Determination of valproic acid (di-N-propyl acetic acid) in plasma by gas-liquid chromatography with pre-column butylation.

A gas-liquid chromatographic procedure for the determination of the anti-convulsant drug valproic acid in plasma or serum is described. Valproic acid is extracted from acidified plasma (or serum) into toluene containing octanoic acid as the internal standard. The toluene layer is back extracted with a methanolic solution of tetramethylammonium hydroxide; an aliquot of the methanolic layer is mixed after phase separation with N,N-dimethylacetamide and 1-iodobutane. After centrifugation the clear supernatant (which is stable for at least 24 h) is analysed by gas-liquid chromatography, on a 3% OV17 column. The procedure is rapid and sensitive, and the back-extraction step with methanolic tetramethylammonium hydroxide results in clean chromatogram obtained from blank plasma. Sample volumes of 50 microliter suffice for the determination of therapeutic levels of VPA.