1198
Anal. Chem. 1984, 56, 1198-1199
on the graphite tube than we had feared and it is possible to make more than 50 measures with the same tube. But the results are very dependent on the quality of the graphite tubes, and quality can vary in the same batch of pyrolytic tubes. Sometimes replacing peak height by peak area mode can improve reproducibility and linearity. The results obtained by electrothermal atomic absorption spectrometry are in good accordance with those obtained by inductively coupled plasma spectrometry using the conditions described in a previous paper (2), modified for a simultaneous spectrometer, JY 48, with background correction by automatic displacement of the entrance slit.
In conclusion, in our experience, addition of a high concentration of nitric acid dramatically improved aluminum determination in concentrates used to obtain dialysis fluid. Registry No. Aluminum, 7429-90-5;nitric acid, 7697-37-2. LITERATURE C I T E D (1) Alfrey, A. C.; Le Gendre, G. R.; Kaehny, W. D. New Engl. J . Med. 1076, 294, 4 , 184-188. ( 2 ) Allah, P.; Mauras, Y . Anal. Chem. 1070,51, 2089-2091.
RECEIVED for reivew October 12, 1983. Accepted February 9, 1984.
Dual Sample Injection for Gas Chromatographic Determination of Sulfur Species with Flame Photometric Detection Floyd A. Barbour,* Robert E. Cummings, and Frank D. Guffey University of Wyoming Research Corporation, P.O. Box 3395, University Station, Laramie, Wyoming 82071 The processing of oil shale whether by in situ or surface retorting generates gaseous sulfur products. During the development of near production-size processes, the release of these products needs to be monitored not only for compliance with air quality standards but also as a data base for input into the design or selection of a product gas cleanup system. The need for continuous monitoring demands that the system be fully automated and reliable. The use of a flame photometric detector (FPD) in gas chromatography (GC) lends itself nicely to the analysis of sulfur gases because the sensitivity for sulfur-containing compounds is in the subnanogram range. The detector has a major drawback in that the dynamic range of the detector only encompasses about 2 decades of sulfur concentrations, which is considerably less than the differences between the major sulfur species (hydrogen sulfide) and the minor species (carbonyl sulfide, methyl and ethyl mercaptan, carbon disulfide, and thiophene) observed in previous monitoring of oil shale retorts (I, 2). Gangwal and Wagner (3) overcame limitations imposed by the small dynamic range of the detector by evacuating the sample loop to an appropriate pressure. However, this technique requires precise vacuum control to evacuate to the same pressure each time and does not eliminate the problem of handling largely different component concentrations in the same gas sample. For this reason we connected two sample valves in series, one with a small sample loop for components present in high concentrations and one with a large sample loop to handle components in lower concentrations. Samples from both valves were sequentially injected during the same determination. EXPERIMENTAL SECTION A Hewlett-Packard 5840 gas chromatograph equipped with a single FPD and conventional six-port gas sampling injection valve was used for the analysis. Modification in the sample injection system was made in order to accommodate a small sample size as well as dual injection by adding a Rheodyne sample valve, type 50. This valve was modified as shown in Figure 1 by replacing the stator with one which contained no holes in the one- and four-port positions, which are normally used for the attachment of the sample loop. A sample loop was then scratched with a scalpel into the rotor side of the stator between the one and four pmitions. This produced a sample valve with a loop approximately
ROTOR
STATOR
Figure 1. Construction of 1-pL sample loop. SAMPLE IN
n
CARRIER OUT
Figure 2. Automatic gas sampling valves showing dual sampling capabilitles.
1pL in size that was attached in series to the conventionalsample valve with approximately a 50-wL loop. The loops were connected, as depicted in Figure 2, so that the sample gas flowed through the larger sample loop and then to the smaller loop. The carrier gas was also plumbed into the two valves in series. Figure 2 shows the small sample valve in the injection position and the large sample valve loop in the fill position. The small sample was injected at the start of the determination and after 2 min the large sample was injected onto the column. The sample loop pressure and flow were controlled by using a water tower approximately 1 m in height. The inlet pressure was controlled to 5.6 kPa above atmospheric (approximately 78 kPa at Laramie, WY) allowing excess sample to bubble through the water and exit through a vent. The sample loops were held
This article not subject to US. Copyright. Publlshed 1984 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
H2S
7-
/cos
CH,SH
i
THIOPHENE
A 0
2
4
6
8
IO
12
14
16
TIME
Figure 3. Chromatogram of sulfur-containing species in oil shale retort production gas by dual sample techniques.
at a back pressure of 4.5 kPa above atmospheric. The sulfur species were chromatographed on Carbopak BHT 100 in a 2 m by 3 mm 0.d. Teflon column. The initial oven temperature of 35 "C was maintained for 3 min and then programmed to 140 "C at 25 OC/min. Certified standard gas mixtures obtained from Matheson were used for calibration. Changes in concentration were accomplished by dilution of the standard gas with nitrogen using mass flow controllers for flow control of the two gases.
RESULTS AND DISCUSSION The valving system was designed to accommodate in a single experiment the analysis of samples containing radically different concentrations of sulfur species. The first concern was testing the 1-hL sample loop for repeatability throughout the desired sulfur concentration range. The FPD response (peak area) is proportional to nearly the square of the amount of sulfur; thus, the square 'roots of the areas were used for repeatability and linearity determinations. The square roots
1199
of the areas at lower hydrogen sulfide concentrations near 100 ppm were repeatable within fZ% of the mean value, while the square roots of the areas for hydrogen sulfide concentrations above 400 ppm were repeatable within f l % . The retention times for all of the monitored sulfur species were observed for variation during an 8-day retort experiment. The retention times for hydrogen sulfide peaks were within f1.5% of the absolute values and other compounds within f0.2%. The l-hL sample loop gave the desired linearity for hydrogen sulfide concentrations up to and in excess of 3000 ppm. The least-squares method used to determine the calibration equation had correlation coefficients (r2)ranging from 0.994 to 0.9998 for various calibrations. Because most of the minor components of interest are present in concentrations in the 5- to 50-ppm range and lower detection limit of the small sample loop was only 100 ppm, the larger sample loop attached to the conventional sample valve was used for these measurements. This allowed determination of minor sulfur species as well as the lower hydrogen sulfide concentrations. Because hydrogen sulfide was the only compound detected by using the small sample loop, both valves were injected sequentially during the same determination. Figure 3 depicts a chromatogram produced by the dual injection. The sample from the larger valve was not injected until hydrogen sulfide from the smaller valve had already eluted so that methane from the larger sample loop, which is about 2% of the retort gas stream, would not coelute and possibly quench the hydrogen sulfide response ( 4 ) from the small loop. The use of sample loops in two injections valves provides analysis of both major (hydrogen sulfide) and minor sulfur species in the same determination. The method has been applied to the analysis of the production gas streams from several retorting experiments to determine base line concentration ranges of all sulfur species present in oil shale retort production gas (2). The use of the sample valving system has been of particular importance in studying sulfur gas control technologies where the concentration of hydrogen sulfide is relatively high before treatment and may be extremely low afterward. Registry No. H,S, 7783-06-4; CH,SH, 74-93-1;C2H,SH, 7508-1; CS2, 75-15-0; COS, 463-58-1; thiophene, 110-02-1.
LITERATURE CITED (1) Ondov, J. M.; Lamson, K. 0.; Stvermer, D. H.; Heft, R. E.; Fallor, R. A,; Ng, D. J.; Morris, 0. J.; Anspaugh, L. R.; Daniels, J. I.; McNabb, J. R. Lawrence Llvermore National Laboratory Publicatlon UCRL-53265, 1982. (2) Guffey, F. D.; Barbour, F. A.; Cummlngs, R. E. Llq. fuels Techno/. 1983, 7(4), 235-257. (3) Gangwal, S. K.; Wagner, D. E. J . Chromatop. Sci 1979, 77, 198-201. (4) Patterson, P. L. Anal. Chem. 1978, 50, 345-3413,
RECEIVED for review September 19,1983. Accepted February 22, 1984.
