Molecular characterization of microgram amounts of oceanic colloidal organic matter by direct temperature-resolved ammonia chemical ionization mass spectrometry

Marine colloidal organic matter in coastal waters off the coast of the eastern United States (from Georges Bank and the Mid-Atlantic Bight) is characterized by means of direct temperatureresolved mass spectrometry (DT-MS). Chemical ionization (NH₃) appears to be a very appropriate ionization method for this material. The analytical data confirm the presence of a signIficant polysaccharide fraction consisting of neutral sugars (normal-, deoxy- and methylated aldoses), N-acetyl aminosugars and, possibly, acidic sugars. The methylated and N-acetyl aminosugars are considered as markers for a bacterial contribution. No evidence is found for polysaccharides with large homopolymeric domains, suggesting an origin distinct from the common structural polysaccharides that comprise major polymers in algae and higher plants. The TIC profiles, which express the number of ions produced as a function of the temperature, often show a bimodal distribution. This indicates that the colloidal matter consists of two fractions with different thermal stabilities and probably also different chemical stabilities. Multivariate "mapping" of the DT-MS spectra reveals that the chemical differences between these two fractions show remarkable conformity amongst samples collected at widespread locations. The relative abundance of the two fractions appears to depend on both depth and location. The more thermally stable fraction is relatively abundant in deep water samples and some surface waters of the Mid-Atlantic Bight.