SWITCHING-VALVE FILTER TECHNIQUE FOR THE DIRECT-INJECTION AND ANALYSIS OF DRUGS IN PLASMA USING HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

An automated technique involving switching valves and a filter assembl y has been developed and evaluated for the on-line precipitation of pr oteins and peptides from plasma samples. In the set-up, the proteins w ere precipitated on-line by injecting the plasma sample into a stream of organic precipitating agent. The precipitates so formed are filtere d on-line by a set of filter assemblies consisting of ordinary in-line HPLC solvent filters. Evaluation of the technique was performed using ibuprofen and a mixture of three estrogens, estradiol, equilin and es trone, spiked in dog plasma. The coefficients of variation (C.V.) for system suitability parameters were below 10%. Absolute recovery of ibu profen in plasma ranged from 80% for 100 mug/ml to 114% for 5 mug/ml s piked concentrations, respectively. Resolution for equilin and estrone , two closely eluting peaks, was 1.79 (C.V. = 5.8%, n = 7). The switch ing-valve-filter assembly had no significant effect on the efficiency of the HPLC system.