Solid phase extraction—Non-aqueous capillary electrophoresis for determination of metformin, phenformin and glyburide in human plasma

Solid phase extraction (SPE) was coupled at line to capillary electrophoresis (CE) for the determination of three basic and neutral diabetic drugs (metformin, phenformin and glyburide) in human plasma. The SPE procedure employed a C18 cartridge to remove most of the water and proteins from the plasma sample. Analyte detectability was increased due to trace enrichment during the SPE process. Elution of metformin, phenformin and glyburide was achieved with methanol + 3% acetic acid. CE analysis was performed using a non-aqueous buffer, acetonitrile + 5 mM ammonium acetate + 5% acetic acid, which afforded rapid separation of metformin from phenformin within 3 min. Glyburide, with a migration time longer than 6 min, did not cause any interference. The present SPE–CE method, with an electrokinetic injection time of 6 s and UV detection at 240 nm, was useful for monitoring down to 1 μg/mL of metformin and phenformin in human plasma. When the electrokinetic injection time was increased to 36 s, the detection limits were improved to 12 ng/mL for metformin and 6 ng/mL for phenformin.