ESI-MS analysis of aquatic humic substances and model compounds for improved interpretation of humic mass spectra
Twelve different aquatic humic samples, were collected and analyzed for DOC and AOX content and ESI/ToF spectra were acquired in positive and negative ion modes. For comparison, three different types of model compounds, fatty acids, carbohydrate, and lignin were also analyzed by ESI/ToF. The fatty acids indicated that under the instrument conditions used, there was little fragmentation with the spectra dominated by quasimolecular ions. Sodium adducts, non-covalently bound complexes and multiply charged species were also observed. Chlorinated fatty acids were also analyzed and produced spectra very similar to the non-chlorinated species. Both types of fatty acids produced higher ion intensity in ESI- than ESI+. The carbohydrate studied (guar gum) produced spectra dominated by fragments with peak spacings characteristic of the hexose subunits. Again, sodium adducts and multiply charged species were observed. The carbohydrate produced much higher ion intensity in ESI- than ESI+. The lignin sample studied produced similar spectra in both ion modes, with peaks at every nominal m/z unit and the odd values at higher intensity than the evens. In ESI-, there were peaks between the nominal m/z values at ~0.33, 0.5 and 0.66 Da, indicating the presence of multiply charged species. The aquatic humic substances produced very complex spectra, similar to those already published in the literature. Comparison to the model compounds indicated the presence of a lignin signature, and characteristics of the carbohydrate spectra. Analysis of the mass defects proved very useful, allowing for more detailed comparisons between spectra. This indicated that the lignin signature observed was likely due to degraded rather than intact lignin, and that any fatty acids present were likely bound to a lignin or carbohydrate derived species. The mass defect plots allowed for correlation between the characteristics of the spectra and the hydrology of the sampling sites. The samples could be distinguished by site and season. Wetland and ground waters were found to vary the least with season, while river waters varied the most. ESI-MS along with detailed data analysis were found to be very useful techniques for analysis of aquatic humic substances.