Deuteromethylation of dimethylxanthines: a gas chromatographic mass spectrometric method for confirmatory analysis in horse urine extracts.

The methylated xanthines caffeine and/or theobromine are commonly encountered in drug-positive samples from racehorses and their metabolism and excretion in the horse and their analysis in urinary extracts has been of particular interest in this laboratory. Due to their polar nature the dimethylxanthines theobromine, theophylline and paraxanthine give unsatisfactory gas chromatographic performance and require derivatization prior to analysis by gas chromatography mass spectrometry. The present paper describes a simple deuteromethylation procedure to render the compounds amenable to analysis by gas chromatography mass spectrometry. Characteristic ions in the mass spectra of the isomeric monotrideuteromethyl-dimethylxanthines provide a means of distinguishing between them. The discriminatory evidence provided by mass spectrometric analysis of the deuteromethylated products is compared to that obtained by gas chromatographic mass spectrometric analysis of the butylated products.