SAMPLE INJECTION IN PROGRAMMED TEMPERATURE GAS CHROMATOGRAPHY AND ITS SIGNIFICANCE FOR TRACE ANALYSIS

Abstract

The measurement of apparent plate heights resulting from various values of the total sample volume has been carried out for C5 to C8n-alkanes on Apiezon L under both isothermal and p.t.g.c. conditions. Plug injection of samples was achieved through the use of a syringe plunger driven at constant speed by a motor. The time for injection of gas samples ranged from 1 to 300 s. All of the results, both isothermal and p.t.g.c, could be correlated to the same plot of Happ/Hc against √nΔVs/VT0 where Happ is the observed plate height, Hc the limiting plate height at low sample volumes, n the true plate number, ΔVs the sample volume, and VT0 the isothermal retention volume at the initial temperature. The experimental values lie somewhat above the theoretical curve calculated by van Deemter et al. It is confirmed that sample volumes less than [Formula: see text] give negligible excess peak broadening. Applying these concepts to trace analysis, very large samples may be used in p.t.g.c. under such initial temperature conditions that the major component is only weakly retained, while traces of heavier components are concentrated into narrow bands and eluted as sharp peaks with the increased sensitivity provided by the large sample. As an example, chromatograms are presented for the p.t.g.c. analysis of natural gas on deactivated alumina, using sample volumes ranging from 2 to 500 ml.