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 east ern United States (from Georges Bank and the Mid-Atlantic Eight) is charact erized by means of direct temperature-resolved mass spectrometry (DT-MS). C hemical ionization (NH3) appears to be a very appropriate ionization method for this material. The analytical data confirm the presence of a significa nt polysaccharide fraction consisting of neutral sugars (normal-, deoxy- an d methylated aldoses), N-acetyl aminosugars and, possibly, acidic sugars. T he methylated and N-acetyl aminosugars are considered as markers for a bact erial contribution. No evidence is found for polysaccharides with large hom opolymeric domains, suggesting an origin distinct from the common structura l polysaccharides that comprise major polymers in algae and higher plants. The TIC profiles, which express the number of ions produced as a function o f the temperature, often show a bimodal distribution. This indicates that t he colloidal matter consists of two fractions with different thermal stabil ities and probably also different chemical stabilities. Multivariate "mappi ng" of the DT-MS spectra reveals that the chemical differences between thes e two fractions show remarkable conformity amongst samples collected at wid espread locations. The relative abundance of the two fractions appears to d epend on both depth and location. The more thermally stable fraction is rel atively abundant in deep water samples and some surface waters of the Mid-A tlantic Eight. (C) 1998 Elsevier Science Ltd. All rights reserved.