A NEW TYPE OF IONIZATION DETECTOR FOR GAS CHROMATOGRAPHY

High resolution is attained in gas chromatography only with small loads of volatile substances and this, in turn, demands a highly sensitive device for measuring the low concentrations of vapor emerging from the chromatograph column. The detection methods so far proposed fall into two principal categories: those measuring the bulk properties of the gas mixture, and those measuring specific properties of the emergent vapors. Detectors in the first category measure differences in the density, thermal conductivity, and ionization cross section of the gas mixture. The principal disadvantage of this type of detector is that the bulk properties of the gas mixture also vary with temperature and pressure and, unless special precautions are taken to neutralize the effects of small changes in these variables, high sensitivities cannot be attained. In these circumstances, it is not surprising that the only detector in this category with a sensitivity sullicient to exploit the full resolution of gas chromatography is the gas density balance (A. J. P. Martin and A. T. James, 1956). In this detector, complex and ingenious methods are used to balance out variations in temperature, pressure, and rate of gas flow. Detectors in the second category include those measuring specific chemical properties, combustibility (R. P. W. Scott, 1956), and specific ionization properties (J. Harley and V. Pretorius, 1957; S. A. Ryce and W. A. Bryce, 1957; and J. E. Lovelock, 1958). Usually, detectors in this category are unaffected by temperature and pressure changes and are sensitive. However, they have the disadvantage of a sensitivity that varies from substance to substance and cannot be calculated from simple physical parameters. The detector described in this paper does not have this disadvantage; it responds similarly to nearly all volatile organic substances. Its sensitivity is such that it will detect 2 X lo-'* moles, and it is insensitive to temperature, pressure, and rate of gas flow. In addition to these desirable properties, it is simple and inexpensive to construct.