Confocal micro X-ray fluorescence analysis for the non-destructive investigation of structured and inhomogeneous samples

Abstract

Confocal micro X-ray fluorescence (CMXRF) spectroscopy is a non-destructive, depth-resolved, and element-specific technique that is used to analyze the elemental composition of a sample. For this, a focused beam of mono- or polychromatic X-rays is applied to excite the atoms in the sample, causing them to emit fluorescence radiation which is detected with focusing capillary optics. The confocal design of the instrument allows for depth-resolved analysis, in most cases with a resolution in the lower micrometer dimension after collecting X-rays from a predefined volume within the sample. The element-specific nature of the technique allows information to be obtained about the presence and concentration of specific elements in this volume. This makes CMXRF spectroscopy a valuable tool for a wide range of applications, especially when samples with an inhomogeneous distribution of elements and a relatively light matrix have to be analyzed, which are typical examples in materials science, geology, and biology. The technique is also commonly used in the art and archaeology fields to analyze the elemental composition of historical artifacts and works of art, helping to provide valuable insights into their provenance, composition, and making. Recent technical developments to increase sensitivity and efforts to improve quantification in three-dimensional samples will encourage wider use of this method across a multitude of fields of application in the near future. Confocal micro X-ray fluorescence (CMXRF) is based on the confocal overlap of two polycapillary lens foci, creating a depth-sensitive and non-destructive probing volume. Three-dimensional resolved element distribution images can be obtained by measuring the fluorescence intensity as function of the three-dimensional position.

Keywords: Confocal; Confocal micro X-ray fluorescence analysis; Depth profile; Non-destructive; Three-dimensional; XRF.

Fig. 1

Schematic build-up of a MXRF and a CMXRF spectrometer (tabletop unit) for spatially resolved analysis. Focusing of X-rays by a polycapillary full lens down to a spot for surface analysis (left). Confocal overlapping of the foci of two X-ray optics creating a depth-sensitive probing volume (right) and sample movement by x-y-z motorized sample stage. Adapted from reference [27] with permission from the Royal Society of Chemistry

Fig. 2

Schematic illustration of the challenges of quantification exemplified by layered samples. Hereby, the influence of, e.g., absorption effects (a) and atomic number Z (b) on the measured depth profiles is demonstrated. Those variables besides others need to be considered in the context of quantification of sample depth and element concentration

Fig. 3

Depth scans (y-axis: weight percentage) of three different car paints. (a) Five-layer paint, (b) two-layer structure imbedded in lead-white matrix, and (c) four-layer paint. Adapted from reference [24] with permission from Elsevier

Fig. 4

(a) Parchment sheet with analysis area “strawberry” (dashed white outline) on front side and “flower” (white outline) on the back side. (b) Different views of the composite 3D distribution with Hg Lα (red), Cu Kα (green), Au Lα (blue), and Pb Lα (yellow) and stretched depth axis by factor 10. Adapted from reference [26] with permission from the Royal Society of Chemistry

Fig. 5

A spongin-based porous microfibrous scaffold of Hippospongia communis bath sponge, after being placed in a model ammoniacal CuCl₂ solution, was covered with a layer of green crystalline material (10× magnification (a), 100× magnification (b)), which has been identified in this study as atacamite [68]. (c) 3D distribution, (d) the side, and (e) top view of copper and bromine distribution in the spongin–atacamite composite, measured in a part of the sample with dimensions of 1.5 × 0.5 × 0.5 mm³