Simultaneous gas chromatographic mass spectrophotometric determination of alpha-fluoro-beta-alanine and 5-flaorouracil in plasma

A gas chromatographic-mass spectrometric (GC-MS) method is reported for simultaneous determination of alpha-fluoro-beta-alanine (FBA), the major end metabolite of 5-fluorouracil (5-FU), and 5-FU in plasma samples isolated from cancer patients. 5-Chlorouracil (5-ClU, 1 mu g/ml) is added to samples as an internal standard. The method relies on protein precipitation of the plasma sample followed by derivatisation with pentafluorobenzyl bromide. Following sample purification with Sep-pak C-18 columns the derivatives are analysed by GC-MS, with FBA, 5-FU and 5-ClU being determined at 36.96-37.03, 46.91-46.98 and 51.99-52.13 min, respectively. The ions measured in each case had m/z of 390, 490 and 506, respectively. The method showed good reproducibility with coefficients of variation between 3 and 10%, with a detection limit of <1 ng/ml for 5-FU and <5 ng FBA/ml plasma. The possibility of sensitive determination of FBA without the use of radioisotopes should permit routine estimation of rates of 5-FU metabolism in individual patients.