Development and validation of a GC-EI-MS method with reduced adsorption loss for the quantification of olanzapine in human plasma

A simple and sensitive GC-EI-MS method using solvent extraction and evaporation was developed for the determination of olanzapine concentrations in plasma samples. Because olanzapine and promazine, which was used as the internal standard (IS), are nitrogenous bases, they can adsorb to the weakly acidic silanol groups on the surfaces of glass centrifuge tubes during solvent extraction and evaporation. Silylation of the glass tubes, addition of triethylamine (TEA), and use of a sample solution with a basic pH could prevent adsorption loss. The extraction method involved mixing plasma (500 μL) in a silylated glass tube with a promazine solution (2 μg/mL, 25 μL) in methanol containing 1% TEA. After addition of aqueous sodium carbonate (0.5 mol/L, pH 11.1, 1 mL) and extraction into 3 mL of dichloromethane/n-hexane (1:1, v/v) containing 1% TEA, the organic phase was evaporated to dryness in a silylated glass tube. The residue was dissolved in ethyl acetate containing 1% TEA (50 μL). For GC-EI-MS analysis, the calibration curves of olanzapine in human plasma were linear from 0.5 to 100 ng/mL. Intra- and interday precisions in plasma were both less than 7.36% (coefficient of variation), and the accuracy was between 94.6 and 110% for solutions with concentrations greater than 0.5 ng/mL. The limit of quantification was 0.5 ng/mL in plasma. The assay was applied to therapeutic drug monitoring in samples from three schizophrenic patients.