Analytical aspects of organic compounds retention indices dependence in reversed phase HPLC on the content of methanol in eluent

As a result of the determination of retention indices (RI) of 11 organic compounds of various polarity in reversed phase (RP) HPLC using methanol as organic solvent in elu­ent, the coefficients of the dependence of RI vs. content of methanol, dRI/dС were de­ter­mi­ned. Formally, these parameters are equivalent to the temperature coefficients of gas chromatographic retention indices, b = dRI/dT. It is shown that the ranges of variations of these coefficients are rather large (from -2.0 up to +4.6 in the current work), which requires re­ve­­a­ling the factors influencing their signs and absolute values. The correlations of coeffi­ci­­ents dRI/dC with retention indices themselves, as well as with hydrophobicity factors, logP (the logarithms of partition coefficients in the system 1-octanol/water) seem to be negligible. The best correlations of dRI/dC were observed with such polarity mea­su­res of organic compounds as homologous increments of hydrophobicity factors (ilogP), and re­ten­tion indices (iRI). Such transformations of pro­perties A as their homologous increments characterize different homologues despite of their locations within the homologous series. Thus, the parameters ilogP and iRI appeared to be the measures of the polarity of the corres­pon­­ding series. This approach per­mits us to reveal that the dependence of retention indices in RP HPLC on the content of me­tha­nol in eluent is determined just by the analytes¢ po­la­rity and their hydrophobic – hydrophilic proper­ties. The ratio dRI/dC > 0 is typi­cal for non-polar analytes, while for polar compo­unds the negative signs of these coeffi­ci­ents are typical, dRI/dC < 0. Hence, deter­mining even the sign of this coefficient provi­des the important information on the chemical origin of analytes.