Mass fragmentographic determination of xanthine and hypoxanthine in biological fluids

The method presented for the simultaneous determination of xanthine and hypoxanthine, uses mass-fragmentography in the electron impact (EI) mode, after the gas chromatographic separation of butylated derivatives. Butylation, rather than methylation, is used in order to avoid interference coming from exogenous caffeine, which is frequently encountered. [7,9-¹⁵N]Xanthine is used as the internal standard, and for each sample, a blank is obtained by xanthine oxidase reaction. In the biological fluids studied the sensitivity was about 50 ng/ml.