* Department of Chemistry, Faculty of Science, Hiroshima University, Higashisenda, Hiroshima 730, Japan
Masahiko Ikeda Nippon Jarrel Ash, Ltd., Shimotoba, Fushimi-ku, Kyoto 612, Japan Although graphite furnace atomic absorption spectrometry has great advantages such as increased sensitivity and minute samples for determination of a trace amount of metals, it suffers from troublesome problems related to matrices and the nature of the graphite surface in the atomization process. T o overcome these problems many improvements in furnace design and performance have been developed: The graphite furnace was coated with a layer of pyrolytic graphite (1-3) or metals (4-6) or fitted out with a graphite platform (7-9) and tantalum or tungsten liner, collar, and wire (10-14). When an analyte metal forms a stable carbide and has a low volatility, enhancements in the sensitivity were obtained by using a pyrolytically coated tube and a metal liner or boat. It was reported that the use of a nonpyrolytic graphite (NPG) surface in place of a pyrolytic graphite (PG) surface enabled charring at higher temperature, to decrease interferences (15), and an increased number of active sites available for graphite reduction (16). For the determination of more volatile selenium, it has been empirically demonstrated that the addition of "matrix modifier" such as nickel(I1) (17)and molybdenum (18) to the sample is effective for reducing the volatility, leading to enhanced sensitivity. This effect was postulated to be due to the formation of relatively nonvolatile metal selenides in the furnace (17-19). In fact we (20) observed that the coextraction of transition metals with ammonium pyrrolidinedithiocarbamate (APDC) into methyl isobutyl ketone (MIBK) greatly enhanced the sensitivity for selenium. Recently, however, Vickery and Buren (21) reported that the role of matrix modifier is not simply to reduce the volatility of the selenium but to modify the graphite surface which leads to more efficient atom formation. In this study, we report that the sensitivity for selenium greatly depends on the nature of the graphite surface, sensitivities obtained by using a nonpyrolytic graphite tube are higher than those obtained by using a pyrolytic graphite tube, and the volatilization loss of selenium in the charring process is less critical for the nonpyrolytic graphite tube. Furthermore, we report that the use of a graphite cloth ribbon placed inside the graphite tube which causes a large contact area with analytes greatly enhances the sensitivity in selenium atomic absorption signals.
EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer Model 5000 atomic absorption spectrophotometer equipped with a deuterium background corrector and a HGA-500 graphite furnace was used with a Model AS-40 autosampler. Peak heights were recorded with a Hitachi Model 56 recorder. A Perkin-Elmer Data System 10 and a Watanabe Model WX4675 plotter were used to record the absorbance signal profiles. A Hamamatsu TV hollow-cathode lamp was used as a light source. The analytical wavelength and slit width were 196.0 nm and 0.7 nm, respectively. The graphite furnaces used were of four types: (i) nonpyrolytic graphite tube (PE-NPG, Perkin-Elmer, part no. 0706991, (ii) pyrolytic graphite coated tube (PE-PG, Perkin-Elmer, part no. 4559-4), (iii) pyrolytic graphite coated tube (G-PG, German Perkin-Elmer, part no. 091054), and (iv) pyrolytic graphite platform tube (platform-PG,
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Table I. Instrumental Parameters drying charring atomization conditioning temp, "C 200 700 2300 2700 0 1 50 30 ramp, s 5 5 2 hold, s 5 300 20 300 int flow gas, 300 mL/min _______ Table 11. Absorbances Obtained by Using Various Types of Graphite Tubesa platPEform G-PG with solution NPGb PE-PGC G-PGC PGd ribbon 0.211 0.075 0.097 0.337 0.520 HC1 0.107 0.292 0.168 0.248 0.342 HNO, 0.131 0.052 0.057 0.113 0.330 %SO, 0.228 0.016 0.045 0.090 0.306 CHC1,-CC1, ( APDC-Se) (0.314)e a The amount of selenium is 2.5 ng and the concentraNonpyrolytic graphite tion of acid is 0.05 mol L-'. tube. Pyrolytic graphite coated tube. Pyrolytic graphite platform tube. e By using PE-NPG with ribbon.
Perkin-Elmer). Graphite cloth made from poly(acrylonitri1e) was used by cutting it to a size 1.5 mm wide, 25 mm long, and 0.3 mm thick, about 5 mg. The furnace program is given in Table I. Maximum power heating was used for the atomization step. Reagents. All solutions were prepared from analytical reagent grade chemicals and distilled water and stored in polyethylene bottles. Standard selenium solution (1000 fig mL-') was prepared by dissolving 1.4053 g of selenium dioxide (SeOz)with hydrochloric acid and diluting to 1000 mL with water. Procedure for Extraction of Selenium(1V)with APDC. Take an aliquot of aqueous solution containing 500 ng of selenium(1V) in a separatory funnel. Add 2 mL of acetate buffer solution (pH 5.0) and 1.0 mL of 4% ammonium pyrrolidinedithiocarbamate (APDC) solution. Shake the solution with 10 mL of mixed solvent (CC14+ CHC13 = 1 + 1). Separate the organic phase for the determination of selenium. In all experiments the amount of selenium introduced in each firing was 2.5 ng with a sample volume of 50 pL for graphite tube sampling. Solutions containing 100- and 200-fold amounts of different foreign ions were employed.
RESULTS AND DISCUSSION Effects of Various Types of Graphite Tubes and Sample Matrices. It has been pointed out that porosity and the gas permeability of the graphite tube cause a source of lowering sensitivity and interference, but these properties can be reduced by the application of a pyrolytic graphite coating to the wall of the tube (1). However, interferences and sensitivity strongly depend on the quality of the pyrolytic graphite coating (22-25), and the quality varies during the lifetime of the tube (24). The use of PG tubes was recommended for difficult-to-volatilize elements such as molybdenum and vanadium (3);it was pointed out that PG is not always the optimum surface for use, especially in the presence of some
0003-2700/84/0356-0829$01.50/00 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56,NO. 4, APRIL 1984 a
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_._.__L__._ Flgure 1. Views of the graphite cloth ribbon position in the graphite tube: (a) graphite tube; (b) graphite cloth rlbbon.
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Flgure 3. Effect of forelgn ions on sensitivity for selenium (50 ng/mL) In CHCIS-CCI, organic medlum with (a) and without (b) a graphite cloth ribbon in the G-PG tube.
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Figure 2. Effect of foreign ions on sensltivity for selenium (50 ng/mL) In hydrochloric acid (0.05 mol/L) medlum wlth (a) and without (b) a graphite cloth ribbon in the G-PG tube.
types of matrices such as magnesium chloride on the analysis of lead (15). Table I1 gives absorbances for 2.5 ng of selenium atomized from various types of tubes and solutions and it shows how much the physical and chemical differences in the quality of the graphite tube and of sample matrices affect sensitivities for selenium. It is interesting to note that generally a higher sensitivity is obtained when atomized from nonpyrolytic graphite tubes, except for the nitric acid solution, and that PG tubes yield the lowest sensitivity, especially for the organic matrix, APDC-Se in CHCl3-CC1& The absorbance with the platform-PG for the organic matrix is much lower than that with the NPG, while the tendency in the absorbances for the hydrochloric acid matrix is opposite. Furthermore, it was found that the absorbance for the NPG tube does not decrease with increasing charring temperature up to 1100 OC, whereas that for the PG and platform-PG tubes decreases as shown later. Graphite Cloth Ribbon. The above results seem to suggest that the porous nature of nonpyrolytic graphite is very effective for atomization of volatile selenium. When the nitric acid matrix was used, the highest sensitivity was observed for pyrolytic graphite kbes. This enhancement may be due to the breakage of the pyrolytic coating surface leading to the porous nature. Nitric acid is known to create more active sites on the graphite available for reaction with analytes such as arsenic (25). Such consideration led us to use a graphite cloth ribbon prepared by cutting a graphite cloth which is woven with fine graphite yarn; the ribbon has a much greater surface area. The ribbon was placed inside a G-PG tube as shown in Figure 1 and the sample is deposited on it. The results obtained by using the ribbon are given in the last column in Table 11. Effects of foreign ions are shown in Figures 2 and 3 where the hydrochloric acid solution and APDC extracts with the CHCl,-CCl, mixed solvent were used as a matrix, respectively, and 100- and 200-fold amounts of nickel(II), copper(II), molybdenum(VI), and iron(II1) were added as foreign ions. It should be noted that the use of the graphite cloth ribbon enhances the sensitivity about six times, when comparing with that in the absence of foreign ions, and greatly decreases interferences from foreign ions except for iron(II1) in the hydrochloric acid medium. When a NPG tube was used in place of the PG tube where the ribbon is placed, nearly the same absorbance was obtained; therefore the effect of the ribbon is independent of the quality of the furnace tube. The
500 700 900 1100 1300 1500 t/'G
Figure 4. Effect of charrlng temperature on the absorbance of selenium In the hydrochloric acid medium: with the graphite cloth ribbon and G-PG, (1) with nickel (10 pg/mL), (2) without nickel; without the ribbon, (3) a PE-NPG tube, (4) a platform-PG tube, (5) a G-PG tube.
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Flgure 5. Atomization-time profiles for selenium in hydrochloric acid medium with (-) and without (---) the ribbon and with platform-PG (---) at various temperatures: (1) 550 OC; (2) 700 OC; (3) 900 OC.
increasing effect of matrix modifiers nickel(I1) and copper(I1) on the selenium sensitivity was observed for the G-PG without the ribbon (Figure 2b) in the aqueous phase and with the ribbon (Figure 3a) in the organic phase. Figure 4 shows an effect of charring temperature on absorbance. We see that when the graphite cloth ribbon was used the absorbance does not significantly decrease irrespective of presence or absence of nickel(I1) up to 1100 "C of charring temperature, whereas without the ribbon the absorbance for the PG and platform-PG tubes decreases with increasing charring temperature but that for the NPG tube does not decrease. Thus, it seems that the increasing sensitivity when using the ribbon is attributed to a modification of the graphite surface leading to more efficient atomization. Recently, KoreEkovl et al. (25) pointed out that the retention of arsenic even at temperatures above 1200 "C is due to formation of stable graphite-arsenate lamellar compounds at
Anal. Chem. 1984, 56,831-832
crystal imperfections of the graphite, and such reactions might also be significant for selenium. Atomization profiles for selenium with and without the graphite cloth ribbon and with the platform-PG are also shown in Figure 5. When the sample is deposited on the ribbon, the analyte signal is delayed. This is because the ribbon is heated primarily by radiation from the tube wall which is heated by an electric current passing through it, and a time lag occurs between the heating of the ribbon and the tube, a more stable or steady-state condition being obtained for volatilization of the sample. Such a situation which is based on L'vov's original theory (7) was first observed for the atomization from a platform placed inside a tube (8,26). In Figure 5 a slight time lag is seen for the platform-PG compared with the ribbon. Although the dependence of atomic absorbance sensitivity on the quality of graphite surface and matrix constituents is so complex that it is very hard to induce a general explanation for it, it is likely that there are the following roles of the graphite cloth ribbon to enhance the sensitivity of volatile selenium: (i) formation of stable lamellar compoounds and effective reduction of selenium(1V) by carbon because of greatly increased graphite surface area available for the reactions; (ii) lagging the heating cycle sufficiently for the tube and the analyte gas to come to a steady-state temperature; (iii) keeping the analyte inside the tube by capillary action of the cloth ribbon, this reduces its permeation or diffusion to the outside of the tube; (iv) condensation and reevaporation of selenium on the ribbon graphite surface during the atomization processes as pointed out by KoreEkov-et al. (25). Furthermore, there is an advantage, (v) the carbon tube is just a heat supplier and its life is extended, and the lifetime of the ribbon is also long: no significant change in the sensitivity is observed for a t least 250 firings.
831
Registry No. Selenium, 7782-49-2;graphite, 7782-42-5; molybdenum, 7439-98-7; iron, 7439-89-6; copper, 7440-50-8; nickel, 7440-02-0. LITERATURE CITED (1) L:vov, 6. V. "Atomic Absorption Spectrochemical Analysis"; Adam Hilger, Ltd.: London, 1970. (2) Thompson, K. C.; Godden, R. G.; Thomerson, D. R. Anal. Chlm. Acta 1975, 74, 289-297. (3) Sturgeon, R. E.; Chakrabarti, C. L. Anal. Chem. 1977, 4 9 , 90-97. (4) Zatka, V. J. Anal. Chem. 1978,50,538-541. (5) Runnels, J. H.; Merryfield, R.; Fisher, H. 6.Anal. Chem. 1975, 4 7 , 1258-1263. (6) Hodges, D. J. Analyst (London) 1977, 702, 66-69. (7) L'vov, 6.V. Spectrochim. Acta, Parts 1978,365, 153-193. (8) GrBgoire, D. C.; Chakrabarti, C . L. Anal. Chem. 1977, 49, 2018-2023. (9) Slavin, W.; Manning, D. C. Anal. Chem. 1979,57,261-265. (IO) Fuller, C. W. Analyst (London) 1974,99, 739-744, (11) Renshaw, C. D. At. Absorpt. Newsl. 1973, 72, 158. (12) Aggett, J. T.; Sprott, A. J. Anal. Chim. Acta 1974, 72, 49-56. (13) Manning, D. C.; Slavin, W.; Myers, S. Anal. Chem. 1979, 51, 2375-2378. (14) Daidoji, H.; Tamura, S . So//. Chem. SOC.Jpn. 1982,55,3510-3514. (15) Erspamer, J. P.; Niemczyk, T. M. Anal. Chem. 1982, 54,538-540. (16)Salmon, S. G.; Holcombe, J. A. Anal. Chem. 1882, 54, 630-634. (17) Ediger, R. D. At. Absorpt. News/. 1975, 14, 127-130. (18) Henn, E. L. Anal. Chem. 1975,4 7 , 428-432. (19) Alexander, J.; Saeed, K.; Thomassen, Y. Anal. Chlm. Acta 1980, 720, 377-382. (20) Kamada, T.; Yamamoto, Y. Talanta 1980,2 7 , 473-476. (21) Vickery, T. M.; Buren, M. S . Anal. Lett. 1980, 73A, 1465-1485. (22) Montgomery, J. R.; Peterson, G. N. Anal. Chlm. Acta lg80, 777, 397-401. (23) Volland, G.; Kolblin, G.; Tschopel, P.; Tolg, G. Z . Anal. Chem. 1977, 284, 1-12. (24) Slavin, W.; Manning, D. C.; Carnrick, G. R. Anal. Chem. 1981, 53, 1504-1509. (25) KoreEkovB, J.; Frech, W.; Lundberg, E.;Persson, J.; Cedergren, A. Anal. Chlm. Acta 1981, 130,267-280. (26) Slavin, W.; Manning, D. C. Spectrochim. Acta, Part S 1980, 356, 701-714.
RECEIVED for review August 31, 1983. Accepted November 28, 1983.
Modified Flame Ionization Detector for Supercritical Fluid Chromatography Michael G . Rawdon Texaco Research Center, P.O. Box 509, Beacon, New York 12508 The use of fluids above their critical point as chromatographic mobile phases has been receiving increased attention recently. Supercritical fluids have physical properties intermediate between liquids and gases, which give them favorable behavior for the transport of solutes in a chromatographic column ( I ) . Low boiling liquids such as pentane (2-4) and compressed gases such as carbon dioxide (5,6) have been used successfully. Advantages of supercritical fluid chromatography (SFC) include fast analyses, increased resolution per unit time compared to HPLC, and ease of fraction collecting (1, 6). Detection in SFC is certainly one of the limiting aspects of the technique, as it often is in HPLC. There exists a need for a sensitive, universally responsive detector. In gas chromatography, several detectors are available to meet this need including the flame ionization detector (FID) and the mass spectrometer. Attempts to adapt this technology to liquid chromatography have met with limited success. Moving wire/belt FID's are useful for nonvolatile solutes but have never made a significant market impact. The problems of interfacing a liquid chromatograph to a mass spectrometer are well documented (7,8), and this is currently an area of great effort. Carbon dioxide as the mobile phase in SFC offers the advantage that an FID can be connected directly, since
it gives no response in this detector. There exists literature on the use of an FID for SFC, but details of its construction were not given (9). A patent has been issued for an FID for SFC, but the detector was never commercially available (IO). For laboratories who cannot or choose not to construct such a detector, we describe here the modifications necessary to adapt a commercial gas chromatographic FID for SFC.
EXPERIMENTAL SECTION The chromatograph is a Hewlett-Packard 1082B equipped with two high-pressure pumps, a manual valve injector, and a heated column compartment. Modifications for use with supercritical fluids include the addition of a back pressure regulator (Tescom Corp. Model 26-1721-24-043),a heat exchanger installed between the column and the detector, and cooling caps fitted to the pump heads through which a water-glycol solution is circulated at -20 'C by a refrigerated bath (Neslab RTE-4Z). The FID is a Gow Mac 12-800 with a Gow Mac 40-900 electrometer. Carbon dioxide (air-free or bone dry grade, AGL Welding Supply, Clifton, NJ) is supplied in siphon tube cylinders and dried by passing through activated silica gel. Hydrogen (prepurified) and compressed air are used as received (AGL Welding Supply). Figure 1 illustrates the modified detector. The original flame tip has been flattened with a pair of pliers to act as a fine orifice restrictor for the COP. As hydrogen is no longer premixed with the mobile phase, the original hydrogen line is used as an outlet
0003-2700/84/0356-0831$01.50/0 0 1984 Amerlcan Chemical Society
