Refining resolution settings for analysis of dissolved organic matter in varied natural environments by Fourier-transform ion cyclotron resonance mass spectrometry

Dissolved organic matter (DOM) is a major carbon reservoir and exhibits high chemo-diversity and similarity. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) enables analysis of DOM due to its ultrahigh resolution at various field strengths. Capturing distinguishing features of DOM is especially challenging for lower resolution instruments. Here, we aim to refine resolution settings for various types of DOM. With a low-field 7 Tesla (T) FT-ICR MS, two strategies for tuning resolution were compared with free induction decay (FID) of 1–4 s: the initial mass to charge (m/z) ratio (A) and data size (B). Peak number rises then falls with data size; 16 M leads to loss of low-mass compounds (< 220 m/z). In further, the comparability of intensity-weighted average parameters was evaluated, revealing that m/z, carbon number, H/C, O/C, aromatic index, and double bond equivalent have a coefficient of variation (CV) of < 3%; in contrast, the average number of heteroatoms—P (45%), N (21%), and S (22%)—shows considerable CV (%) with resolution, varying across samples. Furthermore, the minimum required value of resolution varies across samples, ranging from > 300,000 to > 500,000: it increases from riverine water to porewater, and then to seawater DOM, typically exhibiting abundant CHO, CHOS, and CHOP, respectively. For a 7T FT-ICR MS, we propose tailored FID strategies: a 2-s medium FID (resolution > 300,000) for regular DOM, a shorter FID (~ 1 s) for small metabolites with low initial m/z (~ 50–100), and a longer FID (resolution > 500,000) for heteroatom-enriched DOM.