Aromaticity Index with Improved Estimation of Carboxyl Group Contribution for Biogeochemical Studies

Abstract

Natural organic matter (NOM) components measured with ultrahigh-resolution mass spectrometry (UHRMS) are often assessed by molecular formula-based indices, particularly related to their aromaticity, which are further used as proxies to explain biogeochemical reactivity. An aromaticity index (AI) is calculated mostly with respect to carboxylic groups abundant in NOM. Here, we propose a new constrained AIcon based on the measured distribution of carboxylic groups among individual NOM components obtained by deuteromethylation and UHRMS. Applied to samples from diverse sources (coal, marine, peat, permafrost, blackwater river, and soil), the method revealed that the most probable number of carboxylic groups was two, which enabled to set a reference point n = 2 for carboxyl-accounted AIcon calculation. The examination of the proposed AIcon showed the smallest deviation to the experimentally determined index for all NOM samples under study as well as for individual natural compounds obtained from the Coconut database. In particular, AIcon performed better than AImod for all compound classes in which aromatic moieties are expected: aromatics, condensed aromatics, and unsaturated compounds. Therefore, AIcon referenced with two carboxyl groups is preferred over conventional AI and AImod for biogeochemical studies where the aromaticity of compounds is important to understand the transformations and fate of NOM compounds.

Effects of Isoflavone-Rich NADES Extract of Pueraria lobata Roots and Astaxanthin-Rich Phaffia rhodozyma Extract on Prostate Carcinogenesis in Rats

“…ISF was highly populated by aromatic species, which likely correspond to various polyphenols. ASX was mostly composed of lipids and unsaturated species, which, according to AI con [ 12 ], is unlikely to possess an aromatic ring, as expected for this sample.…”

“…The generated CHONS formulae were validated applying sensible chemical constraints for negative ESI: (O/C ratio ≤ 1, 0.3 < H/C ratio ≤ 2.2; element counts [C ≤ 120, H ≤ 200, 1 < O ≤ 60, N ≤ 5, S ≤ 1]; and mass accuracy window <1 ppm). Molecular components were further divided into six chemical classes [ 35 ] based on constrained aromaticity index AI con [ 12 ]: lipids (H/C ≥ 1.5, O/C < 0.3), aliphatics (H/C ≥ 1.5, O/C ≥ 0.3, N = 0), N-containing saturated compounds (H/C ≥ 1.5, O/C ≥ 0.3, N > 0), unsaturated (H/C < 1.5, AI con < 0.5), aromatics (0.5 ≤ AI con < 0.67) and condensed (AI con ≥ 0.67).…”

Probing Molecular-Level Dynamic Interactions of Dissolved Organic Matter with Iron Oxyhydroxide via a Coupled Microfluidic Reactor and an Online High-Resolution Mass Spectrometry System

“…Early eluting DOM molecules (segment 1) had higher O/C ratios and lower H/C ratios, indicating their more hydrophilic nature than late eluting DOM molecules. Based on empirical AI and H/C cutoffs, , DOM formulas were divided into condensed aromatic (AI con ≥ 0.67), aromatic (0.5 < AI con < 0.67), unsaturated/nonaromatic (AI con ≤ 0.5 and H/C < 1.5), and aliphatic (H/C > 1.5) (shown in Figure B), with their intensity-weighted abundance being 1.1, 29.3, 69.2, and 0.4%, respectively.…”

From mass to structure: Modified van Krevelen diagram and adjusted indexes for high‐resolution mass data of organic matter

“…Four methods were used for calculating the aromaticity index, namely AI, AImod, 5 AIcon 20 and Xc. 6 Of them, AI was suggested for compounds in which oxygen was mainly present as carbonyl functional groups; AImod was adjusted on the basis of AI when oxygen could also be bound with π-bonds in carboxyl groups 18 ; AIcon was proposed based on the measured distribution of carboxylic groups 20 ; and Xc was aimed at aromatic compounds with long alkylation. 6 Considering the contribution of halogen elements to the saturation of organic matter, we adjusted these indexes based on the original equations and put forward them as AI 0 , AImod 0 , AIcon 0 and Xc 0 , respectively.…”