Pyrolysis- and chemical degradation-GC/MS analyses of environmental kerogen and humic substances and their applications to geochemistry

Abstract

A brown complex or refractory organic materials, which are often called humic substances and kerogen, are ubiquitous in aquatic and terrestrial environments. They play an important role in carbon and material cycles in the environment. Although a clear understanding of their molecular nature is essential, some knowledge of their chemical structure and the mechanism by which they form and change in time on Earth is still limited. The molecular characterization of these materials is difficult due to their insolubility and complex macromolecular nature. From this review, we highlight the following three types of chemical degradation techniques, which are becoming popular in the molecular characterization of these organic materials: i.e. pyrolysis gas chromatography-mass spectrometry (py-GCMS), pyrolysis in the presence of tetramethylammonium hydroxide coupled to gas chromatography-mass spectrometry (py-TMAH-GCMS), and gas chromatography-mass spectrometry after bond-selective chemical degradation (chemical degradation-GCMS). Both py-GCMS and py-TMAH-GCMS are often used as small-scale analytical methods. py-GCMS is commonly suitable for materials (e.g. kerogen in rocks) that produce non-polar or less-polar materials (e.g. hydrocarbons) on pyrolysis, while this technique may not be suitable for young humic substances, because they tend to produce polar materials by a thermal reaction of heteroatomic (such as oxygen, nitrogen) functional groups. py-TMAH-GCMS overcomes some of the weak points of py-GCMS. Since this technique is based on the cleavage of labile C-O bonds, such as ester, amide and ether bonds with simultaneous methylation (thermochemolysis), it can be applied for characterizing macromolecular organic matter, which is on the way of change from biomacromolecules (e.g. lignin, proteins, carbohydrates and lipids). Modern sequential chemical degradation-GCMS techniques involving the successive cleavage of different types of bonds (S-C bond, ether bond, double bonds and sub-units linked to aromatic moieties etc.) in refractory organic materials are described. © 2007 The Japan Society for Analytical Chemistry.