Solvatochromic Measurements of Mobile Phase Composition in Reversed Phase Liquid Chromatography: Polarity Values, Retention, and Stationary Phase Information

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Abstract
The ET(30) polarity values of binary methanol-water, acetonitrile-water and tetrahydrofuran-water mobile phases used in reversed phase liquid chromatography are reported, and equations are presented for the calculation of polarity values for any composition of these solvents. We summarize previously reported comparisons of these measured polarities with both chromatographic retention and methylene selectivity. Plots of log k' vs. ET(30) polarity were generally found to be better descriptors of retention than commonly used plots of log k' vs. percent organic modifier. For 332 data sets examined, the average r2 value for plotting log k' vs. percent organic modifier was 0.9783 while plotting vs. ET(30) polarity gave a value of 0.9910. Retention in acetonitrile-water mixtures is also compared with theory developed by Dill using binary interaction parameters. Variation in methylene selectivity was found to correlate best with percent organic modifier in methanol-water mixtures, while the ET(30) polarity provided the best correlation in acetonitrile-water mixtures. Solvatochromic solvent polarity measurements allow an independent examination of the effect of changing mobile phase polarity on chromatographic retention, and give useful information about the solvation structure of the stationary phase.