Development and validation of a liquid chromatography–electrosprayionization mass spectrometric method for the determination of dexamethasone in sheep plasma

A quantitative liquid chromatographic–electrospray ionization mass spectrometric method for the determination of dexamethasone in sheep plasma has been developed and validated. The samples were extracted using solid-phase extraction cartridges with mixed reversed-phase materials (oasis-HLB). The chromatographic separation was performed on a reversed-phase XTerrra MS C18 column (3.0 mm×150 mm; 5 (mu)m) using a mobile phase consisting of 65% methanol in water containing 0.1% (v/v) formic acid, pumped at a flow rate of 0.30 ml min^-1. The analyte was detected after positive electrospray ionization using selected ion monitoring (SIM) mode. The probe heater temperature was set at 260 °C, the capillary voltage was set at 3.5 kV and the source block voltage (AQAmax) was set at 30 V. The method was fully validated. Calibration graphs were linear (r better than 0.998, n=11), in concentration ranges 6–1000 ng ml^-1 for dexamethasone. The intra- and inter-day RSD values were less than 24.1% (n=6). The limits of detection and quantitation for dexamethasone were found to be 1 and 6 ng ml^-1, respectively. The efficiency of the solid phase extraction procedure was found to be 92.4% for dexamethasone. The method was further applied to a pilot kinetic study in order to assess the main pharmacokinetic parameters of dexamethasone in sheep.