Application of a spoked channel array to confocal X-ray fluorescence imaging and X-ray absorption spectroscopy of medieval stained glass

Confocal X-ray fluorescence imaging can determine the spatial distribution of elements in a depth-selective manner. The closely related technique of confocal X-ray absorption spectroscopy can reveal the chemical form of an element at a specific volume element in three dimensional space. Confocal measurement offers increased accuracy and avoids physical sectioning or ablative depth profiling methods, thus preserving sample integrity. At present, a polycapillary mounted on the detector is the common optical element used to complete the confocal configuration. Here we demonstrate implementation of a novel collection optic, a lithographically fabricated spoked channel array, to achieve the confocal configuration at beamline 20-ID, advanced photon source. The advantages of employing spoked channel arrays include improved spatial resolution and energy-independent probing volume. By employing this optic, three dimensional distributions of Fe, Ca and Mn and the localized chemical form of Fe were measured in the intact corrosion layer of an ancient stained glass sample obtained from 13th Century Paderborn Cathedral, Germany. Implementation of the optic enabled superior resolution depth selectivity in the intact corrosion layer and measurement of elemental distribution and speciation in a highly localized manner within the layer with improved signal to noise. The results show that both the composition and the chemical form vary as a function of three dimensional location in the corrosion layer. This observation is contrary to expectations from simple corrosion models assuming leaching processes and indicates that metal chemistry in these layers is quite complex.