Quantitative determination of phenothiazine derivatives in human plasma using monolithic silica solid-phase extraction tips and gas chromatography-mass spectrometry

Solid-phase extraction (SPE) using micropipette tips is a useful technique to prepare samples prior to mass spectrometry. However, most commercial SPE tips have loading capacities that are insufficient for quantitative determination. In this paper, we describe a rapid method for quantitative microanalysis of five phenothiazine derivatives, chlorpromazine, levomepromazine, promazine, promethazine and trimeprazine, using a recently introduced C(18) monolithic silica SPE tip, the MonoTip C(18), for extraction from human plasma. The drugs could be extracted within 5 min from 0.1-mL plasma samples, eluted with methanol, and the eluate injected directly into a gas chromatograph prior to mass spectrometry analysis. Only 0.7 mL of solvent was required for each step of the extraction process. The recoveries of the five phenothiazines spiked into plasma were 91-95% and the limits of quantification for each drug were between 0.25 and 2.0 ng/0.1 mL. The maximum intra- and inter-day coefficient of variation was 11%. The validated method was successfully used to quantify the plasma concentration of levemepromazine in a human subject after oral administration of the drug. This new method is expected to have wide applications as a pretreatment for the rapid, quantitative determination of drug concentrations in plasma samples.