Anal. Chern. 1982, 54, 1883-1885 (3) Macfarlane, R. D. NBS Spec. Pub/. 1979, SP 579, 673-677. (4) Radmacher, W.; Mohrhauer, P. Brennst.-Chem. 1955, 36, 236-239. (5) Radmacher, W.; Mohrhauer, P. Bremsf.-Chem. 1956, 3 7 , 26. (6) Bishop, M.; Ward, D. L. Fuel 1958, 3 7 , 191-200. (7) Tarpley, E. C.; Ode, W. ti. R e p . Invest.-US., Bur. Mines 1958, RI 5470. (8) Fleldner, A. C.; Selvig; VV. A.; Taylor, G. B. Tech. Progr. R e p . - U . S . , Bur. Mines 1919, TP 272. (9) Turner, H. G. Trans. A m . Inst. Min., Mefall. Pet. €ng. 1930, 88, 639. (10) McNeal, C. J.; Macfarlane; R. D.; Thurston, E. L. Anal. Chem. 1979, 57,2036-2039. (11) Tingey, G. L.; Lytle; J. MI.; Baer; D. R.; Thomas, M. T. PNL-3650; Pacific Northwest Laboratory, Richland, WA, 1900. (12) Taylor. L. C. E. Ind. Res./Dev. 1981, 23(9), ‘124-128. (13) Joy, W. K.; Ladner; W. FL; Pritchard, E. fuel 1970, 49, 26-38.
1883
’
R. D. Macfarlane is a professor In the Chemistry Department at Texas A&M University, College Station, TX.
J. M. Lytle* G. L. Tingey R. D. Macfarlane’ Battelle, Pacific Northwest Laboratory Richland Washington 99352 RECEIVED for review March 11, 1982. Accepted June 7, 1982. Research sponsored by the United States Department of Energy under Contract DC-AC06-76RLO-1830.
Capillary Column Supercritical Fluid Chromatography/Mass Spectrometry Sir: The analysis of complex mixtures is often limited by the molecular weight range and selectivity or present analytical techniques. Gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS) are ultimately limited by the volatility of the isample components to compounds having molecular weights less than approximately 300-400. High-performance liquid chromatography (IIPLC) can be used to separate higher molecular weight and nonvolatile compounds, but the chromatographic efficiency is much less than that of modern capillary column GC and detector sensitivity and selectivity are often inadequate. Modern HPLC using packed columns is rapidly approaching practical limits on the number of theoretical plates available; recent results suggest, however, that improvements in chromatographic efficiency may be obtained with microbore or capillary columns. The ideal detector for chromatography is the mass spectrometer due to its inherent sensitivity and selectivity. However, interfacing HPLC to mass spectrometry is a difficult task because of the fundamental incompatabilitiee in required liquid flow rates and solvent evaporation or removal (1-3). Thus, there is a need for an alternative approach t o HPLC for the analysis of complex mixtures of nonvolatile or thermally labile compounds which providies increased chromatographic efficiency as well as greater calmpatabilitywith mass spectrometry. Supercritical fluid chromatography (SFC) has potential advantages in many appliications relative to both GC and LC which have been demonstrated by a number of workers over the past 15 years (4-10). The direct interfacing of SFC with mass spectrometry may have significant advantages relative to LC/MS and has attracted some interest (11-14). Randall and Wahrhaftig have previously reported on the construction of a supercritical fluid (dense gas) chromatograph/mass spectrometer interface using conventional packed columns and supersonic rnolecular beam techniques (11-13). This approach, however, suffers from the lower chromatographic efficiency relative to that possible using capillary columns and the complexity of four stages of differential pumping required by large mobile phase flow rates (13). Gouw et al. have also presented results using direct introduction with electron impact ionization but this approach seems to be limited by compound volatility and poor sensitivity ( 1 4 ) . The use of capillary column SFC technology (15-17) can obviate difficulties associ,ated with previous SFC/MS interfaces and allows a simple interface readily adapted to existing GC/MS systems. The combination of SFC with mass spectrometry offers the followiiig potential advantages relative to 0003-2700/62/0354-1883$0 1.25/0
GC/MS or LC/MS methods: (a) High molecular weight, polymeric, heterofunctional, and thermally labile compounds can be separated as well as the more volatile species. (b) Capillary SFC columns can provide greatly enhanced chromatographic efficiency relative to HPLC due to solute diffusivities which are about lo2greater in the supercritical fluid than in the corresponding liquid phase and viscosities similar to the gas phase ( 4 , 5 , 9 ) . (c) Solvating power of the mobile phase can be readily controlled with pressure programming (5). Mixed mobile phases ( 4 ) , gradient, and temperature programming are also feasible. (d) SFC using capillary columns provides low mobile phase flow rates which, coupled with high mobile phase volatility, allows optimum interfacing of SFC and mass spectrometry. Capillary column SFC/MS instrumentation has been developed in our laboratory to investigate and apply SFC methods; preliminary results are presented in this report. EXPERIMENTAL SECTION The SFC/MS instrument incorporates a capillary column SFC, a direct fluid injection interface, and a tandem quadrupole mass spectrometer equipped with a dual electron impact-chemical ionization ion source. Figure 1 gives an overall schematic illustration of the instrumentation. The supercritical fluid chromatograph utilizes a Varian 8500 high-pressure syringe pump (8000 psi maximum pressure) and a constant-temperature oven and transfer line. Short lengths of
