Hydrophilic shielding of hydrophobic, cation- and anion-exchange phases for separation of small analytes: direct injection of biological fluids onto high-performance liquid chromatographic columns.

Shielded hydrophobic phases (SHPs) have an external hydrophilic network that prevents larger protein molecules from interacting with hydrophobic zones. Smaller analytes are not sterically hindered from interacting with the hydrophobic zones and are retained. This mechanism allows the separation of the proteins from the analytes of interest. SHPs have been shown to be useful for the direct-injection analysis of drugs in biological fluids. In this paper, two kinds of shielded phases are discussed: bonded micellar phases and embedded polymeric phases. For some compounds of interest, the hydrophobic retention and selectivity is insufficient to obtain the desired resolution. To increase selectivity without affecting protein exclusion, shielded phases were prepared with ion-exchange groups added to the hydrophobic zones. Such modified phases with cation-exchange and anion-exchange capabilities were examined for additional selectivities. Usage of these additional selectivities will be demonstrated to achieve better analysis and resolution for basic, acidic, and neutral compounds.