Fluorinated bonded stationary phases in micellar liquid chromatography

The usefulness of fluorinated bonded stationary phases in micellar liquid chromatography (MLC) is examined. Different selectivities and higher efficiencies were observed using a fluoro-octyl (FO) column as compared to that of a C18 column for both non-ionic and ionic compounds in MLC. Shortened analysis times for amino acids and small peptides and increased retention for the early eluting sulfonamides were observed on the FO column. The unique phenomenon of the simultaneous enhancement of solvent strength and selectivity that often occurs in the MLC systems with the alkyl-bonded stationary phases was also observed for the FO column. This is due to the existence of the competing partitioning equilibria in MLC and because of the interactive nature of the two eluent parameters, micelle concentration and the volume fraction of organic modifiers, both of which influence solvent strength and selectivity. Consequently, simultaneous optimization of these two parameters is the most effective strategy for the MLC systems with the fluorinated bonded stationary phases. Subsequently, the iterative regression strategy was utilized to optimize these two mobile phase parameters for a group of amino acids and small peptides. Excellent agreement was obtained between the observed optimum chromatogram and the one predicted by the iterative regression strategy using only five initial experiments. The successful application of the iterative regression optimization procedure indicates that the retention pattern in MLC with fluorinated bonded stationary phases is easily predictable. This is a similar behavior to that using hydrocarbonaceous phases and can be attributed to the linear variations in retention with the two mobile phase parameters and to the highly reproducible retention behavior in MLC.