SOLID-PHASE MICROEXTRACTION FOR THE DETERMINATION OF THE FREE CONCENTRATION OF VALPROIC ACID IN HUMAN PLASMA BY CAPILLARY GAS-CHROMATOGRAPHY

The potential of solid-phase microextraction in the bioanalysis of dru gs is demonstrated. The free concentration of valproic acid in human p lasma was determined by equilibrium dialysis followed by solid-phase m icroextraction and capillary gas chromatography. Human plasma samples were dialysed at room temperature. To the dialysate was added an inter nal standard and the pH was adjusted to 2.5. The polymethylsiloxane-co ated fused-silica fibre of the solid-phase microextraction device was inserted into the dialysate for 3 min. The sorbed analytes were then t hermally desorbed at 210 degrees C in the split-splitless injection po rt of the gas chromatograph, separated on a Nukol capillary column and detected with a flame ionization detector. The method was shown to be highly reproducible with a detection limit of 1 mu g/ml of free valpr oic acid in human plasma.