Characterization of Aquatic Particles by Direct FTIR Analysis of Filters and Quantification of Elemental and Molecular Compositions

This paper presents the first characterization of aquatic particles and particulate organic matter (POM) by attenuated total reflectance infrared spectroscopy (ATR-FTIR) using particles deposited on filters. Particles from 30 water samples from the St. Lawrence System (Canada) were analyzed ATR-FTIR spectra revealed changes in numerous organic and inorganic functional group contents. Particles from marine waters contained POM enriched in amide, N-H, and aliphatic groups, while terrigenous POM had more COO-/COOH and aromatic groups. The spectra showed the selective degradation of amide, N-H, aliphatic, and carbohydrate-like structures during the sinking of the particles. Partial least-squares (PLS) regression of the ATR-FTIR spectra was used to quantify 12 important elemental and molecular parameters, such as amino acids, bacterial biomarkers, and degradation indices, Most parameters were quantified with good accuracy compared to conventional methods (<15% error). The spectral regions leading to the best quantifications and the PLS loadings revealed that aromatic cycles, other unsaturated structures, and COO-/COOH groups were degraded at a much slower rate than N-molecules, such as amino acids, and carbohydrates. Marine POM was enriched in CH3 groups. CH3 groups appeared highly labile and abundant in bacterial POM. ATR-FTIR represents a new and powerful method for a rapid, inexpensive, and nondestructive characterization of particles collected by filtration revealing important biogeochemical processes involving POM.