An assay for paracetamol, produced by the O‐deethylation of phenacetin in vitro , using gas chromatography/electron capture negative ion chemical ionization mass spectrometry

A sensitive gas chromatographic/mass spectrometric assay for paracetamol, produced by the O ‐deethylation of phenacetin by microsomal fractions of human liver, has been developed. The method uses the 3,5‐bistrifluoromethyl‐benzoyl derivative of paracetamol and electron capture negative ion chemical ionization. This derivative has a negative ion mass spectrum which contains only a molecular ion at m/z 391 and, when this ion is specifically monitored, an amount of derivative equivalent to 1 pg of paracetamol can be detected. ( 2 H 3 )Paracetamol was available for use as an internal standard and this allowed the development of an assay with a precision of 8% (SD) at 0.5 ng paracetamol per microsomal incubation and a lower limit of quantitative determination of 0.2 ng paracetamol per microsomal incubation.