Advanced Absorption Correction for 3D Elemental Images Applied to the Analysis of Pearl Millet Seeds Obtained with a Laboratory Confocal Micro X-ray Fluorescence Spectrometer

Absorption Correction for 3D Elemental Distributions of Dental Composite Materials Using Laboratory Confocal Micro-X-ray Fluorescence Spectroscopy

Confocal micro-X-ray fluorescence (micro-XRF) spectroscopy facilitates three-dimensional (3D) elemental imaging of heterogeneous samples in the micrometer range. Laboratory setups using X-ray tube excitation render the method accessible for diverse research fields but interpretation of results and quantification remain challenging. The attenuation of X-rays in composites depends on the photon energy as well as on the composition and density of the material. For confocal micro-XRF, attenuation severely impacts elemental distribution information, as the signal from deeper layers is distorted by superficial layers. Absorption correction and quantification of fluorescence measurements in heterogeneous composite samples have so far not been reported. Here, an absorption correction approach for confocal micro-XRF combining density information from microcomputed tomography (micro-CT) data with laboratory X-ray absorption spectroscopy (XAS) and synchrotron transmission measurements is presented. The energy dependency of the probing volume is considered during the correction. The methodology is demonstrated on a model composite sample consisting of a bovine tooth with a clinically used restoration material.

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

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.

Quantitative Analysis and 2D/3D Elemental Imaging of Cocoa Beans Using X-ray Fluorescence Techniques

As an important raw material for the confectionery industry, the cocoa bean (Theobroma cacao L.) has to meet certain legal requirements in terms of food safety and maximum contaminant levels in order to enter the cocoa market. Understanding the enrichment and distribution of essential minerals but also toxic metals is of utmost importance for improving the nutritional quality of this economically important raw food material. We present three X-ray fluorescence (XRF) techniques for elemental bio-imaging of intact cocoa beans and one additional XRF technique for quantitative analysis of cocoa pellets. The interrelation of all the methods presented gives a detailed picture of the content and 3D-resolved distribution of elements in complete cocoa beans for the first time.

Imaging of Potassium and Calcium Distribution in Plant Tissues and Cells to Monitor Stress Response and Programmed Cell Death

In plants, the response to stress, such as salinity, pathogen attack, drought, high concentration of metals, hyperthermia, and hypothermia, is usually accompanied by potassium ion (K⁺) leakage from the cytosol to the cell wall, mediated by plasma membrane cation conductivity. Stress-induced electrolyte leakage co-occurs with accumulation of reactive oxygen species (ROS) and calcium ions (Ca²⁺) and often results in programmed cell death (PCD). The development of X-ray and mass spectrometry (MS) based imaging techniques has enabled insight into the spatial tissue and cell-specific redistribution of major and trace elements during the stress response. In this chapter a workflow for sample preparation, imaging, and image analysis by X-ray and MS based techniques is presented.

Chemical mapping of teeth in 2D and 3D: X-ray fluorescence reveals hidden details in dentine surrounding fillings

X-rays are frequently used for characterizing both tooth tissues and dental materials. Whereas radiographs and tomography utilize absorption contrast for retrieving details, chemical mapping is usually achieved by energy dispersive X-ray (EDX) analysis that is stimulated under vacuum in electron microscopes. However, the relatively dense mineralized composition of teeth, and the frequent inclusion of a large range of elements in filling materials raise the possibility that other X-ray based techniques such as X-ray fluorescence (XRF) spectroscopy may strongly contribute to investigations of a large variety of dental structures. By exploiting the fluorescence excited by micron sized X-rays (µXRF) it is possible to map minute quantities of a large range of elements (from aluminum to uranium), where spectra containing signals from multiple different elements can be resolved non-destructively and concomitantly. The high penetration depth of X-rays makes XRF highly effective at detecting variable compositions with information emerging from tooth tissues situated well beneath the sample surface. The method supports minimal sample preparation and, different from electron microscopy, it facilitates investigation of hydrated dental materials. Direct comparison of µXRF and confocal µXRF (CµXRF) with SEM-EDX reveals micro zones of chemical heterogeneity in the complex 3D architecture of root canal fillings. These methods reproducibly clarify the mutual arrangement of biomaterials in both fresh fillings as well as in repeatedly treated old teeth of unknown history. The results showcase the complementarity of X-ray and electron based elemental mapping for dental materials research. STATEMENT OF SIGNIFICANCE: Chemical characterization of mineralized tissues such as tooth dentine is often performed using energy dispersive X-ray spectroscopy (EDS/EDX) analysis by scanning electron microscopy (SEM). The widespread use of electron microscopes and simplified detector designs have made this form of chemical and structural analysis extremely popular. However, excitation by electrons is limited to the upper microns of the tissue, and these may not well represent the chemical composition of the bulk. Especially when heavier elements are of interest and when dental filling materials exhibit diffusion into the tooth, little is known about the spatial distribution. Here we show how complementary X-ray fluorescence data originating by electron and X-ray excitation can help visualize the distribution and impregnation of heavy elements through teeth, e.g. for root canal treatment.

Selective mineral transport barriers at Cuscuta-host infection sites

The uptake of inorganic nutrients by rootless parasitic plants, which depend on host connections for all nutrient supplies, is largely uncharted. Using X-ray fluorescence spectroscopy (XRF), we analyzed the element composition of macro- and micronutrients at infection sites of the parasitic angiosperm Cuscuta reflexa growing on hosts of the genus Pelargonium. Imaging methods combining XRF with 2-D or 3-D (confocal) microscopy show that most of the measured elements are present at similar concentrations in the parasite compared to the host. However, calcium and strontium levels drop pronouncedly at the host/parasite interface, and manganese appears to accumulate in the host tissue surrounding the interface. Chlorine is present in the haustorium at similar levels as in the host tissue but is decreased in the stem of the parasite. Thus, our observations indicate a restricted uptake of calcium, strontium, manganese and chlorine by the parasite. Xylem-mobile dyes, which can probe for xylem connectivity between host and parasite, provided evidence for an interspecies xylem flow, which in theory would be expected to carry all of the elements indiscriminately. We thus conclude that inorganic nutrient uptake by the parasite Cuscuta is regulated by specific selective barriers whose existence has evaded detection until now.

Characteristic Analysis of Compact Spectrometer Based on Off-Axis Meta-Lens

Ultra-compact spectrometers with high-resolution and/or broadband features have long been pursued for their wide application prospects. The off-axis meta-lens, a new species of planar optical instruments, provides a unique and feasible way to realize these goals. Here we give a detailed investigation of the influences of structural parameters of meta-lens-based spectrometers on the effective spectral range and the spectral resolution using both wave optics and geometrical optics methods. Aimed for different usages, two types of meta-lens based spectrometers are numerically proposed: one is a wideband spectrometer working at 800–1800 nm wavelengths with the spectral resolution of 2–5 nm and the other is a narrowband one working at the 780–920 nm band but with a much higher spectral resolution of 0.15–0.6 nm. The tolerance for fabrication errors is also discussed in the end. These provides a prominent way to design and integrate planar film-based spectrometers for various instrumental applications.