Supercritical fluid chromatography-mass spectrometry of non-ionic surfactant materials using chloride-attachment negative ion chemical ionization

Abstract

Capillary supercritical fluid chromatography interfaced with chemical ionization mass spectrometry (SFC-CI-MS) provides high-efficiency separations and identification for characterization of alcohol ethoxylate and other non-ionic surfactant mixtures. When using the capillary direct interface for SFC-CI-MS, however, the role of the chromatographic mobile phase during ionization must be considered. The presence of carbon dioxide or nitrous oxide in the chemical ionization source of a Hewlett-Packard 5985 gas chromatography-mass spectrometry system configured for SFC-CI-MS significantly influences the appearance of positive and negative ion chemical ionization mass spectra. Positive and negative ion mass spectra produced using methane, isobutane or ammonia all display extensive fragmentation and adduct ion formation in the presence of carbon dioxide or nitrous oxide. Surfactant sample fragmentation can be limited or eliminated through the use of difluorodichloromethane (Freon-12) as the reagent gas for chloride-attachment negative ion chemical ionization. Only the chloride-attachment adduct anion is produced for alkoxylated surfactants and free fatty acids, even with a two-fold excess of carbon dioxide present in the ion source. With CI conditions that permit the production of only molecular species, effects due to restrictor-heater temperature and interface design and operation could be evaluated. A comparison of direct supercritical fluid injection versus heated probe inlet introduction for surfactant samples was also possible. Chloride-attachment negative ion CI-MS following capillary SFC may be the preferred system for obtaining molecular weight and telomer distribution information on commercial alcohol alkoxylates and other non-ionic surfacant mixtures.