Recent advances and applications in the analysis of organic C and N using soft X-ray XANES

Abstract:

Determining the chemical speciation of carbon and nitrogen in environmental samples is important for understanding the role of organic matter in contamination mobility and nutrient cycling. Despite these important applications, bulk soft X-ray absorption spectroscopy, particularly at the carbon K-edge has not been extensively applied to environmental samples until recently. The primary reasons for this gap is the lack of beamline endstations that are suitable for ‘dirty’ samples and the technical challenges related to acquiring and normalizing spectra from dilute samples.

X-ray absorption spectroscopy (XAS) at the C K-edge probes the local bonding environment of C and N. Bulk XAS techniques permit for high throughput, the study of whole soils and high sampling density. These analyses are complementary to X-ray transmission microscopy (STXM) techniques which are limited by low throughput, thin particles (<100 nm) and low sampling density. In many projects, these bulk XAS measurements may be essential to understanding large scale processes in soils such as the global C cycle.

Technical challenges have been largely overcome through the use of customized energy selective silicon drift detectors which enable the carbon signal to be detected separately from the signals from higher order light such as oxygen (i.e., partial fluorescence yield). Accurate normalization is now possible using the X-ray scattering signal from Au coated Si wafers as the Io. The radiation dose was minimized using a monochromator slew scanning mode in conjunction with the development of software tools to automatically sample of multiple pristine spots on a sample. Technical developments and recent applications will be presented, showing how bulk C and N XAS is now positioned to contribute significantly to advancing the characterization of organic matter in soils and environmental samples.