Gas chromatography/negative ion chemical ionization/tandem mass spectrometric quantification of indomethacin in plasma and synovial fluid

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Abstract
A procedure for the quantification of sub‐nanogram per millilitre concentrations of indomethacin in plasma and synovial fluid has been developed using gas chromatography/negative ion chemical ionization/tandem mass spectrometry. The procedure is based on selected ion monitoring of the daughter ion at m/z 312 generated following decarboxylation of the 35Cl isotope peak of the indomethacin anion (m/z 356) formed from the pentafluorobenzyl ester of indomethacin in the ion source of the mass spectrometer. The internal standard, d2‐indomethacin, has a daughter ion at m/z 316 formed from the corresponding 37Cl isotope peak. The limit of quantification was 0.1 ng ml−1 for both plasma and synovial fluid.