Features of the Dependence of the Retention Indices of Sorbates in Reversed-Phase High-Performance Liquid Chromatography on the Content of Organic Solvents in the Eluent

Characterizing the dependence of the retention indices (RIs) of sorbates of different chemical origins on the content (C) of organic solvents (methanol, acetonitrile) in eluents used in reversed-phase HPLC shows that the coefficients dRI/dC of the same compounds for eluents of different compositions differ considerably. The range of variations in coefficients dRI/dC for water-methanol eluents (-5.6 to +4.6) is approximately twice that of water-acetonitrile eluents (-4.5 to +0.8). It is also found that for nonpolar compounds when using water-acetonitrile eluents, the dRI/dC values are typically lower than those for water-methanol eluents. The opposite is observed for polar compounds. The signs and absolute values of coefficients dRI/dC are largely determined by the polarity of the sorbates. dRI/dC > 0 for nonpolar sorbates, and dRI/dC < 0 for sorbates of higher polarity. Determination even of the sign of this coefficient therefore provides important information about the chemical nature of sorbates. It is shown that coefficients dRI/dC do not correlate with either the RI values or hydrophobicity log P. However, they do correlate with homologous increments of hydrophobicity i(logP) and the homologous increments of RI i(RI) for eluents of any composition. Quantities i(logP) and i(RI) characterize the polarity of homologous series of sorbates, rather than the polarity of the particular sorbates.