Highly sensitive determination of 5-fluorouracil in human plasma by capillary gas chromatography and negative ion chemical ionization mass spectrometry.

A sensitive method using gas chromatography and electron-capture negative ion mass spectrometry for the determination of 5-fluorouracil in plasma is described. 5-Chlorouracil was used as internal standard. Sample clean-up consisted of extraction of the 5-halogenated uracil derivatives with 2-propanol--diethyl ether (22:78, v/v) at pH 6, followed by a back-extraction into aqueous buffer at pH 10.5. Pentafluorobenzyl derivatives of 5-fluoro- and 5-chlorouracil were prepared by extractive alkylation with pentafluorobenzyl bromide with dichloromethane as solvent and tetrabutylammonium as counter-ion. The limit of sensitivity employing this technique was 50 fg on-column. Quantitation in human plasma was possible down to 3 X 10(-9) M (0.39 ng/ml). Mass fragmentographic analysis of 5-fluorouracil in plasma of patients after a bolus injection of 500 mg/m2 is presented. Plasma concentrations could be determined for at least 8 h after administration.