Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya, 464-8601, Japan.
Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, VA, 23529, USA.
Rapid Commun Mass Spectrom. 2020 Aug 15;34(15):e8801. doi: 10.1002/rcm.8801.
Soil fulvic acids (FAs) are considered to be a highly reactive pool of soil organic matter. The functions of FAs are related to their chemical structures, the details of which are largely unidentified. To better understand them, Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) must be a useful but generally unused tool.
The structural properties of the components of five FA samples from a variety of soils were determined using FTICR-MS with negative-mode electrospray ionization. The peaks were assigned to molecular formulae, which were categorized into seven compound groups based on the H/C-O/C van Krevelen diagram. Ramp C cross polarization/magic angle spinning nuclear magnetic resonance (NMR) spectra with phase-adjusted spinning side bands were also recorded to estimate the C composition.
From FTICR-MS, molecular formulae were assigned to 1746-2605 peaks across the m/z range of 200-700. Those aligned in the lignin-like, tannin-like, and condensed aromatic regions of the van Krevelen diagram accounted for 49-58%, 4-20%, and 18-39% of the total peak magnitude, respectively. The proportion of the summed peak magnitudes that were detected in the lignin-like and condensed aromatic regions correlated positively to the aromatic C% as estimated by C NMR. From Kendrick mass defect analysis using a carboxyl group, 94 molecular formulae were assigned to condensed aromatic acids, of which the maximum ring number was 4-7, as potential structures.
A high proportion of lignin-like formulae and condensed aromatics, including those probably condensed aromatic acids with small ring numbers, as well as the existence of tannin-like formulae, which were generally lacking in soil humic acids, was suggested as a common feature of soil FAs.
土壤富里酸（FA）被认为是土壤有机质中反应活性较高的一个部分。FA 的功能与其化学结构有关，而其化学结构的细节在很大程度上尚未确定。为了更好地了解它们，傅里叶变换离子回旋共振质谱（FTICR-MS）必须是一种有用但通常未被使用的工具。
使用带有负模式电喷雾电离的 FTICR-MS 测定了来自各种土壤的五个 FA 样品的成分的结构特性。将峰分配给分子公式，根据 H/C-O/C 范克里夫伦图将这些分子公式分为七类化合物组。还记录了斜坡 C 交叉极化/魔角旋转核磁共振（NMR）谱，并带有相位调整的旋转边带，以估计 C 组成。
从 FTICR-MS 中，在 m/z 范围为 200-700 的 1746-2605 个峰上分配了分子公式。那些排列在范克里夫伦图的木质素样、单宁样和浓缩芳烃区域的峰，分别占总峰幅度的 49-58%、4-20%和 18-39%。通过 C NMR 估计，在木质素样和浓缩芳烃区域检测到的总和峰幅度的比例与芳香族 C%呈正相关。从使用羧基的肯德里克质量缺陷分析中，分配了 94 个分子公式给浓缩芳烃酸，其中最大环数为 4-7，作为可能的结构。
建议土壤 FA 的一个共同特征是存在大量的木质素样公式和浓缩芳烃，包括可能具有小环数的浓缩芳烃酸，以及通常缺乏于土壤腐殖酸中的单宁样公式。
