Detecting Structural Environments of Carboxyl Groups in Dissolved Natural Organic Molecules

Jianshu Duan, Ethan J. Sontarp, Satish C.B. Myneni

Research output: Contribution to journalArticlepeer-review


The carboxyl group is one of the most abundant and important functional groups in natural organic molecules (NOM). Its structural environment determines its reactivity and stability and has been examined previously using Fourier-transform infrared spectroscopy (FTIR), specifically the asymmetric stretch of the deprotonated carboxylate group. In this study, we provide a comparative analysis of the carbonyl stretch of the protonated carboxyl group (ν(C═O)carboxyl) and show that it is useful for the detection of carboxyl structural environments where ambiguity exists. When compared to model carboxylic acids, the ν(C═O)carboxyl frequency of dissolved NOM from various locations and climatic conditions shows a narrow distribution in 1720-1732 cm-1. The majority of carboxyls in NOM are α-substituted (hydroxyl, ether, ester, ketone, carboxyl, etc.) aliphatic carboxylic acids, agreeing with previous studies. Additionally, the ν(C═O)carboxyl frequency, as suggested here, provides a greater distinction between aliphatic and aromatic carboxylic acids and is better suited for peptide-rich biological samples. Environmental water samples often contain high salt concentrations that interfere with the detection of carboxyls. Solid-phase extraction using PPL cartridges is an efficient desalting method and can be applied to organic-poor salt-rich samples for proper FTIR analysis.

Original languageEnglish (US)
Pages (from-to)555-563
Number of pages9
JournalACS ES and T Water
Issue number2
StatePublished - Feb 9 2024

All Science Journal Classification (ASJC) codes

  • Chemistry (miscellaneous)
  • Chemical Engineering (miscellaneous)
  • Environmental Chemistry
  • Water Science and Technology


  • Fourier-transform infrared spectroscopy
  • carboxyls
  • dissolved organic matter
  • solid-phase extraction
  • structural environment


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