On the determination of underivatised fatty alcohol ethoxylates by electrospray ionisation-mass spectrometry.

The oligomers of fatty alcohol ethoxylates (FAEs) exhibit large sensitivity differences in mass spectrometry with electrospray ionisation (ESI-MS) and atmospheric pressure chemical ionization (APCI). Standards of the oligomers from m=1 to 7 ethylene oxide units (EOs) and linear alkyl chains from n=10 to 18 carbon atoms were infused to examine the relative sensitivities or response factors in several media. The response factors of the [M+H]+ and [M+Na]+ peaks in 9:1 acetonitrile/water and methanol/water media containing acid buffers increased following irregular patterns when n and m increased. In methanol/water the response factors depended on the parity of m, being larger than the average trend for the oligomers with an even value of m with respect to those having an odd value. This was attributed to the presence of an uncompensated C-O-C or C-O-H dipole in the former oligomers. The advantages of using ESI over APCI and of measuring the [M+H]+ peaks in an acid methanol/water medium containing 0.1 M HCl are discussed. The advantages and limitations of using models of the response factors to evaluate oligomer concentrations with a reduced set of selected standards are examined. The determination of underivatised FAEs using acid media was made compatible with previous HPLC separation by implementing either a triconcentric nebulizer fed with an acid liquid sheath, or a capillary T-union inserted between the column outlet and the biconcentric nebulizer, and fed with an acid stream provided by a syringe pump.