Determination of Trimethoprim in Tissues Using Liquid Chromatography–Thermospray Mass Spectrometry

Abstract

A method is described for the determination of the antibacterial drug trimethoprim in tissues. Minced tissue is homogenised with chloroform–acetone (1 + 1 v/v), filtered, and the filtrate evaporated to an oily residue using a rotary evaporator. The residue is redissolved in methanol–water–acetic acid (50 + 48.7 + 1.3 v/v) and any fats present are partitioned into hexane. The aqueous phase is analysed by liquid chromatography–thermospray mass spectrometry in positive mode with the protonated molecular ion at m/z 291 being monitored. Recoveries ranged between 60% in liver and 79% in muscle. The limit of determination was 25 µg kg^–1 and the limit of detection was approximately 4 µg kg^–1. The method is suitable for monitoring tissues taken under national surveillance schemes for veterinary drug residues.