Elemental imaging at the nanoscale: NanoSIMS and complementary techniques for element localisation in plants

The ability to locate and quantify elemental distributions in plants is crucial to understanding plant metabolisms, the mechanisms of uptake and transport of minerals and how plants cope with toxic elements or elemental deficiencies. High-resolution secondary ion mass spectrometry (SIMS) is emerging as an important technique for the analysis of biological material at the subcellular scale. This article reviews recent work using the CAMECA NanoSIMS to determine elemental distributions in plants. The NanoSIMS is able to map elemental distributions at high resolution, down to 50 nm, and can detect very low concentrations (milligrams per kilogram) for some elements. It is also capable of mapping almost all elements in the periodic table (from hydrogen to uranium) and can distinguish between stable isotopes, which allows the design of tracer experiments. In this review, particular focus is placed upon studying the same or similar specimens with both the NanoSIMS and a wide range of complementary techniques, showing how the advantages of each technique can be combined to provide a fuller data set to address complex scientific questions. Techniques covered include optical microscopy, synchrotron techniques, including X-ray fluorescence and X-ray absorption spectroscopy, transmission electron microscopy, electron probe microanalysis, particle-induced X-ray emission and inductively coupled plasma mass spectrometry. Some of the challenges associated with sample preparation of plant material for SIMS analysis, the artefacts and limitations of the technique and future trends are also discussed.     

SOFT X-RAY SCANNING MICROSCOPY: ITS PRACTICAL USE FOR ELEMENTAL MAPPING AT THE NSLS U15 BEAMLINE

Scanning soft X-ray microscopy offers several advantages over other forms of X-ray microscopy. It subjects the specimen to the lower doses of radiation and by digitally recording the image, gives direct quantitative information on the absorption of the specimen as a function of position. Elemental maps can be produced easily by comparing images taken at different wavelengths, chosen to exploit X-ray absorption edges or resonances. This technique was used to explore the distribution of calcium in 0.2 μm-thick bone specimens, detecting concentrations of 5% by weight, with a spatial resolution of 0.2 μm. Progress in X-ray sources in the form of undulators and in X-ray optics indicates that soon the range of elements that can be mapped will be extended and the spatial resolution and elemental sensitivity will be improved by an order of magnitude.     

X-Ray Mapping and Interpretation of Scatter Diagrams

Electron beam induced quantitative X-ray mapping has become a very useful characterisation tool for determining the elemental distribution in materials, whether using energy dispersive spectroscopy or wavelength dispersive spectroscopy. The X-ray intensity distributions of the elements from an X-ray map allow us to generate two dimensional and ternary scatter diagrams thus converting spatial information into concentration dimensions, which is an important tool for displaying the spatial relationships of elements or correlated elements (phases) in materials. To best understand how to use this tool, we need to understand the production and features of the scatter diagram. The type of clustering observed in the scatter diagram, whether oval, linear or spherical, can give the major and trace element distributions within phases as well as qualitative and quantitative phase information. This paper demonstrates the generation of scatter diagrams, properties of scatter diagrams, interpretation of scatter diagrams and the advantages of scatter diagrams through the use of examples.     

Superstructure formation and variation in Ni-GDC cermet anodes in SOFC

The microstructures and spatial distributions of constituent elements at the anode in solid oxide fuel cells (SOFCs) have been characterized by analytical transmission electron microscopy (TEM). High resolution TEM observations demonstrate two different types of superstructure formation in grain interiors and at grain boundaries. Energy-filtered TEM elemental imaging qualitatively reveals that mixture zones exist at metal-ceramic grain boundaries, which is also quantitatively verified by STEM energy dispersive X-ray spectroscopy. It was apparent that both metallic Ni and the rare-earth elements Ce/Gd in gadolinium-doped ceria can diffuse into each other with equal diffusion lengths (about 100 nm). This will lead to the existence of mutual diffusion zones at grain boundaries, accompanied by a change in the valence state of the diffusing ions, as identified by electron energy-loss spectroscopy (EELS). Such mutual diffusion is believed to be the dominant factor that gives rise to superstructure formation at grain boundaries, while a different superstructure is formed at grain interiors, as a consequence solely of the reduction of Ce(4+) to Ce(3+) during H(2) treatment. This work will enhance the fundamental understanding of microstructural evolution at the anode, correlating with advancements in sample preparation in order to improve the performance of SOFC anodes.     

地外样品研究中电子探针元素面扫描分析技术的应用

由于月壤等地外样品十分珍贵,在实验室研究中优先使用原位、微区、无损的元素分析方法。电子探针元素面扫描是地外样品研究中常用的分析方法之一。该方法可获取样品整体或者感兴趣区域的多种元素分布数据,应用于矿物相识别与含量估算,锆石等定年矿物的快速定位,矿物环带、出溶、反应边结构等特殊岩相和矿物接触关系等分析和研究。本研究中以嫦娥五号月壤、月球陨石、火星陨石研究为例,介绍了目前元素面扫描的应用方法。此外,本文还对比、分析了电子探针面扫描技术与其它面扫描技术的优缺点和适用范围。未来十年,我国将实施一系列月球、火星、小行星等天体采样返回任务。电子探针元素面扫描分析未来将在这些地外样品研究中广泛使用。同时,建议行星科学家围绕所关心的科学问题,合理搭配多种分析方法以实现各种技术优势互补和样品科学价值最大化,服务我国月球与深空探测任务科学产出和行星科学发展。     

Spatial Resolution of a Wavelength-Dispersive Electron Probe Microanalyzer Equipped with a Thermal Field Emission Gun

In this study, a prototype WDS-EPMA equipped with a thermal field emitter (TFE) was used. By using X-ray mapping technique with this instrument, we analyzed sub-micron inclusions in highly pure copper compounds used for manufacturing electronic devices. The analytical conditions of the accelerating voltages were 10 keV and 15 keV; the measured elements were Ni and Si. We measured 160000 points (400 × 400 points) within a couple of hours (1.5 hours at 10 keV, 0.5 hour at 15 keV). The analysis of the X-ray mapping data revealed very small inclusions with diameters estimated to be less than 100 nm. The new EPMA could be used for the elemental analysis of various materials including very light elements in a wide area with a spatial resolution of 100 nm.     

Multi-elemental mapping of a speleothem using laser-induced breakdown spectroscopy

Speleothems represent an important record of the paleoclimate, and more generally past environmental changes thanks to their laminar structure which is related to variations in rainfall and vegetation throughout the seasons and to their elemental as well as structural compositions which are sensitive to climatic and environmental conditions during their growth. Studies of their composition, especially those with spatial resolution, reveal rich information for paleoclimatology. In this paper, we demonstrate that laser-induced breakdown spectroscopy (LIBS) provides a suitable tool for elemental analysis and especially for 2-dimensional elemental mapping of speleothems. Main, minor, as well as trace elements can be analyzed with this technique. The temporal evolution of the induced plasma is first studied in order to determine a suitable detection window for emission spectrum recording following the impact of the laser pulse on the sample. The matrix effect is then evaluated with a scan on the sample surface by measuring the electron density and the temperature of the plasmas at different positions of the analyzed surface. Concentration mapping is performed for minor and trace elements such as Na, Mg, Al, Si, K, Fe and Sr, by measuring relative variations of line emission intensities from these elements. Finally, correlations in concentration among detected elements are determined. Groups of correlated elements can be attributed to different mineralogical phases.     

Comparison of laser ablation-inductively coupled plasma-mass spectrometry and micro-X-ray fluorescence spectrometry for elemental imaging in Daphnia magna

Visualization of elemental distributions in thin sections of biological tissue is gaining importance in many disciplines of biological and medical research. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and scanning micro-X-ray fluorescence spectrometry (micro-XRF) are two widely used microanalytical techniques for elemental mapping. This article compares the capabilities of the two techniques for imaging the distribution of selected elements in the model organism Daphnia magna in terms of detection power and spatial resolution. Sections with a thickness of 10 and 20 μm of the fresh water crustacean Daphnia magna were subjected to LA-ICP-MS and micro-XRF analysis. The elemental distributions obtained for Ca, P, S and Zn allow element-to-tissue correlation. LA-ICP-MS and micro-XRF offer similar limits of detection for the elements Ca and P and thus, allow a cross-validation of the imaging results. LA-ICP-MS was particularly sensitive for determining Zn (LOD 20 μg g⁻¹, 15 μm spot size) in Daphnia magna, while the detection power of micro-XRF was insufficient in this context. However, LA-ICP-MS was inadequate for the measurement of the S distributions, which could be better visualized with micro-XRF (LOD 160 μg g⁻¹, 5 s live time). Both techniques are thus complementary in providing an exhaustive chemical profiling of tissue samples.     

Speciation of Np(V) uptake by Opalinus Clay using synchrotron microbeam techniques

Synchrotron-based X-ray absorption spectroscopy has been used to determine the chemical speciation of Np sorbed on Opalinus Clay (OPA, Mont Terri, Switzerland), a natural argillaceous rock revealing a micro-scale heterogeneity. Different sorption and diffusion samples with Np(V) were prepared for spatially resolved molecular-level investigations. Thin sections of OPA contacted with Np(V) solution under aerobic and anaerobic conditions as well as a diffusion sample were analysed spatially resolved. Micro-X-ray fluorescence (μ-XRF) mapping has been used to determine the elemental distributions of Np, Fe and Ca. Regions of high Np concentration were subsequently investigated by micro-X-ray absorption fine structure spectroscopy to determine the oxidation state of Np. Further, micro-X-ray diffraction (μ-XRD) was employed to gain knowledge about reactive crystalline mineral phases in the vicinity of Np enrichments. One thin section was also analysed by electron microprobe to determine the elemental distributions of the lighter elements (especially Si and Al), which represent the main elements of OPA. The results show that in most samples, Np spots with considerable amounts of Np(IV) could be found even when the experiments were carried out in air. In some cases, almost pure Np(IV) L(III)-edge X-ray absorption near-edge structure spectra were recorded. In the case of the anaerobic sample, the μ-XRF mapping showed a clear correlation between Np and Fe, indicating that the reduction of Np(V) is caused by an iron(II)-containing mineral which could be identified by μ-XRD as pyrite. These spatially resolved investigations were complemented by extended X-ray absorption fine structure measurements of powder samples from batch experiments under aerobic and anaerobic conditions to determine the structural parameters of the near-neighbour environment of sorbed Np.     

EPMA Measurements of Diffusion Profiles at the Submicrometre Scale

 Concentration profiles due to (inter)diffusion in materials may require high spatial resolution. These profiles may be measured by electron probe microanalysis, which allows one to determine the elemental composition with a good accuracy provided measurement ‘artefacts’ can be accounted for. Standard phenomena are usually corrected by commercial softwares that assume a homogeneous elemental composition in the analysed area. However, in the case of a diffusion process on a small scale, the composition is no longer homogeneous and the effect of the hemispherical volume of the X-ray emission on the spatial resolution of the concentration profiles, and consequently on the diffusion coefficients, has to be considered. Moreover, (secondary) fluorescence across interfaces or interphases has to be evaluated. A radial X-ray distribution associated with the characteristic depth distribution, φ(ρz), allows for the definition of a 2D X-ray emission function that enables the computation of the entire process for a given concentration profile.     

Analytical electron microscopy studies of lithium aluminum hydrides with Ti- and V-based additives

The microstructure of LiAlD(4) with TiCl(3).1/3(AlCl(3)) and VCl(3) additives has been studied during different steps of the decomposition process using electron energy loss spectroscopy and energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope. Energy filtered transmission electron microscopy was used to show elemental distributions in the samples. The spatial distribution of the additives and the main elements within the alanate particles was examined with a resolution of a few nanometers. The analysis of the electron energy loss spectra reveals the chemical state of Al, O, and the additives. Ti and V do not appear to mix chemically with Al to a significant degree. V was found in high concentration in just a few particles, while Ti is more uniformly distributed. All the samples showed evidence of oxidation despite procedures being adopted to avoid exposing the material to air. The additives are oxidized in all the samples, and Al(2)O(3) forms a thin layer at the surface of the particles. This paper gives a comparison between samples at different stages of the decomposition process using different additives.     

Using laser ablation/inductively coupled plasma mass spectrometry to bioimage multiple elements in mouse tumors after hyperthermia

In this study, we employed laser ablation/inductively coupled plasma mass spectrometry (LA-ICP-MS) to map the spatial distribution of Gd-doped iron oxide nanoparticles (IONPs) in one tumor slice that had been subjected to magnetic fluid hyperthermia (MFH). The mapping results revealed the high resolution of the elemental analysis, with the distribution of Gd atoms highly correlated with that of the Fe atoms. The spatial distributions of C, P, S, and Zn atoms revealed that the effect of MFH treatment was significantly dependent on the diffusion of the magnetic fluid in the tissue. An observed enrichment of Cu atoms after MFH treatment was probably due to inflammation in the tumor. The abnormal distribution of Ni atoms suggests a probable biochemical reaction in the tumor. Therefore, this LA-ICP-MS mapping technique can provide novel information regarding the spatial distribution of elements in tumors after cancer therapy.     

Local and surface analysis within an analytical electron microscope

Analytical transmission electron microscopes have the ability to display at very high spatial resolution both the structural information of solid specimens prepared as thin films and the spectroscopic information related either to electronic properties or to the elemental composition. An example of study, by electron energy loss spectroscopy, of small spherical silicon particles is given as an illustration of the performances of the technique.     

Recent progress in quantitative and high spatial resolution AES

Progress in high resolution Auger electron spectroscopy (HR-AES or scanning Auger microscopy, SAM) during the past few years is characterized by the use of efficient field emission electron sources, parallel detection capabilities and improved data acquisition, storage and processing, thus enhancing spatial resolution (to about 10 nm), signal to noise figure and quantification of elements in different chemical bonding states, e.g. by routinely using factor analysis. Optimized ion sputtering facilities, particularly sample rotation, enable depth profiling with high, depth independent resolution. The basic features of SAM are discussed with respect to EPMA (electron probe micro-analysis), emphasizing fundamental limitations and future developments.     

Quantitative Bulk and Trace Element X-Ray Mapping Using Multiple Detectors

X-ray mapping using energy dispersive spectroscopy or wavelength dispersive spectroscopy is a very popular characterisation tool for determining the elemental distribution in materials. Furthermore, quantitative X-ray mapping has become a very powerful technique enabling reliable quantitative results that can be an order of magnitude better than traditional analysis. Quantitative X-ray mapping is also far superior to regions of interest X-ray maps where low levels of an element or elemental overlaps are present. The one major drawback with X-ray mapping is the time required to obtain a high resolution X-ray map with good statistics at low levels of concentration. The use of multi-detectors, and just developed dual turret detectors for X-ray mapping, allows improvement in performance at low levels without compromising quantification quality and precision of traces, even in the presence of overlaps. However, for quantitative X-ray mapping to work properly, the characteristics of each detector must be accurately determined so that the final quantification of the individual detectors can be summed. To accomplish this effectively, the full spectrum at each pixel for each energy dispersive detector should be saved. As a final check for consistency between detectors, a technique was developed that involves assigning a different red-green-blue colour for each detector for the same element. By doing this, when we combine the three maps of the same element, we should obtain a grey scale map that indicates total correlation between the three detectors at the most critical final stage of quantification. To reduce contrast noise and further improve the quality of quantitative X-ray mapping images, a filter referred to as a “speckle filter” has been developed that allows the eye to see a more correct elemental concentration relationship.     

Study on distribution of elements in deep-sea Pacific polymetallic nodules via two-dimensional mapping laser ionization orthogonal time-of-flight mass spectrometry

A newly developed two-dimensional mapping high irradiance laser ionization orthogonal time-of-flight mass spectrometer (LI-O-TOFMS) has been applied for the elemental mapping of polymetallic nodules. Two polymetallic nodule standards were used to demonstrate the efficacy of LI-O-TOFMS for the standardless semiquantitative analysis and the spatial distribution of elements in a deep-sea Pacific polymetallic nodule was mapped. It was found that the two groups of elements, Mn–Ni–Cu–Fe and Co–Si–Al, show clear element-dependent spatial relationships. It is hypothesized that these spatial distributions reflect the environmental and physico-chemical conditions during the nodule formation. LI-O-TOFMS is shown to be a powerful tool in elemental analysis of polymetallic nodules, with the detection limits down to 10^− 7 g/g and a dynamic range of 7 orders of magnitude. Based on the images acquired, the contents of elements and their relationships can be revealed visually.     

The characterizations of solid specimens: Approaching the whole problem

The purpose of this paper is to demonstrate that complicated mixtures of solids can be characterized to a rather high degree if a coordinated examination by non-destructive methods is used. The techniques discussed are X-ray fluorescence, scanning electron microscopy, photoelectron spectroscopy, transmission electron microscopy, electron diffraction and X-ray diffraction. The application of these methods to the characterization of corrosion scale on an inconel coupon is illustrated. The types of information accumulated were elemental composition, chemical forms of elements, special distributions of elements and compounds in the scale, sizes of particles that made up the scale, variations in composition of particle surfaces from that of their interiors, and composition of scale-alloy interface.     

Analytical electron microscopy of materials

Analytical electron microscopy enables combined crystallographic and chemical information with a high spatial resolution to be gained from microregions of electron-transparent specimens. This is reached by the combined application of imaging, diffraction and spectroscopic methods, using either a dedicated scanning transmission electron microscope or a conventional high-resolution electron microscope (having a strong objective lens) equipped with suitable X-ray or electron spectrometers. Of the diffraction methods especially the technique of convergent beam diffraction is used, yielding valuable information on crystal structures, lattice parameter changes, symmetry variations and crystal perfection, respectively. For chemical analysis, either energy-dispersive X-ray spectroscopy (EDX) is used or electron energy loss spectroscopy (EELS). Finally, high-resolution electron microscopy in the lateral resolution range of some 0.1 nm allows the reliable geometrical inspection of extreme microregions.     

New approaches to quantitative x-ray microanalysis of thin films and surfaces

Quantitative analysis of thin films using an electron probe micro-analyser requires the accurate knowledge of the depth distributions of X-ray production (Ø(z) curves) and the influence of the thin film- substrate interface on that distribution. A Gaussian expression for Ø(z) curves has been used which is modified at the interface by the ratio of Ø(0) values. In this way, quantitative predictions of the dependence of X-ray intensity on depth and substrate have been made. Analysis requires the use of only pure element or compound standards. The sensitivity for detection of thin films is about one monolayer for most elements. By measuring X-ray intensity as a function of electron energy, concentration profiles up to a depth of approximately 1 um. can be measured with a depth resolution of the order of 1 nm. in favourable cases.