Candidate reference method: Determination of valproic acid in serum by isotope-dilution mass spectrometry

Summary A method has been developed for the determination of valproic acid, without derivatization, in human serum by isotope-dilution mass spectrometry using labelled 2-propyl3,3,3-d₃]valeric5,5,5-d₃] acid as internal standard for accurate quantification of the concentration of valproic acid in the sample. After acidification, the analyte and internal standard are extracted withn-hexane. The amounts of valproic acid in the serum are calculated from the isotope ratio of valproic acid to labelled valproic acid, which is measured by electron impact (EI) and chemical ionization (CI) selected ion monitoring (SIM). The concentrations of valproic acid in sera measured using isotope-dilution mass spectrometry are compared with results from gas-liquid chromatography (GLC) and fluorescence polarization immunoassay (FPIA). The accuracy, precision and recovery of the GC-MS methods are discussed. The coefficient of variation determined from duplicate samples was less than 1.5%. The detection limit was 10 ng mL⁻¹ at a signal-to-noise ratio of 3∶1. Part of this work was presented at the Kongre? der Deutschen Gesellschaft für Laboratoriumsmedizin, Berlin, 1994.