Full-Field MXRF
Figure 5 : Schematic diagrams of a 1:1 capillary optic and a 1:x optic. Reprinted with permission from Oliver Scharf.
elemental fluorescence lines (comparable to different colors in the light optical regime).
Full-field X-ray microscopy is more restricted than X-ray absorption microscopy regarding the optics set between the array detector and the sample to be imaged [ 7 ]. This is because the optical setup needs to be fairly achromatic to guide photons of different energy accurately, which is necessary in MXRF. Pinholes have been successfully used [ 8 , 9 ]. However, polycapillary optics allow for very high transmission of X rays of various energies. These optics may be used as guiding optics, achieving the resolution given by the array detector ( Figure 5a ), or they may project an enlarged image on the detector ( Figure 5b ). For example, the spatial resolution achieved with the SLcam®
is about
50 µm using a 1:1 optic, but it can be improved by enlarg- ing the image to 1:5 and even 1:8 [ 4 ]. By using an algorithm it is possible to achieve sub-pixel resolution, better than 5 µm [ 10 ].
Results
Drying of aqueous drops . A major advantage of full-fi eld XRF imaging over scanning MXRF is rapid recognition of the major features in elemental distributions. T e fi eld of view is usually large, for example, 12×12 mm [ 4 ], and therefore an overview of the sample and “non-targeted” results are obtained. Full-fi eld observation is also favorable for imaging objects in situ , especially in environments where liquids are involved or when the specimen must remain static. Imaging of droplets while drying requires a non-destructive probe operating under ambient conditions. T e experiment shown in Figure 2 could not have been accomplished in an electron microscope that typically must place the specimen under vacuum. T at experiment also had a temporal aspect. While XRF images were acquired every 15 seconds, only a selection of images taken over the 27-min drying process is displayed in Figure 2 . Simultanous imaging of spatially separated areas can also be realized using a full-fi eld setup, allowing the observation of process changes over time. Time-resolved measurements, however, are limited by the acquisition rate and count rate [ 11 ].
Ancient Phoenician object . A Phoenician ivory (eighth- century BCE) from the Badisches Landesmuseum, Karlsruhe, Germany, was examined by full-fi eld XRF imaging using synchrotron radiation, a straight polycapillary optic ( Figure 5a ), and the SLcam ®
energy-dispersive camera/detector by Reiche
et al. [ 12 ]. T is non-destructive analysis provided distributions of the major, minor, and trace elments on the surface of the carved object ( Figure 6 ). Of the major elements in the global
2015 May •
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spectrum from the front surface of the object, Ca and Sr are known to be from the ivory, Cu is likely a pigment that once decorated part of the design, and Fe could be either from a pigment outling the design or picked up from the burial sediments. T ese assumptions about the elements in the object were largely derived from the elemental images produced by the SLcam ®
. T e assumptions about the Fe distribution were gleaned from the
manner in which some Fe deposits were located in the deep crevices of the carving, whereas other Fe deposits appear to follow the the surface cracks. T ese elemental results help to produce a hypothesis concerning the colors employed in the original ancient artwork. Shadowing effects . A static position of the sample is indispensable for diagnostics in total reflection X-ray fluorescence (TXRF) analysis. The term TXRF decribes a certain geometry in XRF elemental analysis that allows for trace element determination in minute amounts of a sample. In TXRF the excitation beam inpinges at a very small angle (in the range of 0.1°) onto the sample carrier surface. The shadowing of parts of the sample by rough surface features is an interference in TXRF, and better understanding of shadowing would improve the method significantly. Imaging of shadings in the TXRF geometry was possible using a full-field micro-XRF setup [ 6 ]. Figure 7 shows shadows in a Cu fluorescence image of a copper plate caused by roughness and particles as the plate was illuminated in total reflection excitation geometry from the bottom of the figure. The Cu image was captured using a color X-ray camera.
Figure 6 : Analysis of an ancient Phoenician carved ivory object. Elemental images overlay of Ca K-alpha image (ivory), Cu K-alpha image (blue), Pb L-alpha image (yellow), Ti K-alpha image (purple), and Fe K-alpha image (red). Full width = 13 mm. Reprinted from [12] with permission from the American Chemical Society.
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