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The separation of volatile substances by gasliquid chromatography (James & Martin, 1952; James, Martin & Smith, 1952; James, 1952; James & Martin, 1954; Ray, 1954a, b) requires that the substances be detected as they leave the chromatogram in the stream of gas that acts as the mobile phase of the system. A number of methods of detection of varying sensitivity have already been developed (James & Martin, 1952; Ray, 1954a; Griffiths, James & Phillips, 1952; Griffiths & Phillips, 1954), but we have felt it important to use a detector of high sensitivity whose response was independent of the chemical structure of the substances being separated. From many points of view measurement of the density of the gas stream emerging from the chromatogram is attractive, since response is dependent on the molecular-weight differences of gab and vapour only, and calibration is relatively simple. Claesson (1946) has described an apparatus for comparing the density of a standard gas and that issuing from a chromatogram in which a liquid manometer is used to detect pressure differences between columns of gas 6 ft. long. The sensitivity of this apparatus suffices only for measuring the high concentrations encountered in displacement analysis. The pressure differences due to the flow of gas were kept very small by the use of large-diameter tubes; this resulted in the volume of the apparatus being very large. In the design of an instrument based on the same principle (the gas-density meter) we have compensated for the pressure difference 'due to flow by employing tubes arranged as a bridge circuit in which two points can be found, vertically displaced from one another, where the pressure difference is independent of the rate of flow of gas. It is possible to combine two such bridges in one of which chromatogram gas flows and in the other a reference stream, so that the difference in head due to the density difference of the two gases is directly measurable.

Gas–liquid chromatography: the gas-density meter, a new apparatus for the detection of vapours in flowing gas streams