Combination of flow injection with capillary electrophoresis. Part 5. Automated preconcentration and determination of pseudoephedrine in human plasma

The combined flow injection solid-phase extraction capillary zone electrophoresis (FI-SPE-CZE) system described previously was refined and modified for determination of traces of pseudoephedrine in human plasma samples for pharmacokinetic studies following oral administration. The plasma sample was deproteinized off-line by precipitation with trichloroacetic acid. The supernatant plasma was adjusted to pH 4.2 with acetate buffer and 2 ml of plasma sample was loaded on a C-18 micro-column in the FI system. After washing the column with water to remove residual salt, the sample was eluted with a solution of 40% 62.5 mmol l(-1) acetate buffer (pH 3.5) and 60% acetonitrile (v/v), and by means of a FI zone-sampling procedure, the most concentrated 45-mu l zone of the eluate (preceded by a water zone) was introduced via a carrier and running buffer of 200 mmol l(-1) acetic acid-sodium acetate (pH 4.4) containing 0.1% poly(ethylene glycol)-6000 (PEG-6000) into an electrokinetic split-flow sample introduction interface for separation by CZE. The requirements of sensitivity and selectivity for pharmacokinetic studies were met by on-line preconcentration, modification of the sample matrix, increasing the sample volume, and exploiting field-amplified electrostacking effects. A detection limit of 12 mu g l(-1) (3 sigma) was achieved at a sample throughput of 10 h(-1), and with cimetidine as internal standard, a relative standard deviation of 1.2% (n = 10) at 0.5 mu g l(-1) was obtained. Recoveries were in the range 95-98% and 92-101% for inter-day and intra-day studies, respectively. (C) 1999 Elsevier Science B.V. All rights reserved.