X-Ray Microprobe and Spectroscopy

X-ray Microprobe

X-ray fluorescence (XRF) allows the non-destructive identification of the elemental composition of materials, especially for heavy elements. With the powerful X-ray source of an APS undulator, intense X-ray beams around 1 micron in size can be used for collecting XRF data from heterogeneous samples such as soils, minerals, rock sections, and plant leaves and roots. These XRF measurements are fast enough that samples can be rastered back and forth through the focused X-ray beam to build up spatial maps of the elements with micron-level resolution. These compositional maps allow the study of partitioning of trace metals in a variety of samples, including toxic elements in wetlands, the transport of nutrients in soils and roots, diffusion of metals through volcanic magmas, pigments in paintings and other cultural artifacts, and the compositions of micro-meteorites and lunar samples.

At the X-ray microprobe beamline at 13-ID-E, we can tune the X-ray energy to select which heavy elements (above sulfur) we excite for X-ray fluorescence. Generally, we can detect trace elements down to the parts-per-million level, though that can vary (in both directions) depending on sample composition.

For some samples, we can also measure the X-ray diffraction of a sample at the same time as we measure the X-ray fluorescence to give spatial images of both the elemental composition and major crystalline phases of a sample.

X-ray absorption spectroscopy

X-ray absorption fine structure (XAFS) probes the chemical and local atomic environment of a selected element by scanning the X-ray energy across the binding energy of a core electron (or “absorption edge”) of that selected element. X-ray absorption near-edge spectra (XANES) is very sensitive to the valence state and coordination environment of the element. The extended XAFS oscillations well past the absorption edge can be used to give quantitative information about the species, coordination number, and distance to the neighboring atoms of the selected element.

At the X-ray microprobe beamline at 13-ID-E, we can make XAFS measurements with very high spatial resolution. Coupled with XRF maps, this allows not only identifying the elemental composition of samples at micron resolution, but also identifying the valence and chemical state of metals and other heavy elements at both high abundance and trace levels, down to a few ppm.

The combination of XRF mapping and micron-scale XANES allows us to study the chemical state of inclusions trapped in glasses that preserve the chemistry of volcanic magmas, determine the oxygen levels of samples from the deep earth, samples returned from moon, or samples from other planets in micro-meteorites. It also allows the determination of chemical state of metals used by plants, marine organisms, and so on.