This annual review of X-ray fluorescence covers developments over the period 1999–2000 in instrumentation and detectors, matrix correction and spectrum analysis software, X-ray optics and microfluorescence, synchrotron XRF, TXRF, portable XRF and on-line applications as assessed from the published literature. The review also includes a survey of applications, covering sample preparation, geological, environmental, archaeological, forensic, biological, clinical, thin films, chemical state and speciation studies. During the current review period, further advances have taken place in the development of high resolution semiconductor detectors and in the design and application of XRF instrumentation for space and planetary research. Overall, the papers reviewed here again confirm the important contribution the XRF technique makes to a wide range of scientific endeavour.

X-ray fluorescence spectrometry

The performances of a number of commercially available components which together can be employed to assemble a compact micro-XRF spectrometer were compared. The purpose of such a spectrometer is to permit local, non-destructive analysis of various materials; its compact nature will allow it to be used on-site, i.e. at the location where the investigated objects/materials are present rather than in a laboratory. The performances of two polycapillary lenses (of different manufacture) and one monocapillary tube were compared in combination with a mini-focus x-ray tube with Mo anode. The use of an Si-drift chamber and an Si PIN-diode detector was investigated and the achievable minimum detection limits for trace elements in glass and silver matrices were determined. Finally, the effect of using mini-focus tubes (with 250 Âµm focus) as opposed to the use of a more conventional x-ray source (1 mm focus) on the obtained beam size was investigated. Copyright Â© 2001 John Wiley & Sons, Ltd.

Component selection for a compact micro-XRF spectrometer

The active chemical state of zinc (Zn) in a zinc-copper (Zn-Cu) catalyst during carbon dioxide/carbon monoxide (CO2/CO) hydrogenation has been debated to be Zn oxide (ZnO) nanoparticles, metallic Zn, or a Zn-Cu surface alloy. We used x-ray photoelectron spectroscopy at 180 to 500 millibar to probe the nature of Zn and reaction intermediates during CO2/CO hydrogenation over Zn/ZnO/Cu(211), where the temperature is sufficiently high for the reaction to rapidly turn over, thus creating an almost adsorbate-free surface. Tuning of the grazing incidence angle makes it possible to achieve either surface or bulk sensitivity. Hydrogenation of CO2 gives preference to ZnO in the form of clusters or nanoparticles, whereas in pure CO a surface Zn-Cu alloy becomes more prominent. The results reveal a specific role of CO in the formation of the Zn-Cu surface alloy as an active phase that facilitates efficient CO2 methanol synthesis. Description Zinc’s state in methanol synthesis Methanol can be synthesized from carbon monoxide (CO), carbon dioxide (CO2), and molecular hydrogen (H2) over copper–zinc (Cu–Zn) catalysts, but studies have disagreed about the chemical state of Zn. Although x-ray photoelectron spectroscopy (XPS) can determine its oxidation state, many studies have been limited to reaction pressures of a few millibars, where the rates are low. Amann et al. performed XPS at 180 to 500 millibars for CO2 and CO hydrogenation over a Zn/ZnO/Cu(211) surface at high turnover rates. Stoichiometric mixtures of CO2 and H2 formed ZnO, but for CO and H2, Zn became more metallic and formed Cu alloys. In industrial synthesis, CO2 and H2 are mixed with CO, and the presence of CO would generate Cu–Zn alloy sites active for CO2 reduction to methanol. —PDS CO induces Zn–Cu surface alloy sites that are active for CO2 hydrogenation in catalytic methanol synthesis.

https://www.science.org/doi/pdf/10.1126/science.abj7747?download=true

The state of zinc in methanol synthesis over a Zn/ZnO/Cu(211) model catalyst

Polycapillary halflenses are widely used to focus X-ray radiation onto a small spot. Additionally they can reduce the field of view of a semiconductor detector when placed in front of one. In 3D micro X-ray fluorescence spectroscopy (3D Micro-XRF) with synchrotron radiation, two polycapillary halflenses are used in a confocal geometry. Up until now, characterization measurements in the focal plane have only been performed in the case of the lens focusing parallel radiation. Assumptions have been made, that in the other case, when isotropic radiation from a spot source is transported to a detector, the acceptance distribution in the focal plane is also Gaussian. We performed measurements with an electron beam as well as a proton beam which confirm this assumption. In addition, a comparison between measurements in collecting and focusing mode show differences in spot size and transmission. These differences exemplify the fact that there is not one global spot size or transmission function of a polycapillary halflens. Illumination and divergence effects can alter both characteristic lens parameters.

Performance of a polycapillary halflens as focussing and collecting optic—a comparison

Heterogeneous catalyst surfaces are dynamic entities that respond rapidly to changes in their local gas environment, and the dynamics of the response is a decisive factor for the catalysts' action and activity. Few probes are able to map catalyst structure and local gas environment simultaneously under reaction conditions at the timescales of the dynamic changes. Here we use the CO oxidation reaction and a Pd(100) model catalyst to demonstrate how such studies can be performed by time-resolved ambient pressure photoelectron spectroscopy. Central elements of the method are cyclic gas pulsing and software-based event-averaging by image recognition of spectral features. A key finding is that at 3.2 mbar total pressure a metallic, predominantly CO-covered metallic surface turns highly active for a few seconds once the O2:CO ratio becomes high enough to lift the CO poisoning effect before mass transport limitations triggers formation of a √5 oxide.

/pdf/stroboscopic-operando-spectroscopy-of-the-dynamics-in-3vb6zv1i2v.pdf

Stroboscopic operando spectroscopy of the dynamics in heterogeneous catalysis by event-averaging

Polycapillary x-ray optics are gaining increased attention as their transmission characteristics and manufacturing difficulties are better understood. This paper reports experimental results obtained with an optic that was designed for use with both Mo Kα (17.5 keV) and Cu Kα (8 keV). The spectral gain function from 4 to 30 keV was determined from measurements made with an Si(Li) detector, and the results were compared with Monte Carlo simulations. The spatial variation in the spectral transmission was examined with both an Si(Li) detector and a magnifying x-ray CCD camera. The results show that even though the individual capillaries are tapered, their performance is well described by a simple parabolic relationship between the radius of curvature and the maximum energy transmitted derived for capillaries of constant diameter. Copyright © 1999 John Wiley & Sons, Ltd.

Jerome Gormley

Papers

Polycapillary x-ray optic spectral gain and transmission

Grazing incidence X-ray photoemission spectroscopy of SiO2 on Si