Direct-injection high performance liquid chromatography ion trap mass spectrometry for the quantitative determination of olanzapine, clozapine and N-desmethylclozapine in human plasma

A specific and sensitive direct‐injection high performance liquid chromatography electrospray ionization tandem mass spectrometry (HPLC/ESI‐MS/MS) method has been developed for the rapid identification and quantitative determination of olanzapine, clozapine, and N‐desmethylclozapine in human plasma. After the addition of the internal standard dibenzepin and dilution with 0.1% formic acid, plasma samples were injected into the LC/MS/MS system. Proteins and other large biomolecules were removed during an online sample cleanup using an extraction column (1 × 50 mm i.d., 30 µm) with a 100% aqueous mobile phase at a flow rate of 4 mL/min. The extraction column was subsequently brought inline with the analytical column by automatic valve switching. Analytes were separated on a 5 µm Symmetry C18 (Waters) analytical column (3.0 × 150 mm) with a mobile phase of acetonitrile/0.1% formic acid (20:80, v/v) at a flow rate of 0.5 mL/min. The total analysis time was 6 min per sample. The inter‐ and intra‐assay coefficients of variation for all compounds were