Characterization of organic matter in an interlayer clay-organic complex from soil by pyrolysis methylation-mass spectrometry

Abstract

The composition of organic matter in clay-organic complexes isolated from a New Zealand Spodosol has been investigated using the novel analytical combination of (a) labscale (500 mg) off-line pyrolysis, (b) direct inlet (5 mg) off-line methylation with tetramethylammonium hydroxide (TMAH) and pyrolysis-field ionization mass spectrometry (Py-FIMS), and (c) Curie-point pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) with off-line and conventional on-line derivatisation with TMAH. In complex I the organic matter was associated with both external and interlayer surfaces of a swelling clay. Complex II was obtained by removing the external portion with H 2O 2 leaving the interlayer organic matter essentially intact. For both complexes, a high proportion of the carbon (64–86%) and nitrogen (76–91%) was volatilized during pyrolysis. Analysis of the pyrolysates showed mono- and dicarboxylic acids, alkanes, alkenes, n-alkylmonoesters, and N-containing compounds to be major constituents. Thermal methylation of the material, prior to (off-line) and during (on-line) pyrolysis gave rise to a wide range (between C 6 and C 35) of methyl esters of aliphatic and aromatic acids, methoxy derivatives of phenols and phenolic acids. In accord with previous 13C-nuclear magnetic resonance (NMR) spectroscopic measurements, the organic matter in both complexes was highly aliphatic in composition. Although there was little difference between complex I and complex II in the composition of the aliphatic constituents, complex II was greatly depleted in lignin-derived aromatics. In addition, the interlayer material in complex II had a relatively low thermal stability.