Investigation of chromatographic behaviour of ethoxylated alcohol surfactants in normal-phase and reversed-phase systems using high-performance liquid chromatography mass spectrometry

Retention behaviour of ethoxylated alcohols was investigated on octadecyl silica, unmodified silica and aminosilica columns. Good separation according to the alcoholic alkyl length is achieved in reversed-phase systems. The elution order of the oligomers with different numbers of oxyethylene groups depends on the type of the organic solvent (methanol or acetonitrile) and on its concentration in the mobile phase. Different retention behaviour of lower and higher oligomers was observed. The distribution of the oligomers according to the number of oxyethylene units is suppressed, but it overlaps with the peak distribution according to the alkyl length in mobile phases containing high concentrations of acetonitrile. In normal-phase systems, the alkyl length affects the retention much less than the number of oxyethylene units. Better separation than on unmodified silica gel columns can be achieved on a chemically bonded aminosilica column in 2-propanol-n-hexane and especially in acetonitrile-water-dichloromethane mobile phases. Possible retention mechanisms in the systems studied are discussed. The retention is strongly affected by solvation of the oxyethylene groups by the mobile phase. The retention factors of higher oligomers with bimodal mass distribution can be described using a simple equation. HPLC-MS with atmospheric pressure chemical ionization allows sensitive detection, easy identification and reconstruction of the chromatographic peaks in chromatograms of complex mixtures of oligomers with different numbers of methylene and oxyethylene groups. (C) 1998 Elsevier Science B.V. All rights reserved.