Separation of very hydrophobic analytes by MEKC. II. Carbon number equivalents as analyte descriptors--influence of the composition of the separation electrolyte.

The use of the parameter carbon number equivalent, N(c)(), as analyte descriptor for neutral hydrophobic solutes in EKC is investigated. The carbon number equivalent of an analyte is defined analogously to the retention index, RI, however, it is directly calculated from the regression line plotting for the members of a homologous series the logarithm of the retention factor against the number of methylene units N(c) in the alkyl chain. This plot can be obtained by using simultaneously an iteration procedure for the calculation of t(MC). For separation electrolytes containing SDS as micellar phase employing ACN, urea, and CaCl(2) as additives within a wide span of concentrations the parameter N(c)() is quasi-independent of the composition of the separation electrolyte, being of importance concerning identification of analytes and improving repeatability and reproducibility. With known octanol-water partition coefficients for the members of the homologous series taken as reference it is possible to transform the conventional time scale of an electropherogram into a scale of N(c)() or a scale of log P(OW). Due to the quasi-independence of N(c)() from the composition of the separation electrolyte these transformed electropherograms can be used as normalized electropherograms for identification purposes. It is also shown that the linear dependence of log P(OW) on N(c) can be the basis of the determination of octanol-water partition coefficients of solutes within a very large range (-4 < or = log P(OW) < or = 9).