Micellar proportion: a parameter to compare the hydrophobicity of the pseudostationary phases or that of the analytes in micellar electrokinetic chromatography

Micellar proportion, t(prop,mic) = t(mic)/t(m), a quantity expressing how much time is spent by the analyte in the micellar phase related to its whole migration time (t(m)) has been introduced by utilizing the micellar phase residence time (t(mic)). The t(prop,mic) values have been determined for analytes of different chemical structures (alkyl benzene and alkyl phenone homologous series, alcohols, strongly hydrophobic peptides) studied by micellar elektrokinetic chromatography (MEKC) using various cationic and anionic pseudostationary phases. A good linear correlation was obtained between t(prop,mic) and the calculated hydrophobicity (CLOGP) of the analytes for all pseudostationary phases (CLOGP = A.logt(prop,mic) + B). Considering a given pseudostationary phase, t(prop,mic) as a relative quantity is a suitable parameter to characterize and compare experimentally the behavior of the various analytes in MEKC. Applying a set of probe molecules with known hydrophobicity, the CLOGP(50) value (showing the value of hydrophobicity of a virtual molecule spending exactly 50% of its migration time in the pseudostationary phase) has been calculated for each pseudostationary phase applied here. This experimentally determinable numerical value (characterizing the pseudostationary phase) can be utilized to compare the hydrophobicity and hence retention ability of the pseudostationary phases. The t(prop,mic) value was found to be applicable to compare the methylene selectivity of the different pseudostationary phases as well: logt(prop,mic) = A.Z + B, where Z is the number of carbon atoms of the alkyl chain in the alkyl benzene homologous series.