Pyrolysis methylation—mass spectrometry of whole soils

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Abstract
Summary: In‐source pyrolysis field ionization mass spectrometry (Py‐FIMS) and Curie–point pyrolysis gas chromatography/mass spectrometry (Py‐GC/MS) have been applied to the characterization of whole soils methylated off–line and on–line by two methylation reagents. Samples of a gleysolic Ap–horizon and a podzolic Bh horizon (Corg concentrations 2–3%) were investigated by conventional on–line and developed off–line methylation with tetramethylammonium hydroxide (TMAH) and by off–line methylation with diazomethane. For the first method, the soils were dried, milled and pretreated with TMAH for 10 min at 250°C at ambient pressure outside the pyrolyzers. For the second method, the dried and milled soils were methylated with an ether solution of diazomethane for 12 h at room temperature (∼20°C). Pyrolysis methylation with TMAH enabled aliphatic C2–C39 monocarboxylic acid methyl esters, C4–C30 dicarboxylic acid dimethyl esters and benzenecarboxylic acid methyl esters to be detected. Methoxybenzenes from phenols, benzenediols and benzenetriols, methoxybenzenecarboxylic acid methyl esters from phenolic acids and furancarboxylic acid methyl esters from carbohydrates were also identified. Nitrogen–containing compounds in soil organic matter were obtained as N,N–dimethylamides. Using diazomethane as methylation reagent, distinct Py‐FIMS signals were observed for aliphatic C2–C32 monocarboxylic acid methyl esters and C3–C24 dicarboxylic acid dimethyl esters. Additionally, methoxybenzenes originating from lignins, methoxybenzenecarboxylic acid methyl esters from phenolic acids and N,N–dimethylamides from amides were detected. The more acid podzolic Bh horizon showed higher relative intensities for dicarboxylic acid dimethyl esters and methylated phenolic acids compared to the gleysolic Ap horizon. Similarly, benzenecarboxylic acids are connected mainly by ester linkages to the macromolecular network of soil organic matter. Both methylation procedures support conventional Py‐FIMS and Py‐GC/MS and give valuable additional information on the occurrence of aliphatic and aromatic carboxylic acids, substituted phenols, benzenediols, benzenetriols, phenolic acids and amides in soil organic matter.