The determination of pentobarbital and other barbiturates in blood plasma by gas—liquid chromatography with on-column and pre-column butylation

Two g.l.c. methods for the determination of pentobarbital and other barbiturates are reported. In the first method the plasma samples are extracted with toluene; the toluene layer is back-extracted with a small volume of a tetrabutylammonium hydroxide solution of which an aliquot is injected into the gas Chromatograph. This method is simple and rapid, and sensitive enough for monitoring pentobarbital plasma concentrations in the therapeutic range. A number of barbiturates are not well extracted from toluene by tetrabutylammonium hydroxide because of ion-pair partitioning. The extraction efficiency of barbiturates from toluene is much improved by back-extraction of the toluene layer with a tetramethylammonium hydroxide solution; clean chromatograms are obtained from blank plasma when the toluene layer is back-extracted, instead of evaporated, prior to derivative formation. This modification is introduced in the second method; an aliquot of the tetramethylammonium hydroxide layer is subjected to a pre-column butylation procedure which converts the barbiturates completely without any decomposition in the injection port; the sensitivity is at least as good as that of the on-column technique