On-line combination of dialysis and column-switching liquid chromatography as a fully automated sample preparation technique for biological samples. Determination of nitrofuran residues in edible products.

The potential of dialysis coupled on-line with trace enrichment by column-switching high-performance liquid chromatography as an automated sample treatment technique in drug residue analysis has been investigated. The nitrofuran veterinary drugs furazolidone, nitrofurazone, nitrofurantoin and furaltadone were used as model compounds. Critical parameters, i.e., dialyser dimensions, air segmentation, dialysis time, flow-rates and enrichment column breakthrough, were evaluated. Depending on the analytical purpose, the technique can be set up in either a highly sensitive or a high-speed mode. High dialysis efficiencies (greater than 85%) can be obtained in a stopped-flow dialysis of only 3 min. Aqueous biological sample volumes (eggs, meat, milk) of 100 microliters to 4 ml can be injected with only sample treatment. A routine monitoring method for residues of nitrofuran drugs in edible products was set up. A 4-ml sample was dialysed in a pulsed mode with an efficiency of about 30% and concentrated on a short C18 column. Recoveries compared with standards were 75-85% (coefficient of variation 2-7%). Limits of determination ranged from 1 to 10 micrograms/kg. At this concentration level, ca. 30 samples can be monitored per day.