Operation of the flame-ionization detector (FID) under vacuum outlet conditions permits the benefits of short column vacuum outlet gas chromatography to be realized. A relatively simple modification was made to the standard FID, allowing connection of the FID tower outlet to a rotary vacuum pump for vacuum operation. Pure oxygen was utilized in place of air for the flame gas mixture; observations of the behavior of the FID flame are noted and discussed. The modified detector allows flame stability down to 130 torr. When running the FID under normal gas mixtures, the flame is stable only to 230 torr. Sample chromatograms of atmospheric and vacuum outlet FID are presented at both the optimum average linear velocities and optimum practical gas velocities (OPGVs). Subambient column inlet pressures are required to achieve the optimum average linear velocity and OPGV for chromatograms run on short columns with vacuum outlet.
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Bernier et al. (1993) studied this question.