Possibilities of determination and prediction of solute capacity factors in reversed-phase systems with pure water as the mobile phase

Abstract

A sorption and an elution experimental method were developed and tested for the determination of solute capacity factors in pure water as the mobile phase in reversed-phase systems. The experimental values determined by the sorption method are generally higher than those determined by the elution method, but the two types of values are strongly correlated. These correlations may be used for corrections of the experimental capacity factors to suit either chromatographic or sorption conditions. The log k′ dependencies on methanol concentration are adequately described by quadratic equations for various pesticides tested over a range of mobile phase compositions from 2 to 90% methanol in water, but the extrapolation to zero methanol concentration yields systematically lower extrapolated capacity factors, k′ H 2O . The extrapolated values depend strongly on the range of the experimental k′ and on the range of methanol concentrations used for the determination of the experimental data points, and neither linear nor quadratic extrapolation yield correct k′ H 2O values. The accuracy of predicted extrapolated k′ H 2O improves as the concentration range of methanol approaches zero, but the time of determination and experimental difficulties increase simultaneously. The accuracy of the predicted k' H 2O values from the low methanol concentration range can be improved using correlations with the k′ H 2O values determined experimentally. The correlation equations are determined for a set of (at least three) standard compounds and apply only for a given sorbent. Correlations of log k′ H 2O with solute lipophilic indices, n ce, yield better predictions than similar correlations with logarithms of solute solubilities, with comparable errors in the predicted k′ H 2O values as for the values extrapolated from the data measured in the range of medium methanol concentrations.