Acidification impacts on the molecular composition of dissolved organic matter revealed by FT-ICR MS

Solid-phase extraction (SPE) is a traditional pretreatment procedure widely used for dissolved organic matter (DOM) desalination and enrichment prior to the Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis, and the extracts are usually acidified to pH = 2 with hydrochloric acid (HCl) before passing through the cartridge. However, little is known about the effects of acidification on the integrity and molecular composition of DOM. Here, the differences in the molecular compositions in acidified and nonacidified DOM samples of soil, seawater and atmospheric aerosol were performed by FT-ICR MS. The results showed that the quantity and intensity of aromatic compounds with high oxygen content (e.g., polyphenols, tannin-like and highly oxygenated organic compounds) were greatly enhanced after acidification, while highly saturated compounds (lipid-like and aliphatic/peptide-like) were absent. The underlying reason is the variation of solubility and hydrolysis of DOM under acidic conditions. Meanwhile, the effect of acidification on the molecular composition of DOM was also dependent on their original environmental media. Based on these results, we suggest that the extracts of soil samples are selectively acidified according to the focus of research, while the extract is acidified for seawater samples and the pH of the extract can be unadjusted for aerosol samples before the SPE procedure. These findings provide a reference for the selection of suitable pretreatment methods for different experimental purposes and for the comprehensive characterization of samples with different properties.