Salt-mediated retention of proteins in hydrophobic-interaction chromatography. Application of solvophobic theory

Retention behavior in hydrophobic-interaction chromatography was examined within the framework of the solvophobic theory. The principal parameters which determine the effect of salt on the retention are salt molality and the molal surface tension increment of the salt. According to the theory, in the absence of special binding effects, increase in salt molality in the mobile phase or change of salt to one of greater molal surface tension increment will result in increased retention of proteins in hydrophobic chromatography. The theory is expanded to treat retention in gradient elution with linear decrease in salt concentration that is equivalent to linear increase in eluent strength. The results of the simple model lead to an expression with two parameters: the adjusted isocratic retention volume of the eluite with the gradient former and the slope of plot of logarithmic adjusted elution volume against salt molality, lambda. The latter parameter is linearly dependent on molal surface tension increment if no specific interactions between the eluite and the stationary phase and/or salt are present. In practice, deviations are to be expected from the predicted behavior due to such effects. The results of calculations are consistent with experimental results obtained with several proteins as the eluites and various salts in the eluent. Although unique values of the critical parameter lambda could not be obtained from the data, the trends showed that lambda is strongly correlated with the value of the molal surface tension increment. The prediction that increase in salt concentration in the initial eluent leads to increase in retention volume was found to be generally true, even when the isocratic retention volumes obtained with use of eluent having low salt concentration were small. Use of NaClO4 in the starting eluent led in some cases to decrease in retention volume with increase in the salt concentration at the beginning of the gradient elution. This effect may be due to specific binding effects.