Combined assay for phenacetin and paracetamol in plasma using capillary column gas chromatography-negative-ion mass spectrometry.

A gas chromatographic-mass spectrometric assay has been developed for the measurement of phenacetin and its major metabolite paracetamol in plasma. Phenacetin and unconjugated paracetamol are analysed in a single chromatographic run while total paracetamol is measured separately after enzymatic hydrolysis. The two compounds, and the deuterated analogues used as internal standards, are analysed as their trifluoroacetyl derivatives and the mass spectrometer is operated in the electron-capture negative-ion chemical ionisation mode. The negative-ion mass spectra of the derivatives contain fragment ions, formed by loss of an acetyl group from the respective molecular ions, which are the base peaks in the spectra. When these ions are specifically monitored, amounts of derivative equivalent to 1 pg of parent compound can be detected. This allowed the development of an assay for phenacetin, unconjugated paracetamol and total paracetamol in plasma having a precision of 2.6, 1.4 and 2.4%, respectively, and preliminary results for a subject given a 100-mg oral dose of phenacetin are reported.