Direct coupling of a dense (supercritical) gas chromatograph to a mass spectrometer using a supersonic molecular beam interface

Abstract

A detecting mass spectrometer has been successfully coupled to a dense gas (supercritical fluid) chromatograph to produce an instrument (DGC/MS) that may be an alternative to high performance liquid chromatograph/mass spectrometer instruments (HPLC/MS) and gas chromatograph/mass spectrometer instruments (GC/MS) for analysis of involatile and/or thermally labile compounds. The mobile phase in DGC is a gas held at temperatures above the critical temperature and at pressures sufficient to obtain nearly liquid-like densities. DGC combines advantages of GC and HPLC: rapid separations, moderate operating temperatures, and analysis of involatile compounds. An advantage unique to DGC is the solvent power dependence upon pressure. While several groups have studied DGC, its development has been limited by the lack of a sensitive and selective detector. Hence, work has been directed towards the design and construction of a DGC/MS resulting in a trial instrument capable of chromatographic pressures of at least 300 atm and temperatures from 10° to 60°C. The DGC/MS coupling has been accomplished by the use of a supersonic molecular beam interface. This application of molecular beam formation appears to be unique in its requirements of a large pressure ratio (∼108), low flow rates, and low final pressures. The authors outline characteristics of supersonic jets and molecular beams pertinent to the design of such an instrument. The interface which uses pumping speeds of 2400 and 1200 l/s in the beam forming chambers is described in detail, while the other components—the detecting mass spectrometer, the dense gas supply, and the DGC—are briefly described. Preliminary work with this instrument has established the feasibility of DGC/MS as an analytical technique and further development is recommended.

Capillary supercritical fluid chromatography-mass spectrometry using a "high mass" quadrupole and splitless injection

“…Simple ammonia chemical Ionization spectra, dominated by the ammonium adduct Ion, are seen to the mass limit of the Instrument. A signal-to-nolse ratio of approximately 4 Is measured for the ammonium adduct Ion of 0.8 ng of methyl arachldate.…”

“…Mass spectrometry (MS) has been shown to provide sensitive, universal, and specific detection for SFC (3)(4)(5). Consistent progress in understanding the SFC-MS interface and in applications of SFC-MS has been reported by Smith, Wright, and co-workers (6)(7)(8)(9)(10)(11)(12).…”

Pulsed supersonic expansion of nonvolatile solids

“…Methods explored so far include laser desorption/ionization or matrix assisted laser desorption/ionization, 1-5 electrospray generation, 6,7 and the expansion of supercritical solutions into a molecular beam. [8][9][10][11][12][13][14] In the latter approach the key idea is to dissolve the substance at high pressure, exploiting the supercritical fluid's solvent power, and precipitate it by expansion. The use of supercritical fluids as solvents offers various advantages: One is that a solvent such as carbon dioxide is compatible with hydrocarbon-free ultrahigh vacuum conditions.…”

Mass spectrometry with direct supercritical fluid injection

“…Theoretical predictions of the precise extent of cluster formation are still inadequate; however, an empirical method of "corresponding jets" has been developed (33) , which uses scaled parameters, and has been successfully employed. Randall and Wahrhaftig (21) have applied this method to the expansion of supercritical C02 and obtained the following empirical equation: N = (6 X io")Pfi*«Da867"5-4…”

“…The method of direct fluid injection mass spectrometry (DFI-MS) is being explored in our laboratory for the purpose of interfacing capillary column supercritical fluid chromatography (SFC) with mass spectrometry (19). The direct interfacing of SFC was discussed more than a decade ago (20) and Randall and Wahrhaftig (21) have described an approach using supersonic molecular beams. Unfortunately, this approach appeared to suffer from instrumental complexity and extensive solvent cluster formation.…”