In-Air Micro-PIXE Mapping with Chemical Contrast.

Currently, chemically specific X-ray fluorescence imaging is restricted to synchrotron facilities and based on the chemical selectivity of X-ray absorption spectra. In this article, we demonstrate the capability of high-energy-resolution micro-PIXE spectroscopy to perform two-dimensional (2D) chemical state mapping of sulfur and phosphorus within spatially inhomogeneous model samples. The approach is based on the parallel-beam wavelength-dispersive tender X-ray emission spectrometer combining polycapillary X-ray optics with diffraction on a flat crystal analyzer, which is used to record chemically sensitive Kβ X-ray emission. This was used to maximize the fluorescence signal of one chemical species vs the other and record 2D maps with pronounced chemical contrast. The ratio of intensities recorded at two preselected X-ray emission energies was used as a unique spectral signature of the particular chemical state to yield pure chemical state maps with high lateral resolution provided by the focused proton beam. The presented approach is not restricted to micro-PIXE but is also applicable to other micro X-ray fluorescence imaging techniques commonly applied in various research fields.