Design of a photoionization detector for high-performance liquid chromatography using an automated liquid-to-vapor phase interface and application to phenobarbital in an animal feed and to amantadine.

An automated liquid-to-vapor phase interface system forms the basis for a new high-performance liquid chromatography (HPLC)-photoionization detection (PID) system. The system incorporates a six-valve interface enabling peak trapping, solvent switching and thermal desorption of the solute of interest into a vapor phase PID. For reversed-phase HPLC, the eluted solute peak is isolated on a Tenax trap after dilution of the effluent with water; the water is then evaporated, following which the trapped solute is flash-evaporated into the PID system. For normal-phase HPLC, the column effluent is diluted with hexane, the solute peak is concentrated on a short column packed with a propyl-amino/cyano bonded phase and the solvent is evaporated. The solute is then eluted with water onto the Tenax trap, and the above procedure for reversed-phase HPLC followed. All operations are controlled with a microcomputer. The advantages of the new detector system include completely automated operation, fast sample preparation, high sensitivity, and inherent selectivity. The system was applied to phenobarbital, which was extracted with acetonitrile from spiked laboratory animal feed, and to amantadine. The phenobarbital assay used a normal-phase separation with hexane-methyl tert.-butyl ether-methanol eluent. The manual sample preparation time was 5 min and the limit of detection was 2 ng phenobarbital injected; a conventional HPLC assay with UV detection required a longer sample preparation time and had a detection limit of 700 ng. Amantadine was assayed using a reversed-phase HPLC system with a water-methanol-triethylamine-orthophosphoric acid mobile phase. The detection limit was 25 ng injected.