Humic substances studies and new approaches to analysis of small molecules by MALDI-TOFMS

Abstract

Humic substances (HS) are complex natural materials that are thought to be produced by decomposition of biogenic matter. HS are known to be involved in various processes such as amending soil fertility, metal chelation and sequestration of organic contaminants; they also are precursors of harmful water disinfection byproducts (DBPs). To understand the role of HS in all these processes it is imperative to understand their structural features. A MALDI-TOF MS method has been developed to characterize these materials and the results showed clear evidence of the oligomeric nature of HS. However, the inherent complexity of HS demands a multi pronged approach, thus thermally assisted hydrolysis-methylation-GC-MS was employed to aid in elucidating possible structural constituents of HS. Thermochemolysis results demonstrated a strong similarity between natural organic matter standard and humic acid model synthesized from polymerization of 4-oxo-2-butenoic acid, derived from oxidation of furfural, a well known product of dehydration of polysaccharides. These results suggest the concept that polysaccharides are important precursors in HS formation. -- Studies of DBPs resulting from chlorination and chloramination of HS were also carried out. Based on our understanding of key structural features of HS and rigorous mass spectral analysis, haloketones were found to be one of the major classes of DBPs formed and numerous other potentially new DBPs were also identified. Their formation was replicated using selected plant flavanoids as model compounds for HS. Most of the resulting DBPs were polar, thermally labile and of high molecular weight and hence not amenable to analysis by the standard technique, GC-MS. To circumvent these problems, we have developed a new, rapid, selective, quantitative and very sensitive (detection limits in the sub parts per billion range) "reactive matrix"-LDI MS method fix the analysis of carbonyl and α-dicarbonyl compounds. These techniques can be extended to the analysis of carbonyl compounds of importance in other fields: e.g., metabolomics, clinical diagnostics, food science, etc. Finally with good understanding of the structural properties of HS obtained from our studies, aquatic fulvic acid has been demonstrated as a novel universal MALDI matrix.