Sample stacking microemulsion electrokinetic capillary chromatography induced by reverse migrating pseudostationary...

For the first time, a capillary electrophoretic (CE) method with sample stacking induced by a reverse migrating pseudostationary phase (SRMP) technique has been developed and validated for sensitive determination of phenobarbital (PB) and its p-hydroxyphenobarbital (PHPB) metabolite in rat urine samples. Separation and determination were optimized on a fused-silica capillary with a total length of 50 cm (effective length 40 cm) and 75 μm ID. The microemulsion background electrolyte consisted of 0.8% (v/v) ethyl acetate, 6.6% (v/v) butan-2-ol, 1.0% (v/v) acetonitrile, 2.0% (w/v) sodium n-dodecyl sulfate (SDS), and 89.6% (v/v) of 7.5 mM ammonium formate at pH 8. When this preconcentration technique was used, the sample stacking and the separation processes took place successively with changing the voltage with an intermediate polarity switching step. For practical application, a solid-phase extraction (SPE), C(18) sorbent with n-hexane/ethyl acetate (1 : 1%, v/v) as the elution solvent was used for sample purification and concentration. The SPE method gave good extraction yields for all the analytes, with absolute recovery values of 96.9% and 99.1% for PB and PHPB, respectively. The regression equations for PB and PHPB showed excellent linearity over a concentration range of 55-1386 ng mL(-1) for PB and PHPB (r = 0.998). The developed microemulsion electrokinetic capillary chromatography (MEEKC) method for separation of the studied compounds with SRMP as the electrophoretic preconcentration technique allowed detection limits in urine samples at 16.8 ng mL(-1) for PB and PHPB which are 15-fold lower than the reported CE method in the literature. The precision results, expressed by the intra-day and inter-day relative standard deviation (RSD) values range from 3.6 to 7.1% (repeatability) and from 3.2 to 7.2% (intermediate precision) for PB and PHPB, respectively, which were in line with Food and Drug Administration (FDA) criteria.