Department of Analytical Chemistry, Helmholtz Centre for Environmental Research (UFZ), 04318 Leipzig, Germany.
Institute of Analytical Chemistry, University of Leipzig, 04103 Leipzig, Germany.
Anal Chem. 2021 Jan 26;93(3):1740-1748. doi: 10.1021/acs.analchem.0c04426. Epub 2020 Dec 28.
Natural organic matter (NOM) is a highly complex mixture of natural organic molecules. The recent developments in NOM molecular characterization methods have shown that ESI-FT-ICR hyphenated with liquid chromatography (LC) is a promising approach to also obtain chemical information (such as polarity and molecular size) about NOM molecules. However, due to changing solvent composition during gradient elution in LC-FT-ICR-MS, ionization conditions also change throughout the chromatographic separation process. In this study, we applied a post-LC column counter gradient (CG) to ensure stable solvent conditions for transient ESI-MS signals. Suwanee River Fulvic Acid (SRFA) standard and a peat pore water were used as representative dissolved NOM samples for method development and validation. Our results show that in polar NOM fractions (which elute with <50% methanol) the TIC intensity and number of assigned molecular formulas were increased by 48% and 20%, as compared to the standard gradient (SG) method. Further application of a Q-isolation and selective ion accumulation for low abundance fractions revealed over 3 times more molecular formulas (especially for CHNO, CHOS, CHNOS formula classes) than in full scan mode. The number of detected highly polar NOM compounds (with elemental ratios H/C < 1, O/C > 0.6) were more than 20 times larger for CG-LC mode as compared to direct infusion (DI) (5715 vs 266 MF). We conclude that the application of a postcolumn counter gradient in LC-FT-ICR-MS analyses of NOM offers novel insight into the most polar fractions of NOM which are inaccessible in conventional DI measurements.
天然有机物 (NOM) 是一种高度复杂的天然有机分子混合物。最近在 NOM 分子特征描述方法方面的发展表明，ESI-FT-ICR 与液相色谱 (LC) 联用是一种很有前途的方法，也可以获得 NOM 分子的化学信息（如极性和分子大小）。然而，由于 LC-FT-ICR-MS 中梯度洗脱过程中溶剂组成的变化，离子化条件也会在整个色谱分离过程中发生变化。在这项研究中，我们应用了 LC 柱后反梯度 (CG)，以确保瞬态 ESI-MS 信号的稳定溶剂条件。苏瓦尼河腐殖酸 (SRFA) 标准品和泥炭孔隙水被用作代表性的溶解 NOM 样品，用于方法开发和验证。我们的结果表明，与标准梯度 (SG) 方法相比，在极性 NOM 馏分（<50%甲醇洗脱）中，总离子流强度和分配分子公式的数量分别增加了 48%和 20%。进一步应用 Q 分离和选择性离子累积对低丰度馏分进行分析，揭示了比全扫描模式多 3 倍以上的分子公式（特别是对于 CHNO、CHOS、CHNOS 分子式类别）。与直接进样 (DI) 相比，CG-LC 模式下检测到的高极性 NOM 化合物（H/C<1，O/C>0.6）数量多 20 倍以上（5715 比 266 MF）。我们得出结论，在 NOM 的 LC-FT-ICR-MS 分析中应用柱后反梯度提供了对传统 DI 测量无法获得的 NOM 最极性馏分的新见解。
