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tions of individual porphyrins are performed (23,33,34),and some researchers preferred esterification methods due to the simplicity of isocratic elution (20). Further significant improvement in the detectability of porphyrin methyl esters separated on TLC plates requires major reduction in the high background specular scatter, which may be achieved by using a pulsed laser and gated detection electronics (15). In our experiment it should be realized that only 20 mW of continuous wave radiation is necessary to produce relatively large fluorescence signal from porphyrin methyl esters, which could be easily generated from a low-cost, air-cooled argon-ion laser. Work is in progress in our laboratory to improve upon the detectability of various porphyrins present in human serum after separation by HPLC and the use of an optical fiber flow cell to optimize signal-to-noise ratio (27,+35).Hopefully, the ability to detect trace or ultratrace amounts of certain porphyrins in human serum would allow the detection of the early onset of certain types of cancers.
LITERATURE CITED Harris, J. W.; Keliermeyer, R. W. The Red Cell; Haward University: Cambridge, MA, 1970;pp 3-63. Baum. R. M. Chem. Eng. N e w s 1988, 66, 18-22. Xu, X.; Meng, J.; Hou. S.;Ma, H.; Wang, D. J. Lumin , 1988, 40 & 4 7 ,
219-220. Askevold, R. Scand. J. Clin. Lab. Invest. 1951, 3 , 318-319. With, T. K. Clln. Biochem. 1968, 2 , 97-104. Doss, M. Z . Klin. Chem. Klin. Biochem. 1970, 3 , 197-207. Petryka, 2. J.; Watson, C. J. J. Chromatogr. 1979, 779, 143-149. Evans, N.; Jackson, A. H.; Matlin, S. A,; Towill. R . J. Chromatcgr. 1976, 125, 345-355. Ford, R. E.; Ou, C. N.; Ellefson, R. D. Clin. Chem. 1981, 27, 397-401. Johnson, P. M.; Perkins, S.L.; Kennedy, S.W. Clin. Chem. 1988, 3 4 ,
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(13) Issaq, J. H. Chromatography 1987, 2 , 37-41. (14) Zakaria, M.; Gonnord, M. F.; Guiochon, G. J. Chromatogr. 1983, 271. 127-1 30. (15) Berman, M. R.; Zare, R. N. Anal. Chem. 1975, 4 7 , 1200-1202. (16) Bicklng, M. K. L.; Kniseley, R. N.; Svec, H. J. Anal. Chem. 1963, 55, 200-204. (17) Huff, P. B.; Sepaniak, M. J. Anal. Chem. 1983, 55. 1992-1994. (18) Kawazumi, H.; Yeung, E. S.Appl. Spectrosc. 1989, 43, 249-253. (19) Day, R . S.;Pimstone, N. R.; Eales, L. Int. J. Biochem. 1978, 9 , 897-902. (20) Sagen. E.; Romslo, I. Scand. J. Clin. Lab. Invest. 1985, 45, 309-314. (21) Adams. R. F.; Siarin. W.; Wiliims, A. R. Chromatcgr. News/. 1976, 4 , 24-27. (22) Crosby, D. G.; Abaronson, N. J. Chromatcgr. 1966, 25, 330-335. (23) Day, R. S.; Pimstone, N. R.; Eaies, L. Int. J. Biochem. 1978, 9 , 897-904. (24) Kennedy, S.W.; Wingfield, D. C. Toxlcol. Lett. 1986, 37, 235-241. (25) Reyftmann, J. P.;Morliere. P.;Goldstein, S.;Santas, R.; Dubertret, L.; Lagrange, D. Phofochem. Photobiol. 1984, 40, 721-729. (26) Leiner, M. J. P.; Hubmann, M. R.; Wolfbeis, 0. S. Anal. Chim. Acta 1987, 198,13-23. (27) Sepaniak, M. J.; Yeung, E. S. J. Chromatcgr. 1980, 790, 377-383. (28) Doss. M. I n Progress in Thin-Layer Chromatography and Related Methods; Nlederwieser, A., Pataki, G., Eds.; Ann Arbor Science: Ann Arbor, MI, 1972;Voi. 111, p 162. (29) Belenkii, B. G.; Gankina, E. S.;Adamovich, T. B.; Lobazov, A. Ph.; Nechaev. S. V.; Solonenko, M. G. J. Chromatogr. 1886, 365,
3 15-320. (30) Sawlcki, E.; Golden, C. Microchem. J. 1969, 74, 437-442. (31) Andrade, J. D. Surface and Interfacial Aspects of Biomedical Polymers; Plenum: New York, 1985;Vol. 2, Chapters 1 and 2. (32) Field, L. R.; Sanders, L. J. Chromatogr. Scl. 1980, 78, 133-136. (33) Kennedy, S. W.; Wigfield, D. C.; Fox, G. A. Anal. Biochem. 1986, 157, 1-7. (34) McCarroll, N. A. Clin. Chem. 1988. 3 4 , 2390-2391. (35) Roach, M. C.; Harmony, M. D. Anal. Chem. 1967, 59, 411-415.
RECEIVED for review May 30, 1989. Accepted July 26, 1989. This work was supported by BRSG 240-H0050 awarded by the Biomedical Research Support Grant from the U.S. Public Health Service.
Radio Frequency Plasma Detector for Sulfur Selective Capillary Gas Chromatographic Analysis of Fossil Fuels R. J. Skelton, Jr.,l H.-C. K. Chang? P. B. Farnsworth, K. E. Markides, and M. L. Lee* Department of Chemistry, Brigham Young University, Provo, Utah 84602
Flame photometric detectlon Is often wed for sulfur selective gas chromatographlc analysis. However, nonllnear response and quenchlng are slgnlficant drawbacks to this detector. Due to the relatively low abundances of sutfur compounds In extremely complex fossll fuels, capillary gas chromatography coupled with element selective radio frequency plasma detection was used to provlde sulfur selective (>IO3) analysis of petroleum dlstlllates and coal extracts. This detector was evaluated as a sulfur selective detector and was found to possess low llmlts of detectlon (0.5 pg/s) and good llnear response (4 decades). Varlous numbers of phenyl groups attached to the base thlophenlc rlng had little effect on the Intensity of the sulfur emlsslon signal. Coelutlon of hydrocarbons was found to affect sulfur response only at high concentratlons.
* Author
t o w h o m correspondence should be addressed. B o x 1380, C u r r e n t address: Shell Development Company, P.O. Houston, TX 77251. Current address: Department of Chemistry, Virginia Polytechnic I n s t i t u t e a n d State University, Blacksburg, VA 24061.
INTRODUCTION Polycyclic aromatic compounds (PAC) are known to be major constituents in coal and petroleum products. Polycyclic aromatic hydrocarbons (PAH) are by far the most common PAC in these materials. However nitrogen-, oxygen- and sulfur-containing PAC are also often found in significant quantities. Because of the widespread interest in and use of fossil fuels, detailed study of their compositions has become an important task ( I ) . The sulfur content of these fuels is of major importance when one is considering environmental pollution during processing or usage. Sulfur gases that result from combustion or conversion processes are particularly noxious. Organic sulfur compounds from these processes have been classified according to functional groups: thiol (-SH), sulfide (-S-), disulfide (-S-S-), and thiophene. Many of these compounds have been identified in fossil fuels (2-8), but the major organosulfur compounds are thiophenic in nature, and many are known to possess mutagenic activities (9-12). In addition to pollution, organic sulfur compounds are known to affect storage stabilities of petroleum products, and they foul the
0003-2700/89/0361-2292$01.50/00 1989 American Chemical Society
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catalysts used in the cracking process. The determination of these compounds has often been accomplished with gas chromatography (3-7). T h e flame photometric detector (FPD) is often w e d with gas chromatography because of its excellent selectivity for sulfur. This detector has existed for over 2 decades (13),and it continues to have widespread use. The F P D is well suited to the lahoratory, because of its ease of use and reasonable mt. I n addition, it can also be used &s a phoephorus selective detector, greatly increasing the number of applications for which it is suited. These advantages have made this the detector of choice for sulfur selective detection after gas Chmmatography. Electrochemical techniques and the microwave induced plasma detedor provide other means for sulfur detection (14); however, their use is limited hy either complexity or expense. Although the FPD is an excellent detector, it is not without drawbacks. The FPD is based on S2hand emission in a cool, hydrogen-rich flame. Because the detector responds to a molecular band, the response is of the form R = [SI". The exponent n is theoretically 2, but the response to sulfur ranges between first and second order, depending on the heteroatom environment (15). The detector response is also quenched by coeluting water or hydrocarbons (16,17). Dual-flame photometric detectors have been used to overcome these problems to a degree, but the single-flame versions are more common in the laboratory. Recently, we reported the construction and initial evaluation of a multielement selective radio frequency plasma detector (18)that is similar to the helium afterglow detector reported by Rice e t al. (19). Because this detector is based on atomic emieaion f"a helium plasma, high sensitivity is coupled with good selectivity and linear response. This study was undertaken to determine the operational characteristics of this plasma for sulfur detection, including detector response dependence on molecular structure, quenching, linearity, and sensitivity. The detector was applied to the analysis of sulfur heterocycles occurring in coal extracts and petroleum distillates. EXPERIMENTAL S E C T I O N Radio Frequency Plasma Detector. The radio frequency p h a detector has been previously reported (18)and will be only briefly described here. The detector consists of a helium radio frequency plasma doped with a small quantity of oxygen and contained inside a 1-mm-i.d. quartz tube. Research grade helium (99.9999%, Scott Specialty Gases, Plumsteadsville, PA) was used with a flow of 70 mL/min. High-purity oxygen (
