EXPERIMENTAL PROTOCOL FOR THE ASSESSMENT OF SOLVENT STRENGTH AND SELECTIVITY OF LIQUID-PHASES USED IN GAS-CHROMATOGRAPHY

A thermodynamic approach to the practical characterization of solvent strength and selectivity of a wide range of molecular and ionic stationary phases for gas chromatography is discussed. The experimenal protocol is outlined in detail and the principal sources of error with their magnitude identified. The solvent strength parameter (the partial molar Gibbs free energy of solution for a methylene group per unit solvent volume) is proposed as a universal parameter for measuring solvent strength although it is indicated that in the general ranking of liquid phases anomalous positioning of ionic phases and perfluorocarbon phases may occur. The partial molal Gibbs free energy of solution for selected test solutes is proposed as a scale of solvent selectivity. Nitrobenzene and n-octanol are demonstrated to be acceptable test solutes to characterize solvent orientation and proton acceptor capacity, respectively. Benzodioxan is shown to be an unacceptable test solute for assessing solvent proton donor capacity and dibutylformamide is tentatively identified as a promising replacement. Tabulation of the above constants provides new insight into the solvent properties of the most common liquid phases used in gas chromatography. In particular, it is demonstrated that the liquid organic salts provide enhanced orientation and proton acceptor capacity compared to non-ionic liquids and represent an important class of new solvents for extending the selectivity range of solvents used in gas chromatography. A need for new phases with strong proton donor capacity is also identified. © 1990.