High temperature liquid chromatography and liquid chromatography-mass spectroscopy analysis of octylphenol ethoxylates on different stationary phases

Temperature was investigated as active parameter in the liquid chromatography (LC) analysis of octylphenol ethoxylates. Significant differences in selectivity were observed when the oligomers were analyzed by reversed phase LC (RPLC) on silica-, zirconia- and polystyrene/divinylbenzene based stationary phases at low (ambient), medium and elevated temperature with acetonitrile/water as mobile phase. As ascertained by LC-mass spectroscopy (MS), in most cases the elution order of the oligomers was completely reversed comparing ambient and high temperature separations. On a graphitized carbon type column, the selectivity remained unchanged, regardless the analysis temperature. Also in normal phase LC, the elution order remained unaffected by temperature variations both for acetonitrile/water and methanol/water mixtures as mobile phase. Surprisingly, when reversed phase LC on a octadecylsilicagel column at different temperatures was repeated with methanol instead of acetonitrile as mobile phase ingredient, the reversal of elution order did not take place. Results are evaluated in terms of thermodynamic parameters.