Simultaneous determination of paracetamol and caffeine in human plasma by LC-ESI-MS

A rapid, selective and convenient liquid chromatography-mass spectrometric method for the simultaneous determination of paracetamol and caffeine in human plasma was developed and validated. Analytes and theophylline [internal standard (I.S.)] were extracted from plasma samples with diethyl ether-dichloromethane (3:2, v/v) and separated on a C-18 column (150 x 4.6 mm ID, 5 mu m particle size, 100 angstrom pore size). The mobile phase consisted of 0.2% formic acid-methanol (60:40, v/v). The assay was linear in the concentration range between 0.05 and 25 mu g mL(-1) for paracetamol and 10-5,000 ng mL(-1) for caffeine, with the lower limit of quantification of 0.05 mu g mL(-1) and 10 ng mL(-1), respectively. The intra- and inter-day precision for both drugs was less than 8.1%, and the accuracy was within +/- 6.5%. The single chromatographic analysis of plasma samples was achieved within 4.5 min. This validated method was successfully applied to study the pharmacokinetics of paracetamol and caffeine in human plasma.