Aromaticity index with improved estimation of carboxyl group contribution for biogeochemical studies

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...

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Bibliographic Details
Published in:Environmental Science & Technology
Main Authors: Zherebker, A., Rukhovich, G.D., Sarycheva, A., Lechtenfeld, Oliver, Nikolaev, E.N.
Format: Article in Journal/Newspaper
Language:English
Published: American Chemical Society (ACS), Washington, DC 2022
Subjects:
NOM
humic substances
FTICR MS
carboxylic groups
aromaticity index
deuteromethylation
isotopic labeling
Blackwater River
permafrost
Online Access:https://www.ufz.de/index.php?en=20939&ufzPublicationIdentifier=25706
https://doi.org/10.1021/acs.est.1c04575
Description
Summary: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.

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