Survey of organic molten salt phases for gas chromatography

Abstract

The organic molten salts tetra- n-hexylammonium benzoate, tetra- n-heptylammonium chloride, 1-methyl-3-ethylimidazolium chloride, ethylpyridinium bromide, tri- n-butylbenzylphosphonium chloride, tetra- n-butylammonium tetrafluoroborate, and sodium isovalerate are evaluated as stationary phases for gas chromatography. They all behave as stable isotropic liquids over a temperature range of at least 50°C, and in most cases, greater than 100°C. As a series these salts provide a range of liquid phases spanning temperatures from room temperature to ca. 290°C. The principal solute-retaining forces are orientation and proton donor interactions, the magnitude of which depends on the identity of the salt. The physical and chemical properties of the organic molten salts are compatible with their use in gas chromatography to separate a wide range of sample types including alcohols, halocarbon compounds, substituted benzene and naphthalene derivatives, and essential oils. Calcium nitrate tetrahydrate is also evaluated as an example of a hydrated melt. Excessive retention of polar solutes and poor peak shape make its general use impractical.