2617
Anal. Chem. 1984, 56,2617-2618
for instance, cortisol showed a limit of 1 ng. These results are superior to those reported in the literature where normal (2-4)and reversed (5-11)phase chromatography with UV (2-10)and fluorescence (10,ll)detectors are discussed. With regard to cortisol alone, an early report by Trefz et al. (12) may possibly be comparable to our results. Their detection limit of cortisol was as low as 500 pg with a S I N ratio of 16. However, they did not examine chromatographic analysis using serum cortisol as a low concentration. In the present work, selection of the extraction solvent was made so that the least possible polar solvent could be used to obtain an extraction peak within a capacity ratio of 1 (see Figure 3), resulting in no peak following prednisolone and a minimum background over the corticosteroid area on the final HPLC chromatogram (see Figure 5 ) . Registry No. Corticosterone, 50-22-6; cortisone, 53-06-5;6amethylprednisone, 91523-05-6; prednisone, 53-03-2; dexamethasone, 50-02-2; cortisol, 50-23-7; 6a-methylprednisolone, 83-43-2; prednisolone, 50-24-8.
LITERATURE C I T E D Oka, K.; Mlnagawa, K.; Hara, S.; Noguchi, M.; Matsuoka, Y.; Kono, M.; Irimajiri, S. Anal. Chem. 1984, 56, 24-27. Loo, J. C. K.; Butterfleld, A. G.; Moffatt, J.; Jordan, N. J. Chromatogr. 1977, 143, 275-280. Rose, J. Q.; Jusco, W. J. J . Chromatogr. 1979, 762,273-260. Frey, F. J.; Frey, B. M.; Benet, N. 2 . Clin. Chem. (Winston-Salem, N . C . ) 1979, 25, 1944-1947. Wortmann, W.; Schnabel, C.; Touchstone, J. C. J . Chromatogr. 1973, 84, 396-401. Scott, N. R.; Dlxon, P. F. J . Chromatogr. 1979, 164, 29-34. Reardon, G. E.; Cardarelia, A. M.; Canalis, E. Ciin. Chem. (WinsfonSalem, N . C . ) 1979, 25, 122-126. Kabra, P. M.; Tsai, L.-L.; Marton, L. J. Clin. Chem. (Winston-Salem, N.C.) 1979, 25, 1293-1296. Canalis, E.; Cardarella, A. M.; Reardon, G. E. Clin. Chem. (WinstonSalem, N . C . ) 197% 25, 1700-1703. Gotelli, G. R.; Wall, J. H.; Kabra, P. M.; Marton, L. J. Clin. Chem. ( Winston-Salem, N . C . ) 1981, 27, 441-443. Horikawa, R.; Tanimura, T.; Tamura, 2. J . Chromatogr. 1979, 768, 526-529. Trefz, F. K.; Byrd, D. J.; Kochen, W. J . Chromatogr. 1975, 107, 161-169.
RECEIVED for review March 29,1984. Accepted June 6, 1984.
Graphlte-Furnace Atomic Absorption Method for Trace-Level Determination of Total Mercury B a r b a r a J. Keller* a n d M a r k E. Peden
Illinois State Water Survey, Analytical Chemistry Unit, Box 5050, Station A, Champaign, Illinois 61820 Anthony Rattonetti
Consultant i n Atomic Spectroscopy, Suite 411,55 Sutter Street, Sun Francisco, California 94104 Because of mercury's acute toxicity to biological systems, the United States Environmental Protection Agency (USEPA) has set a limit of 2 hgL-' as the maximum drinking water contaminant level (I).In response, analytical chemists have placed a great deal of emphasis on developing a sensitive technique to determine trace amounts of mercury. The generally accepted and USEPA-approved procedure is an atomic absorption cold-vapor method (2). While this method is quite sensitive, it requires numerous reagents, a 2-h digestion, a special cold-vapor apparatus, and a large sample volume to achieve a low-detection limit (0.2 pgL-l). Rattonetti (3) described a method to determine mercury in water and urine that employed a carbon-furnace atomizer atomic absorption technique. He found that a matrix of 5% nitric acid (HN03) and 0.1% potassium dichromate (K2Cr2O7) stabilized the mercury, allowing a pyrolyzation temperature of up to 300 "C. Rattonetti showed that in this procedure the mercury is atomized before most of the matrix, thereby minimizing potential interferences. Because of the need to verify the applicability of Rattonetti's method for the determination of organic mercurials, more research was conducted by the Illinois State Water Survey (ISWS). EXPERIMENTAL SECTION Apparatus. An Instrumentation Laboratory (IL) Model 151 atomic absorption spectrophotometer equipped with an IL Model 455 furnace atomizer was used to determine mercury levels. The light source was a Varian Techtron mercury hollow cathode lamp. Samples were delivered to the uncoated furnace cuvette with a 5O-pL plastic-tipped Eppendorf pipet, and peak heights were measured on a Linear Instruments Model 261 high-speed potentiometric recorder. Reagents. Reagent grade KzCrzO7and HgO and Baker InstraAnalyzed HNO, were used. Reagent water was first deionized
Table I. IL 455 Furnace Atomizer Settings Conditions purge gas argon purge gas flow 6 SCFH mode auto
graphite cuvette round, uncoated sample vol 50 pL detection limit 0.6 pg-L-'
Atomization Program temp, "C time, s
100
20
225 20
275 30
1200
1200
0
0
1
0
Table 11. USEPA Reference Samples Hg concn, series no.
pg.L-'
70 recovery
70 re1 dev
WS378 (inorganic) 1172 (organic + inorganic)
4.0
95.0 87.5
6.3
20
9.0
22
1172 1172
4.8
97.9 100.7
4.7
8 8
2.4
14
3.8
n
and then passed through a Barnstead Nanopure System. The conductance of the product water was 12 MQ or greater. Procedure. All working standards and samples were prepared in a matrix of 5% (0.8 N) HN03and 0.1% K2Cr207. Of the sample 50 pL was pipetted into the furnace, and the atomization sequence was initiated. Mercury determinationswere made at the 253.7-nm wavelength, with a band-pass of 2.0 nm and employing deuterium background correction. Furnace settings are given in Table I. The USEPA reference standards used were trace metals series No. WS 378 for inorganic mercury and total mercury series No. 1172 for total (inorganic and organic) mercury. Series No. 1172 was prepared at various dilutions so the method could be tested at different concentration levels.
0003-2700/84/0356-2617$01.50/0 0 1984 American Chemical Society
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perature of 750 "C (Figure 1). The 1200 "C atomization temperature used in this study increased peak heights, providing greater sensitivity. Uncoated furnace cuvettes were used in preference to pyrolytic graphite cuvettes because of improved sensitivity and precision. The signal-to-noise ratio for uncoated tubes was 20 times higher than that for pyrolytic tubes. In addition, the high-acid matrix caused the samples to "creep" out of the coated tubes and the pyrolytic coating degraded rapidly. Problems with the use of pyrolytic graphite cuvettes with high-acid matrices have been noted previously (3-5). The argon flow was critical to achieve maximum peak height and curve linearity. Increasing the flow over 5-6 SCFH depressed peak heights for higher mercury concentrations due to mercury's high vapor pressure. This procedure is much simpler than the conventional cold-vapor method and requires no digestion to determine total mercury. Employment of this method could considerably simplify the monitoring of drinking water for the 2 pg-L-l maximum contaminant level. Registry No. Mercury, 7439-97-6; water, 7732-18-5. LITERATURE C I T E D (1) "Illinois Pollution Control Board Rules and Regulations"; Public Water Supplies: Chicago, IL, 1979, Chapter 6. (2) Hatch, W. R.; Ott, W. L. Anal. Chem. 1968, 4 0 , 2085. (3) Rattonetti, A. Report No. 12; Instrumentation Laboratory, Inc.: Wiimington, MA, Feb 1980. (4) Shum, G. T. C.; Freeman, H. C.; Uthe, J. F. Anal. Chem. 1979, 51, 414. (5) Peden, M. E. "Flameless Atomic Absorption Determinations of Cadmium, Lead, and Manganese in Particle Size Fractionated Aerosols", Natl. Bur. Stand. (U.S.) Spec. Pub/. 464, 1976, 367.
RECEIVED for review February 15, 1984. Accepted June 15, 1984.
Determination and Optimization of Flow Rates in Vacuum Capillary Gas Chromatography Noel
W.Davies
Central Science Laboratory, University of Tasmania, G.P.O. Box 252C, Hobart, Tasmania, Australia 7001 The direct insertion of capillary gas chromatography columns into mass spectrometer ion sources is a common method of coupling these instruments (1-6). This results in the pressure a t the column outlet being effectively zero and the advantages of this such as increased speed of analysis over atmospheric outlet operation, total transfer of effluent to the ion source, and the relative ease of transfer of labile compounds have been well publicized. Cramers et al. (7) presented a detailed analysis of the various factors involved, including the gain in speed of analysis over atmospheric outlet for a given system and the potential loss in efficiency with vacuum outlet pressure. In any capillary gas chromatographic analysis, the flow rate of carrier gas is a critical parameter for optimized performance. Average linear velocity (ij) is often the form in which this is expressed and relates to a particular carrier gas diameter, and outlet pressure. An increased average velocity will be required for optimum performance if outlet pressure is lowered (6, 7). However, the optimum volume flow rate, reduced to atmospheric pressure, remains independent of outlet pressure (7). Optimum average carrier gas velocity a t atmospheric outlet pressure (ijopt,etm)can be found from literature values, while
in many situations the corresponding volume flow rate is also known. To obtain the optimum performance with vacuum outlet, DOpGatmmust be adjusted to the correct value for vacuum outlet (Dopt,vac) or the volume flow must be measured. It is generally impracticable to measure flow in directly coupled gas chromatography/mass spectrometry, since the outlet of the source pump is the only available point for measurement, after isolation of all other pumps to ensure passage of total effluent. The erratic liberation of gas makes accurate measurement difficult, particularly for very low flow rates. The discussion below shows how flow rate can be simply measured from readily accessible parameters, the air peak retention time (t,,,,,), the length of the column ( L ) ,and the carrier gas viscosity (a), without requiring column diameter or inlet pressure terms. Conversely, the air peak retention time required to give a particular flow rate can be predicted. Alternatively, Dopt,vac and hence ta,opt,vac, the optimum air peak time with zero outlet pressure, can be calculated from ijopt,atmafter calculation or accurate measurement of the inlet pressure required for the latter. Calculation of Flow Rates. Poiseuille's equation for viscous flow in long cylindrical columns can be written (7)
0003-2700/64/0356-2618$01.50/00 1984 Amerlcan Chemical Society
