Pyrolysis and soft ionization mass spectrometry of aquatic/terrestrial humic substances and soils

The rapid, reproducible, chemical characterization of complex environmental materials such as plants, humic substances and whole soil can be performed by controlled thermal degradation. Except for drying and milling no pre-treatment of the samples is required. Biomacromolecular cleavage during a short degradation step directly in the ion source of a mass spectrometer results in the production of high-mass chemical subunits. Short reaction times and small amounts of sample favour the generation of large, thermal fragments, i.e., chemical building blocks, which can be identified and correlated with the structure of the polymeric biomaterials investigated. The principal aim is to monitor the primary, thermal fragmentation by high molecular ion intensities of the pyrolyzates and to avoid consecutive, mass spectrometric fragmentation as far as possible.For the detection and identification of the pyrolysis (Py) products, a combination with time-/temperature-controlled mass spectrometry (MS) is used. Typical heating rates are 0.2–10°C/s and the temperature range is 50–800°C. Soft ionization techniques such as field ionization (FI), field desorption, chemical ionization (CI) and, to some extent, fast atom bombardment are employed in the positive and negative modes. The results of direct Py-MS are supported by high-resolution mass measurements using electric or photographic detection and Curie-point pyrolysis in combination with gas chromatography-electron ionization/FI/CIMS and library searches for the identification of the pyrolysis products.Fingerprinting and time-resolved Py-MS of aquatic and terrestrial humic substances are reported. The methodology for the investigations of dynamic processes during the volatilization and thermal decomposition of these complex biomaterials is illustrated. Weight loss curves and the temperature function of accurate molecular weight averages for aquatic fulvic and humic acid are derived from the Py-FIMS data. Initial results on the differentiation of soil horizons in a moder profile by Py-FIMS and pattern recognition are presented. In particular, the chemometric evaluation appears promising for future Py-MS studies of humic substances and whole soils, but also for fossil fuels, synthetic polymers and food. In an integrated approach, the linking of conventional chemical and spectroscopic data with the high-mass signals in pyrolysis-mass spectra will be the focus of forthcoming work. Preliminary results for combining wet-chemical data with those of ¹³C nuclear magnetic resonance, Fourier transform infrared and electron spin resonance spectroscopy are put forward in this survey. Finally, initial results of pilot studies to detect biocides such as atrazine directly in soils using Py-FIMS are demonstrated.