A silver-loaded thiol stationary phase was used for the trace enrichment of pyrimidine nucleobases (uracil, thymine and cytosine) and some related compounds (5-fluorouracil, bromacil, uridine). The critical parameters for the sorption of these compounds on Ag(I)-thiol are the pH value of the sample solution and the flow-rate with which they are flushed through the pre-column. Complete recovery for all compounds except uridine was obtained at pH 11 .O at a flow-rate of 0.6 ml/min. The desorption mechanism was based on protonation of the analytes. Efficient on-line desorption to the Cl8 analytical column was achieved by injecting a plug of 60 ~1 HN03 (pH 1.2) onto the Ag(I)-thiol pre-column. After separation the analytes were detected by UV detection at 269 nm. For 5-fluorouracil a detection limit of 3 + lo-' M was obtained for the preconcentration of 1.0 ml with a reproducibility of 2.3% (n = 6,5 . 10e7 M). The column efficiency measured in terms of plate number for 5-fluorouracil was 11 000 for direct injections and 9000-10 000 for 1 .O-ml preconcentrations, indicating that the desorption step causes no appreciable band broadening. Applications of the described method for the determination of 54uorouracil in plasma and for the herbicide bromacil in surface water are presented.
' Author deceased.