Background: Free valproic acid is shows no characteristic absorption in the ultraviolet region (above 235 nm), therefore its direct quantification and also the quantification of the corresponding metabolites from human plasma has proven to be challenging. Aim: The aim of our study was to develop and validate an effective LC-MS method for the determination of valproic acid in human plasma without using solid phase extraction as sample preparation, with a short analysis time and high sensitivity.
Materials and methods: Valproic acid was analyzed on a reversed – phase column (Zorbax SB – C18, 100 mm x 3 mm I.D., 3.5 μm) under isocratic conditions using a mobile phase of a 40:60 (v/v) mixture of acetonitrile and 0.1% (v/v) acetic acid in water. The flow rate was 1 mL/min and the column temperature 45 ºC. In these chromatographic conditions, the retention time was 2.3 minute for valproic acid. The detection of the analyte was in single ion monitoring mode using a triple quadrupole mass spectrometer with electrospray negative ionization. The monitored ion was 143.1 m/z derived from 144.2 m/z valproic acid. The sample preparation was very simple and consisted in plasma protein precipitation from 0.2 mL plasma using 0.6 mL methanol.
Results: Calibration curves were generated over the range of 2–200 µg/mL with values for coefficient of determination greater than 0.996 and by using a weighted (1/x) quadratic regression. The values of precision and accuracy for valproic acid at quantification limit were less than 3.3% and 7.2%, for within- and between-run assays, respectively. The mean recovery of the analyte was 104%. Valproic acid samples demonstrated good short-term, post-preparative and freeze-thaw stability.
Conclusion: The method is very simple and allows obtaining a very good recovery of the analyte. The validated LC-MS/MS method could be applied to pharmacokinetics and therapeutic drug monitoring study regarding valproic acid in humans.