| Humic acids are a group of natural organic macromolecules and ubiquitous in the environment. They play an important role in global carbon cycle, affect the distribution, fate, transport, and bioavailability of organic and inorganic pollutants. In addition, they are involved in the mineral composition of soil and the improvements of the soil fertility. All these behaviors are related to their structural composition. Therefore, it has a great significance to characterize the molecular structures of humic acids.The structural characterizations of aqueous humic acid from Pahokee peat was conducted by Electrospray Ionization Mass Spectrometry (ESI-MS). This method avoided the limitation of other analytical methods for solid samples. The verification of side alkyl substitutes bonding to aromatic rings (Calkyl-Carom) in five humic acids, which are two peat humic acids (PHAI and PHAII), two soil humic acids (SHAI and SHAII) and a commercial humic acid (AHA) were characterized using Ruthenium Ion Catalyzed Oxidation (RICO) method. The aromatic structures of humic acids were also discussed in this study.The ion peaks distributed continuously with the mass charge ratios, with most ion fragments below m/z 1500. The two atomic mass unit difference of the odd ion peaks in the low mass charge ratios resulted from neutral mass lose of complex molecular. The strength of ion peaks was characterized by "wave pattern" maxima atm/z 250, 350, 500 and 700, representing that the dimeric, trimeric and tetrameric structures of lignin degraded products. Based on ESI-MS/MS mechanism of 17 model compounds of lignin monomers, the structures of ion fragments between m/z 70 and 200 was deduced. M/z 70-110 were the "core structure" of Pahokee humic acid, while m/z 110-200 were mainly the monomer of p-hydroxy phenols, vanillyl phenols, syringyl phenols and their derivations. So, the parent source were gymnosperm and angiosperm.n-Alkanoic acids were the major components in the RICO products of humic acids, with obviously odd/even predominance. The highest abundances of four natural humic acids were the n-C16 acid, whereas n-C26 acid for commercial sample AHA. Carbon numbers of monocarboxylic acids range from C7 to C34, consequently the carbon numbers of side alkyl chain bonding to aromatic rings were C6 to C33. The distributions of n-alkanoic acid for PHAI, SHAI, and SHAII were characterized by bimodal peak, whereas AHA was characterized by unimodal distribution with long chain alkanoic acids (>C20). Alkyl-a, ω-dicarboxylic acids were only detected in sample PHAI. Most of n-alkanoic acids ranging from C14 to C18 were bonding to aromatic rings (Calkyl-Carom) as side alkyl substitutes, whereas few of them are occurred as free fatty acid. Hopane acid methyl esters were firstly founded in humic acids, which ranged from C28 to C31 with exception for C29. Hopane acid methyl esters had only R configuration, i.e., organism configuration, suggesting that the maturity of humic acids was low. Thirteen kinds of aromatic carboxylic acids (1-6 carboxyl groups) were also detected in these humic acids. Aromatic tetracarboxylic acid and pentacarboxylic acid were relatively high in four of five humic acids, showing that more polycyclic aromatic hydrocarbons such as anthracene, phenanthrene, triphenylene existed. Relatively high contents of aromatic hexacarboxylic acid implied that more complex PAH such as triphenylene, dibenxopyrene, coronene and perylene existed in humic acids. |
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