Total-Effluent Gas Chromatography-Mass Spectrometry William Henderson1 a n d Gordon Steel Department of Chemistry, Unioersity of California, Berkeley, Calif. 94720
A GC-MS system which allows introduction of a total GC effluent of up to 20 ml/min directly into the ion source of a mass spectrometer is described. The effect of flow rate on ion source and analyzer pressures, overall sensitivity, and resolving power of the system are evaluated. It has been shown that 10-lo-lO-ll gram/sec sample flow rates are obtained for cholesterol trimethylsilyl ether at helium flow rates of 1-7 ml/min. Within that flow rate range, only a 12% reduction in mass spectrometer resolving power was noted. The system has been shown to be suitable for a wide range of molecules from low molecular weight gases to high molecular weight steroid and carbohydrate derivatives. SINCE THE ORIGINAL WORK of James and Martin (I), the application of gas-liquid chromatography to the separation and identification of components of complex mixtures of organic materials has increased enormously. A wide variety of column types and column packings are now used, extending from conventional packed columns to open tubular capillary columns, support coated capillary columns, and, more recently, to packed capillary columns. Open tubular capillary columns have been extensively employed in the high resolution analyses of the extremely complex hydrocarbon mixtures frequently encountered in petroleum analysis (2) and in other organic geochemical investigations (for example, 3, 4 ) and in flavor and fragrance analyses. However, it is only relatively recently that their full potential in biochemical and biomedical analyses has been recognized; for example, Novotny and Zlatkis ( 5 ) have explored the use of high resolution wall-coated glass capillary columns for the separation of a wide range of oxygenated molecules. In recent years, the development and availability of commercial combination instruments where a gas-liquid chromatograph is linked in a variety of fashions to the ion source of a mass spectrometer has resulted in probably the most powerful analytical tool available to the organic chemist at the present time. However, combined GC-MS instruments have been used predominantly with packed G L C columns and the advantages of coupling high resolution capillary columns to a mass spectrometer with a few exceptions have not been realized. In general terms, the slow increase in the use of capillary columns in combined GC-MS instruments has been due to two factors; first, the use of capillary columns in organic and biochemical GLC analyses has been very restricted; second, the use of capillary columns in GC-MS instruments (and to some extent packed columns)
Present address, Instrument Products Division, DuPont Company (U.K.) Ltd., Wilbury Way, Hitchin, Hertfordshire, England. (1) A. T. James and A. J. P. Martin, Biochem. J . , 50, 679 (1952). (2) D. H. Desty, A. Goldup, and W. T. Swanton, Proc. Instr. SOC. Amer. 3rd Nat. Symp. Progr. Trends Chem. Petrol. Itistr.; 105, (1961). (3) W. Henderson, V. Wollrab, and G. Eglinton, in “Advances in Organic Geochemistry 1968,” P. A. Schenck and I. Havenaar, Ed., Pergamon Press, Oxford, 1969, p 181. (4) K . A. Kvenvolden, J. G . Lawless, and C. Ponnamperuma, Proc. Nat. Acad. Sci. US.,68,486 (1971). ( 5 ) M. Novotny and A. Zlatkis, Chromatogr. Reo., 14, 1 (1971). 2302
involves a number of problems and limitations which have resulted in loss of resolution and sensitivity. One of the major problems to be overcome when coupling a gas chromatograph to a mass spectrometer has been to concentrate the organic material in the gas chromatographic effluent by removal of the helium carrier gas in order to maintain a workable pressure in the ion source of the mass spectrometer. The many elegant ways of interfacing the two instruments, have been described extensively elsewhere, namely: (a) use of a molecular separator of one kind or another to enrich the sample with respect to the chromatographic carrier gas (6-10); (b) use of a capillary restriction a t the exit of the column to allow a small aliquot of the total effluent to directly enter the mass spectrometer ion source (11-13); (c) direct coupling of the column to the mass spectrometer, thus limiting the carrier gas flow to less than 0.5 ml/min for conventional mass spectrometers. All of these systems have been described extensively; suffice to say that while they have found wide application in many areas of research they all have faults, most notably a reduction in overall sensitivity due to incomplete preservation of the solute. The development of a system, which allows the use of high resolution capillary columns coupled with a mass spectrometer, capable of analyzing complex mixtures of trace quantities of high molecular weight components directly is of some interest; particularly, since the application of capillary gas chromatography to many biochemically oriented research programs seems to be increasing in recent years. This paper describes a combined gas chromatographmass spectrometer system which was designed specifically for capillary column use, but which may also be used with packed columns provided certain requirements are met. It involves direct introduction of the total gas chromatographic effluent into the ion source of the mass spectrometer and has been used effectively on an extremely wide range of sample types, including fixed gases, hydrocarbons in the carbon number range of
