Calculation of modified capacity factor in micellar electrokinetic chromatography

In addition to the expression, k' = (t(m)-t(o))/t(o) (1-t(m)/t(mc)), we propose the expression k'' = (t(m)-t(o))/(t(mc)-t(o)) to calculate the capacity factor in micellar electrokinetic chromatography (MEKC), where t(m), t(o), and t(mc) are the migration time of the analyte, the flow marker, and the micelles, respectively. The k' and k'' values that were obtained from simulated data as well as from MEKC analysis of different peptides (in 100 mM sodium dodecyl sulfate/0.1 N sodium berate buffer at pH 11.0) were calculated and compared. The k' value is equal to zero for an analyte remaining in the aqueous phase whereas it is equal to one for an analyte always staying in the micellar phase. By applying k'' a finite capacity factor can be obtained for an analyte, indicating its partition between the two moving phases (aqueous and micellar) even in those cases when t(m) equals t(mc). The slope of the curve k'' as a function of t(m) is constant through the whole migration window and therefore peak compression does not occur when applying k'' to calculate the capacity factor. A given difference in k'' corresponds the same difference in migration times and this value does not depend on the position within the migration window. Since k' is a normalized parameter it is easy to evaluate the significance of a given difference in capacity factor or to estimate the relative position of an analyte with a given capacity factor in the migration window by applying k''. Therefore, k'' seems to be an adequate parameter to calculate the capacity factor in MEKC and, similar to k', it also refers to the hydrophobicity of the analyte.