Structure of “unknown” soil nitrogen investigated by analytical pyrolysis

Abstract

Curie-point pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and in-source pyrolysis-field ionization mass spectrometry (Py-FIMS) were applied for the first time to the structural characterization of organic nitrogen in hydrolyzates and hydrolysis residues resulting from the classical 6M HCl hydrolysis of mineral soils. Two well-described soils of widely different origin (i.e., a Gleysol Ah and a Podzol Bh) were investigated. Py-GC/MS was performed using a nitrogen-selective detector to detect and identify N-containing pyrolysis in the hydrolyzate (e.g., pyrazole and/or imidazole, N,N-dimethylmethanamine, benzenacetonitrile, propane- and propenenitriles) and the hydrolysis residue (e.g., pyrroles, pyridines, indoles, N-derivatives of benzene, benzothiazol, and long-chain aliphatic nitriles). Moreover, temperature-resolved Py-FIMS allowed us to record the thermal evolution of the N-containing compounds during pyrolysis. These were characterized by a particularly high thermostability compared to their thermal release from whole soils. The combination of pyrolysis with mass spectrometric methods permitted analyses of the identities and thermal stabilities of complex nitrogen compounds in hydrolysis residues of whole soils, which cannot be done by wet-chemical methods. Pyrolysis-methylation GC/MS with tetramethyl-ammonium hydroxide (TMAH) as methylating agent enabled the identification of N,N-dimethylbenzenamine and so confirmed the identification of benzeneamine by Py-GC/MS in nonmethylated hydrolysis residues. The described N-derivatives of benzene and long-chain nitriles are usually not detectable by pyrolysis-mass spectrometry of plants and microorganisms. These compounds are characteristic of soils, terrestrial humic substances and hydrolysis residues and seem to be specific, stable transformation products of soil nitrogen.