Using thermal slicing ramped pyrolysis gas chromatography mass spectrometry to investigate composition and structure of natural organic matter

Natural organic matter (NOM) in aquatic systems represents one of Earth’s largest reservoirs of reduced carbon, yet its molecular structure remains poorly understood. We introduced a novel technique, thermal slicing ramped pyrolysis gas chromatography mass spectrometry (TSRP-GC-MS), coupled with computing, to analyze the thermal lability and molecular structure of NOM. This method identifies and categorizes pyrolyzates produced at low temperatures (<370°C) based on their functional groups. The release patterns of pyrolyzates are integrated and transformed into activation energy based on distributed activation energy model (DAEM) to offer insights into how organic compounds are assembled within the structure. For instance, we quantified the bonding energy of n-alkanes in asphaltenes from light sweet crude oil, showing an increase from ~108 kJ/mol in native asphaltenes to ~140 kJ/mol in photo-irradiated samples, indicating tighter binding in the latter, which may partially explain their resistance to biodegradation. This approach can be extended to create energy matrices for different chemical classes in any NOM samples. For marine sinking particles, the release energy ranges from ca. 120 kJ/mol for acetic-, aldehyde-, and sterol-like structures, to ca. 160 kJ/mol or higher for alkane- and alkene-like classes. These energies shift as NOM lability changes. Overall, TSRP-GC-MS offers a powerful tool to quantify the binding strength of different chemical classes in NOM, providing novel insights into its structure and stability.

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Primary Presenter: Kaijun Lu, Coastal Carolina University (klu@coastal.edu)

Authors:

Jianhong Xue, The University of Texas at Austin Marine Science Institute  (jxue@utexas.edu)

Zhanfei Liu, The University of Texas at Austin Marine Science Institute  (zhanfei.liu@utexas.edu)