Improved direct injection method and extractive methylation method for determination of valproic acid in serum by gas chromatography.

Two simple methods for the determination of valproic acid in serum by gas chromatography are described. One is a direct injection method. To the serum sample are added an acetonitrile solution including an internal standard and hydrochloric acid. The sample is deproteinized and after centrifugation valproic acid in the supernatant is measured in the free form by direct injection into the gas chromatograph. The other is an extractive methylation method. To the serum sample are added an internal standard solution and a counter-ion solution, and the mixture is extracted into methylene chloride containing methyl iodide. Valproic acid is extracted into methylene chloride and simultaneously the methylation reaction proceeds. After centrifugation, an aliquot of the lower layer without evaporation is injected into the gas chromatograph. Both methods have been applied successfully to monitoring routine serum levels of valproic acid.