Stable isotope labelling and determination of ketroprofen in human plasma and urine by gas chromatography negative ion chemical ionization mass spectrometry

A method for the quantitative measurement of the non-steroidal anti-inflammatory drug ketoprofen in human plasma and urine is presented. The assay is based on gas chromatography/negative ion chemical ionization mass spectrometry. The preparation of stable isotope-labelled ketoprofen for use as an internal standard is described. After solvent extraction from the acidified matrix, urine samples were analysed as pentafluorobenzyl esters, whereas plasma samples required further derivatization to the hydroxylamine-trimethylsilyl derivatives. The detection limit was found to be 5 pg in both cases. The method was applied to the pharmacokinetics of ketoprofen in man after epidermal application.