Retention properties of internal-surface reversed-phase silica packing and recovery of drugs from human plasma

The properties of internal-surface reversed-phase silica packings developed for use in high-performance liquid chromatography, were investigated with regard to the effects of mobile phase conditions (pH, ionic strength, nature and concentration of organic modifiers, ion-pairing agent and surfactant) on the retention of some selected compounds having different polarities. The results indicated that pi-electron interactions play the main role in solute retention, although it is weaker than that on phenylsilica supports, and that weak cation-exchange properties exert a secondary effect on the retention of ionic solutes. The ion-pairing effect was found to be markedly weakened in mobile phases containing an anionic counter ion. In contrast, the addition of a surfactant (SDS) caused a marked increase in the retention of certain drugs at low pH and expanded the pH range of the mobile phase applicable to plasma samples. Based on these findings, the separation and recovery of several drugs (probenecid, lidocaine, cefpiramide and cefaclor) from human plasma were investigated, with emphasis on their protein bindings. The contour chromatogram of human plasma is also demonstrated in relation to the selection of the detection wavelength.