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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.

The mass spectra of re-butane-2-¿, «-butane-1,l,l-d3, «-butane-1,1,1,3,Z-d¡ and re-butane-1,1,1,2,2,3,3-¿7 using 80 volt electrons at 100°are compared with that of «-butane. The results are discussed with respect to the isotope effect, rearrangement phenomena, and activated complexes. The rate of H-D exchange in the parent molecule-ions is slower than the rate of C-C bond break, but faster than the rate of loss of H2 or HD. Such exchange occurs between atoms on adjacent carbons and on carbons 1-3 and 1-4.

An improved continuous approximation to the number of states of a collection of harmonic oscillators of different frequencies has been developed and applied to the quasi-equilibrium theory of mass spectra. The classical approximation previously used in this theory has been shown to be invalid at low energies. The difficulties in the application of the quasi-equilibrium theory of mass spectra are to a great extent resolved by the use of this more reasonable approximation.