GLC microanalyses of phenacetin and acetaminophen plasma levels.

A GLC method utilizing a flame-ionization detector is described for the simultaneous analysis of acetaminophen and phenacetin in plasma. p-Bromoacetanilide is used as an internal standard. The drugs are extracted with ether from plasma diluted with 1 M phosphate buffer (pH 7.4). The ether extract is evaporated to dryness under nitrogen, and the residue is dissolved in 300 microliter of ethyl acetate. The ethyl acetate is transferred to a microcentrifuge tube (0.4 ml), and the sample is evaporated in a vacuum centrifuge. Then the residue is redissolved in 0.2 M trimethylanilinium hydroxide in methanol for GLC analysis. Extraction efficiency of added phenacetin and acetaminophen in plasma at concentrations of 1-10 microgram/ml was complete, and the limit of detection in plasma was less than 0.1 microgram.