Capillary gas chromatographic-mass spectrometric determination of stable isotopically labelled and unlabelled theophylline in serum and urine and of 1,3-dimethyluric acid in urine.

A capillary gas chromatographic-mass spectrometric method for the determination of [13C,15N2]theophylline and unlabelled theophylline (TP) and of 1,3-dimethyluric acid (1,3-DMU), a major metabolite of TP, is described. TP and the metabolite were extracted separately from serum or urine, purified by high-performance liquid chromatography and converted into alkyl derivatives (monoethyl-TP and dibutyl-1,3-DMU). The internal standards used for the respective determinations were [2H6]TP and [2H6]1,3-DMU. Detection was performed by monitoring the molecular ions of the alkyl derivatives. The method needed no complex corrections for contributions and provides good accuracy and precision.