Nitrogen K-edge XANES – an overview of reference compounds used to identify `unknown' organic nitrogen in environmental samples

The chemical nature of soil organic nitrogen (N) is still poorly understood and one-third to one-half of it is typically classified as `unknown N'. Nitrogen K-edge XANES spectroscopy has been used to develop a systematic overview on spectral features of all major N functions in soil and environmental samples. The absolute calibration of the photon energy was completed using the 1s → π* transitions of pure gas-phase N<sub>2</sub>. On this basis a library of spectral features is provided for mineral N, nitro N, amino acids, peptides, and substituted pyrroles, pyridines, imidazoles, pyrazoles, pyrazines, pyrimidines and purine bases. Although N XANES was previously considered `non-destructive', effects of radiation damage were shown for two compound classes and an approach was proposed to minimize it. This new evidence is integrated into a proposal for the evaluation spectra from environmental samples with unknown composition. Thus a basis is laid to develop N K-edge XANES as a complementary standard research method to study the molecular composition and ecological functions of `unknown N' in soil and the environment.