Improved liquid-chromatographic assay of labetalol in plasma

This is an assay for labetalol in plasma by "high-performance" liquid chromatography, with 5-(2-[4-(4-chlorophenyl)ethyl]) salicylamide hemihydrate as the internal standard. Plasma samples (500 microL) are extracted with acetonitrile, evaporated under nitrogen, reconstituted in the mobile phase, and injected onto a PRP-1 (Hamilton) column packed with particles of poly(styrene-divinylbenzene) copolymer. Fluorescence, enhanced by post-column introduction of NH4OH, was measured in the effluent (excitation wavelength 340 nm, emission wavelength 418 nm). Retention times for labetalol and the internal standard were 1.99 and 3.32 min, respectively. Inter- and intraday CVs for high and low concentrations of the drug were less than 7.5%. The assay standard curve is linear from 1 to 250 micrograms/L. Some commonly co-administered drugs were tested and did not interfere.