Christine Vanhoof

This review covers developments in and applications of XRF techniques such as EDXRF, WDXRF, TXRF, XRF microscopy using technologies such as synchrotron sources, X-ray optics, X-ray tubes and detectors in laboratory, mobile and hand-held systems.

This review of 89 references covers advances in X-ray fluorescence spectrometry and its special applications, published from April 2024 to March 2025 inclusive. It provides critical insights into developments in instrumentation, methodologies and data handling, representing significant progress in XRF spectrometry. Applications of cultural heritage are also covered. Highlights of this review period include notable research findings. A method was developed to overcome self-absorption effects in confocal XRF spectrometry, enabling quantitative and distortion-free 3D elemental analysis by combining sample density information from μCT with mass attenuation coefficients from absorption measurements. Additionally, compositional data from fundamental parameter quantification of reference-free XRF spectrometry for ‘dark matrix’ elements like C, N and O in the soft X-ray region were incorporated. Using SR-XRF spectrometry, quantitative nano-characterisation of ion beam-implanted samples, particularly focusing on Ga dopants in silicon, was achieved. This method detected a minimum of 3000 Ga atoms per pixel with a 1 s integration (171 nm² spot) and 650 Ga atoms with a 25 s integration, corresponding to LODs of 18 impurities per nm² and 3.8 impurities per nm², respectively. The determination of elemental profiles in size-segregated airborne particulates with high time-dependent resolution (in 2 thicknesses (2.3 nm). We can expect a considerable increase of research in this area in the future. This year, reviews were published highlighting the advancements and applications of hand-held XRF spectrometry techniques over time, specifically for measuring bone lead and for determining the elemental composition of food samples. A comprehensive review provided an overview of synchrotron applications for cultural heritage over the past decade.

Three-dimensional chemical imaging by XRF spectrometry techniques continues to advance in both experimental methods and quantitative data evaluation and reconstruction strategies. These techniques are gaining interest across various research fields, ranging from material science and environmental and Earth sciences to life science and biomedical imaging. Two primary techniques associated with 3D XRF spectrometry are reviewed in this update: XRF spectrometry CT and confocal XRF spectrometry. There has been an increase in the building of in-house specialised 2D XRF spectrometry instruments. Attention to various components, e.g. coating of optics, has improved performance. There was an increase during the review period in the use of SR-XRF spectrometry in conjunction with complementary X-ray spectroscopic and imaging techniques for integrating spatially resolved elemental data with information on speciation and structural and morphological images. Applications of μXRF spectrometry continued to expand in fields such as biomedical, environmental and materials sciences and cultural heritage research. These applications were primarily carried out at specialised hard-X-ray micro- and nano-probe facilities by combining SR-XRF spectrometry with micro- and nano-XAS, XRD analysis, ptychography and various forms of tomographic techniques. The TXRF spectrometry technique continues to be successfully implemented in medical research because of its outstanding performance as a microanalytical method. Changes in the elemental profiles of small organs from, e.g. rats, can be detected. The introduction of a versatile pipetting instrument made possible significant advances in the strategic identification of errors in sample morphology. MacroXRF spectrometry continues to play a significant role in cultural heritage applications. Instrumentation is constantly expanding with new functionalities such as simultaneous measurement with reflectance image spectroscopy and luminescence imaging spectroscopy. The investigation of papyrus fragments was enhanced by upgrading a novel mobile macroXRF spectrometer scanner with new high-performing mechatronics and a high-throughput detection system.

The utilisation of SR sources has significantly enhanced the analytical capabilities of XRF spectrometry techniques. With the latest generation of facilities, SR-XRF spectrometry achieves remarkably high nm-scale resolution with excellent LODs at ppb levels. A noteworthy trend is the increasing use of SR-XRF spectrometry together with other X-ray spectroscopic and imaging techniques. This provides complementary information on elemental speciation as well as structural and morphological characteristics of samples. Sub-μm SR-XRF spectrometry has been extensively applied in diverse fields such as environmental and planetary studies, biomedical research, materials science and cultural heritage investigations. Methods for handling the huge datasets produced by macroXRF spectrometry have become essential for processing and classifying the element distributions collected from the analysis of paintings. Machine-learning-based correlations of element maps have been developed for the automatic identification of patterns as an alternative method of processing macroXRF spectrometry data from cultural heritage samples. The microanalytical capabilities of TXRF spectrometry have led to a steep increase in applications to biomedical problems with successful analyses of minute amounts of samples (ca. 20 mg) of, e.g., blood, placenta and heart tissue. The suspension-assisted preparation of theses samples and of mineralogical materials was improved in many studies by extending the common internal standard calibration with uni- and multivariate approaches. The development of a scan-free grazing-exit XRF spectrometer improved accuracy in the analysis of periodic surface structures. The degree of protonation of different thiol- or hydroxyl-bearing organic monolayers was successfully determined using grazing-incidence XRF and TXRF spectrometries in combination with other techniques.

A highlight this year was the construction of elemental maps of microalgae cells at the organelle level using XRF-CT with a beam size of just 15 nm. These probably represent the highest ever spatial resolution achieved for XRF-CT images. An innovative approach for reconstruction algorithms used a deep convolutional neural network to correct for the self-absorption effects in the XRF-CT sinogram domain. Such corrections will enable the use of conventional tomographic reconstruction algorithms for XRF-CT analysis. A remarkable advantage of the proposed method was that it could correct for self-absorption effects without any prior knowledge of the scanning setups. A particularly innovative dual-energy X-ray-beam ptycho-fluorescence imaging method combined XRF spectrometry and X-ray ptychography scanning with simultaneous data-collection. To overcome the incompatibility of the different excitation and scanning conditions required for fast ptychography and high spatial resolution XRF spectrometry, two coaxial beams of different sizes were used on the sample simultaneously. In comparison to sequential scans, this combined approach has the potential to reduce the data collection time by a factor of 25. The very good performance of laboratory 2D-XRF spectrometry for the analysis of minute amounts of radioactive material has made it a potential alternative to TXRF spectrometry. Reconstruction algorithms are now capable of significantly enhancing imaging resolution in μXRF spectrometry. The forensic classification of incredibly small amounts of samples such as single fibres was accomplished using elemental profiles determined by TXRF spectrometry. Although characterisation of the low-Z matrix in milk and polymers by conventional TXRF spectrometry is challenging, it was successfully achieved by studying the Compton and Rayleigh scatter with both univariate and multivariate approaches. Portable XRF spectrometry is a well-established technique for a wide range of applications. Emphasis continues to be placed on improving calibration and reducing matrix and inter-element effects. The sample type remains a challenge for portable XRF analysis. Computer vision and statistical methods such as spectral angle mapper are being increasingly applied to maximise the information that can be extracted from the data. For example, an innovative approach exploited the elemental features from XRF spectra to improve the interpretation of molecular information provided by reflectance spectroscopy, and vice versa.

That the boundaries of analysis using XRF spectrometry performed at SR sources continues to be pushed back is evidenced by the nanoscale spatial resolution with near single-atom sensitivity for the most efficiently detected elements in nanobeam applications and high speed 2D/3D imaging capabilities. The 4^th generation SR facilities currently under development, such as the ESRF Extremely Brilliant Source (EBS), will be at the forefront of these advances. A clear trend is that SR-XRF spectrometry is increasingly applied with complementary X-ray spectroscopic/imaging techniques to combine spatially resolved elemental information with speciation and structural/morphological imaging. The wide range of applications of scanning (sub)microXRF spectrometry for elemental imaging published in the period covered by this review included biomedical, environmental, materials science and cultural heritage studies. Most applications involved XAS and XRD methods at hard X-ray micro- and nano-probe facilities. Full field microXRF spectrometry has made very promising advances with respect to the optics used. Coded apertures have potential for overcoming the low count rates that often restrict the full potential of laboratory-based full-field setups. The availability of commercial full-field detectors will increase the user community and thereby foster advancement in full field microXRF spectrometry. The TXRF spectrometry of ambient air is becoming more and more sophisticated and the advantages of this micro analytical tool over ICP-MS in terms of short sampling times with high particle size resolution are becoming ever more apparent. For example, the optimal sampling time for aerosols for TXRF analysis was well below 12 hours, whereas that for ICP-MS analysis was about 24 hours. High-quality grazing incidence XRF analysis in the laboratory has become more feasible with the development of prototype TXRF instrumentation and the availability of commercial XRD setups with energy dispersive detectors. Portable XRF spectrometry has undergone significant technological improvements in recent years and is now applied in a wide range of applications. This is reflected in a significant number of valuable review papers dealing with different aspects of the portable XRF technique. The growth in the use of macroXRF scanning systems in cultural heritage investigations has required development of new software and methodologies for efficient handling of the huge data files generated. In several contributions the possibilities of a new scanning station equipped with real time macroXRF spectrometry was demonstrated.

The XRF-CT technique continues to gain attention with respect to both its development and applications. The emergence of fast scanning approaches in conjunction with ED-detectors of high counting rate capability has allowed XRF-CT to evolve towards a truly 3D elemental imaging technique. At SR sources, the technique is regularly applied at sub-μm resolution together with complementary imaging modalities such as absorption and phase-contrast CT, XRD-CT, 2D/3D XAS and ptychography. The TXRF spectrometric analysis of suspensions and undigested samples (e.g. clay, cosmetics and nanoparticles) continues to receive much attention. In the analysis of fly ash samples, elemental sensitivities were affected by the angle of the measurement. In the use of TXRF spectrometry for the analysis of particle-like large specimens, the influence of the standing wave field was considered to be negligible. However, more work needs to be done on this source of uncertainty to improve our understanding of the influence of the standing wave field on these kind of samples. Novel equipment capable of very good angle resolution will be useful for such studies. Pre-concentration procedures are becoming more sophisticated with the introduction of new substrates for ligands capable of binding heavy metals and, in some cases, of being applied directly on the TXRF spectrometry reflector for immediate analysis. A sophisticated explanation of the excitation spectrum reported for μ-XRF spectrometry included a new description of the energy distribution of electrons in the target. Another highlight was the application of high-resolution monochromatic μ-XRF spectrometry using DCC optics for excitation and focusing as well as for increasing the spectral resolution of the Pu and U L-lines. Although the use of portable XRF spectrometry systems is now well established, new dedicated systems continue to be developed including those with a triaxial configuration. The focus is now primarily on the development and use of portable TXRF spectrometry systems for an expanding range of applications. In the area of cultural heritage applications, macroXRF spectrometry is a well-established imaging technique for the analysis of paintings that is now also being used to image stained-glass panels. The extensive study using a number of micro- and macro-techniques of Vermeer’s painting of The Girl with a Pearl Earring illustrated the power of this approach. Not only could the different shades of blue in the painting be characterised but also the materials and techniques used to achieve various effects in the painting.

The CHROMIC project (effiCient mineral processing and Hydrometallurgical RecOvery of by-product Metals from low-grade metal contaIning seCondary raw materials) aims to recover chromium from steelmaking and ferrochrome slags to regain valuable resources and simultaneously reduce potential environmental impacts. To develop the recovery flowsheets and reliably calculate metal recovery, an accurate assessment of chromium concentration and distribution is essential. Therefore, model streams were thoroughly characterized using a combination of analytical techniques. In all materials, chromium is present in distinct but often small spinel phases, intergrown with other minerals and showing a considerable zonation in Cr content with higher amounts in the cores. The small size of the Cr-rich particles makes recovery by mineral processing challenging. Measured chromium content was found to differ largely based on the chemical dissolution method applied. The analysis of insoluble residues and comparison with a standard reference material evidenced that standard acid dissolution procedures based on HCl/HNO₃/HBF₄ and HNO₃/HF/H₂O₂ are insufficient to fully dissolve spinel structures, leading to severe underestimations of chromium content. A sodium peroxide treatment is required for a full dissolution of spinels. This is noteworthy, since most of the legislation for the reuse of slags is currently based on acid dissolution methods.

This review describes advances in the XRF group of techniques published approximately between April 2018 and March 2019. The review is selective with the aim of providing a critical insight into developments in instrumentation, methodologies and data handling that represent a significant advance in XRF spectrometry. It is not the intention of the review to cover comprehensively the applications of XRF techniques except in those cases where the non-destructive and remote sensing nature of XRF analysis makes it particularly valuable and the method of choice. These applications concern samples which are irreplaceable and of great cultural value such as works of art and archaeological artefacts.

This review covers developments in and applications of XRF techniques such as EDXRF, WDXRF, TXRF, XRF microscopy using technologies such as synchrotron sources, X-ray optics, X-ray tubes and detectors in laboratory, mobile and hand-held systems.