Simultaneous determination of methylphenobarbital enantiomers and phenobarbital in human plasma by on-line coupling of an achiral precolumn to a chiral liquid chromatographic column.

A fully automated liquid chromatographic (LC) method for the simultaneous determination of methylphenobarbital enantiomers and phenobarbital in human plasma has been developed. The method is based on the use of a precolumn packed with an internal-surface reversed-phase packing material (LiChrospher ADS) for sample clean-up coupled to LC analysis on a cellulose tris(4-methylbenzoate) based chiral stationary phase (Chiralcel OJ-R). A 100-microliter plasma sample was injected directly on the precolumn packed with LiChrospher RP-18 ADS using a mixture of pH 5.0 phosphate buffer-methanol (97:3, v/v) as washing liquid. The analytes were then eluted in the back-flush mode with the LC mobile phase. The enantiomeric separation of methylphenobarbital was achieved on Chiralcel OJ-R). The retention times were modelled using a D-optimal design with ten experimental points in order to optimise the LC mobile phase for the separation of phenobarbital from the enantiomers of mephobarbital. The factors selected were the acetonitrile content, the pH and the sodium perchlorate concentration in the mobile phase. A Derringer's desirability function was used to find an optimal and robust solution within the experimental domain. The mobile phase selected consisted of a mixture of pH 7.0 phosphate buffer-acetonitrile (60:40, v/v). The elution profiles of phenobarbital, methylphenobarbital and blank plasma samples on the precolumn and the time needed for analyte transfer from the precolumn to the analytical column were then determined. Finally, the method developed was validated.