oa Acidification-Driven Molecular Fractionation of Dissolved Organic Matter During Direct Analysis of Fresh Water Using Uplc-Orbitrap Ms

Dissolved organic matter (DOM) is a highly complex mixture of natural organic molecules. Ultrahigh resolution mass spectrometry coupled with liquid chromatography (LC) is an emerging tool for chemical analysis. Acidification pretreatment, whether through solid-phase extraction (SPE) or direct injection, significantly affects the distribution of assigned molecular formulas in DOM. However, the fractionation dynamics of DOM molecules in response to pH gradients remain largely unexplored. This study investigates the effects of acidification on DOM at molecular level in freshwater across pH gradients of non-acidified, 5, 3, 2, and 1. The results show that acidification-induced molecular formulas undergo a transition from aliphatic to aromatic species as pH values decreases. Condensed aromatic-like compounds (mainly nitrogen-containing) were exclusively detected when the pH value drops to 1. We find that, compared to the SPE-DOM, DOM molecules in water at the optimized acidification degree (pH 1) exhibit high sensitivity to oxygenated and condensed aromatic-like compounds, suggesting that terrigenous-like signature could be largely overlooked in previous routine analysis. The method developed in this study has great applications in terms of our understanding of the sources and fate of terrigenous DOM in lakes, estuaries, and even offshore regions.