High-resolution Chemical Mapping Research Articles

Optimising confocal Raman microscopy for spectral mapping of cement-based materials

Raman spectroscopy combined with confocal imaging, i.e. confocal Raman microscopy (CRM) is a relatively new technique with huge potential for high-resolution chemical mapping of phase composition and spatial distribution in cement-based materials. However, the effects of sample preparation and various operating parameters on mapping quality has not been systematically studied. This paper optimises CRM for spectral mapping of carbonated and non-carbonated cement-based materials. The effects of sample preparation and scanning parameters on the detection of four main phases (calcite, portlandite, ettringite and unreacted cement) were investigated. Results show that although freshly cut cementitious samples can be analysed as-is, the Raman signals improve with short gentle drying and surface grinding/polishing prior to analysis. Increasing laser power, exposure time and scan accumulation, and short laser wavelength yields higher signal-to-noise (SNR) ratio in the obtained spectrum. The use of a 4.15 mW laser power, 2 s exposure time and scan accumulation of 2 with 532 nm laser represents a good operating condition for Raman analysis of cement-based materials. This produces SNR > 10 for all investigated phases at short testing time and low risk of laser-induced damage. Microcracking caused by localised heating during closely-spaced mapping can be limited by impregnating the sample with epoxy to protect the microstructure. We show for the first time that CRM can be used to quantify the volume fraction of calcium carbonate and portlandite at high resolution when combined with SEM. The advantages and limitations of CRM for mapping cement-based materials are discussed.

Magmatic-hydrothermal evolution of the El Laco iron deposit revealed by trace element geochemistry and high-resolution chemical mapping of magnetite assemblages

The Plio-Pleistocene El Laco magnetite ore bodies in the Chilean Altiplano represent an unusual subvolcanic/aerial type of an iron oxide-apatite (IOA) deposit. The textures of these magnetite ore bodies have sustained a long-standing geological controversy on the origin of iron oxide deposits with models spanning the spectrum from purely igneous to magmatic-hydrothermal processes. In particular, the link between the geochemical processes taking place in the source magma and the subsequent evolution of the overlying magmatic-hydrothermal system are still not well understood. Here we address this problem by focusing on microtextural and geochemical features of iron ore and silicate gangue minerals retrieved from drill core samples from El Laco ore bodies, as well as from the andesite volcanic host rocks. We report a comprehensive geochemical dataset of magnetite assemblages at El Laco obtained using a combination of EPMA, LA-ICP-MS, and synchrotron radiation µ-XRF methods. Microtextural and geochemical data of magnetite reveal consistent variations with depth. Magnetite in the andesitic host rocks and from deep/intermediate levels of the ore bodies have a high concentration of trace elements including Ti, V, Ni, Mn, Zn, Cr, Al, Ga, Cu and Co in comparison with magnetite from the upper sections of the ore bodies. Clinopyroxene is present in the andesite host rocks as well as the ore bodies, showing chemical differences between both types, with Mn and Fe contents that are higher in the former, and Na and Ca concentrations enriched in the latter. We interpret the enriched-to-depleted compositional trends in magnetite and clinopyroxene as resulting from a transition from purely igneous conditions to a fluid-dominated, cooling magmatic-hydrothermal system. Detailed microtextural analysis of magnetite and clinopyroxene from the ore bodies support this notion, revealing multiple growth stages punctuated by dissolution-reprecipitation processes. Our data further supports a genetic model that explains the formation of the El Laco iron oxide deposit through the injection, upward migration and venting of magmatically-derived Fe-rich fluids.

DNAcycP: a deep learning tool for DNA cyclizability prediction.

DNA mechanical properties play a critical role in every aspect of DNA-dependent biological processes. Recently a high throughput assay named loop-seq has been developed to quantify the intrinsic bendability of a massive number of DNA fragments simultaneously. Using the loop-seq data, we develop a software tool, DNAcycP, based on a deep-learning approach for intrinsic DNA cyclizability prediction. We demonstrate DNAcycP predicts intrinsic DNA cyclizability with high fidelity compared to the experimental data. Using an independent dataset from in vitro selection for enrichment of loopable sequences, we further verified the predicted cyclizability score, termed C-score, can well distinguish DNA fragments with different loopability. We applied DNAcycP to multiple species and compared the C-scores with available high-resolution chemical nucleosome maps. Our analyses showed that both yeast and mouse genomes share a conserved feature of high DNA bendability spanning nucleosome dyads. Additionally, we extended our analysis to transcription factor binding sites and surprisingly found that the cyclizability is substantially elevated at CTCF binding sites in the mouse genome. We further demonstrate this distinct mechanical property is conserved across mammalian species and is inherent to CTCF binding DNA motif.

Integrated Tapping Mode Kelvin Probe Force Microscopy with Photoinduced Force Microscopy for Correlative Chemical and Surface Potential Mapping.

Kelvin probe force microscopy (KPFM) is a popular technique for mapping the surface potential at the nanoscale through measurement of the Coulombic force between an atomic force microscopy (AFM) tip and sample. The lateral resolution of conventional KPFM variants is limited to between ≈35 and 100nm in ambient conditions due to the long-range nature of the Coulombic force. In this article, a novel way of generating the Coulombic force in tapping mode KPFM without the need for an external AC driving voltage is presented. A field-effect transistor (FET) is used to directly switch the electrical connectivity of the tip and sample on and off periodically. The resulting Coulomb force induced by Fermi level alignment of the tip and sample results in a detectable change of the cantilever oscillation at the FET-switching frequency. The resulting FET-switched KPFM delivers a spatial resolution of ≈25nm and inherits the high operational speed of the AFM tapping mode. Moreover, the FET-switched KPFM is integrated with photoinduced force microscopy (PiFM), enabling simultaneous acquisitions of high spatial resolution chemical distributions and surface potential maps. The integrated FET-switched KPFM with PiFM is expected to facilitate characterizations of nanoscale electrical properties of photoactive materials, semiconductors, and ferroelectric materials.

Early-Stage Melt-Rock Reaction in a Cooling Crystal Mush Beneath a Slow-Spreading Mid-Ocean Ridge (IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge)

Microtextural and chemical evidence from gabbros indicates that melts may react with the crystal framework as they migrate through crystal mushes beneath mid-ocean ridges; however, the importance of this process for the compositional evolution of minerals and melts remains a matter of debate. Here we provide new insights into the extent by which melt-rock reaction process can occur in oceanic gabbros by conducting a detailed study of cryptic reactive melt migration as preserved in an apparently unremarkable, homogeneous olivine gabbro from deep within a section of the plutonic footwall of the Atlantis Bank core complex on the Southwest Indian Ridge (International ocean discovery program Hole U1473A). High-resolution chemical maps reveal that mineral zoning increases toward and becomes extreme within a cm-wide band that is characterized by elevated incompatible trace element concentrations and generates extreme more/less incompatible element ratios. We demonstrate that neither crystallization of trapped melt nor diffusion can account for these observations. Instead, taking the novel approach of correcting mineral-melt partition coefficients for both temperature and composition, we show that these chemical variations can be generated by intergranular reactive porous flow of a melt as it migrated through the mush framework, and whose composition evolved by melt-rock reaction as it progressively localized into a cm-scale reactive channel. We propose that the case reported here may represent, in microcosm, a preserved snapshot of a generic mechanism by which melt can percolate through primitive mafic (olivine gabbro) crystal mushes, and be modified toward more evolved compositions via near-pervasive reactive transport.

Synchrotron macro ATR-FTIR microspectroscopy for high-resolution chemical mapping of single cells.

Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy has been used widely for probing the molecular properties of materials. Coupling a synchrotron infrared (IR) beam to an ATR element using a high numerical aperture (NA) microscope objective enhances the spatial resolution, relative to transmission or transflectance microspectroscopy, by a factor proportional to the refractive index (n) of the ATR element. This work presents the development of the synchrotron macro ATR-FTIR microspectroscopy at Australian Synchrotron Infrared Microspectroscopy (IRM) Beamline, and demonstrates that high quality FTIR chemical maps of single cells and tissues can be achieved at an enhanced spatial resolution. The so-called "hybrid" macro ATR-FTIR device was developed by modifying the cantilever arm of a standard Bruker macro ATR-FTIR unit to accept germanium (Ge) ATR elements with different facet sizes (i.e. 1 mm, 250 μm and 100 μm in diameter) suitable for different types of sample surfaces. We demonstrated the capability of the technique for high-resolution single cell analysis of malaria-infected red blood cells, individual neurons in a brain tissue and cellular structures of a Eucalyptus leaf. The ability to measure a range of samples from soft membranes to hard cell wall structures demonstrates the potential of the technique for high-resolution chemical mapping across a broad range of applications in biology, medicine and environmental science.

The influence of skeletal micro-structures on potential proxy records in a bamboo coral

Assessing the physicochemical variability of the deeper ocean is currently hampered by limited instrumental time series and proxy records. Bamboo corals (Isididae) form a cosmopolitan family of calcitic deep sea corals that could fill this information gap via geochemical information recorded in their skeletons. Here we evaluate the suitability of high-resolution chemical imaging of bamboo coral skeletons for temperature and nutrient reconstruction. The applied elemental mapping techniques allow to verify the suitability of the chosen transect on the sample section for paleo-reconstructions and enhance the statistical precision of the reconstruction. We measured Mg/Ca via electron microprobe at 1 µm resolution and Ba/Ca via laser ablation ICP-MS at 35 µm resolution in a historic specimen of Keratoisis grayi from the Blake Plateau off Eastern Florida. Long-term growth temperatures of 7.1 ± 3.4 °C (± 2 SD) that are in agreement with recent ambient temperature range can be reconstructed from Mg/Ca ratios provided that anomalously Mg-enriched structural features around the central axis and isolated features related to tissue attachment are avoided for reconstruction. Skeletal Ba/Ca measurements reflect mean seawater barium [Ba]SW concentrations ([Ba]SW = 51 ± 24 nmol kg-1 (± 2 SD)), in agreement with instrumental data (47 nmol kg-1). We show for the first time that Ba/Ca forms concentric structures in a bamboo coral skeleton section. Our investigations suggest that, while bamboo coral skeletons do record environmental parameters in their mean chemical composition, the magnitude of environmental variability reconstructed from high-resolution chemical maps exceeds that expected from instrumental time series. This necessitates additional investigation of the factors driving bamboo coral skeletal composition.

In Situ Stimulated Raman Scattering (SRS) Microscopy Study of the Dissolution of Sustained-Release Implant Formulation.

Localized drug delivery systems (DDSs) provide therapeutic levels of drug agent while mitigating side effects of systemic delivery. These systems offer controlled release over extended periods of time making them attractive therapies. Monitoring drug dissolution is vital for developing safe and effective means of drug delivery. Currently, dissolution characterization methods are limited to bulk analysis and cannot provide dissolution kinetics at high spatial resolution. However, dissolution rates of drug particles can be heterogeneous with influences from many factors. Insights into finer spatiotemporal dynamics of single particle dissolution could potentially improve pharmacokinetic modeling of dissolution for future drug development. In this work, we demonstrate high-resolution chemical mapping of entecavir, a hepatitis B antiviral drug, embedded in a slow release poly(d,l-lactic acid) formulation with stimulated Raman scattering (SRS) microscopy. By tracking the volume change of individual micron-sized drug particles within the polymer matrix, we establish an analytical protocol for quantitatively profiling dissolution of single crystalline particles in implant formulations in an in situ manner.

Nanoencapsulation of hexavalent chromium with nanoscale zero-valent iron: High resolution chemical mapping of the passivation layer

Solid phase reactions of Cr(VI) with Fe(0) were investigated with spherical-aberration-corrected scanning transmission electron microscopy (Cs-STEM) integrated with X-ray energy-dispersive spectroscopy (XEDS). Near-atomic resolution elemental mappings of Cr(VI)–Fe(0) reactions were acquired. Experimental results show that rate and extent of Cr(VI) encapsulation are strongly dependent on the initial concentration of Cr(VI) in solution. Low Cr loading in nZVI (<1.0wt%) promotes the electrochemical oxidation and continuous corrosion of nZVI while high Cr loading (>1.0wt%) can quickly shut down the Cr uptake. With the progress of iron oxidation and dissolution, elements of Cr and O counter-diffuse into the nanoparticles and accumulate in the core region at low levels of Cr(VI) (e.g., <10mg/L). Whereas the reacted nZVI is quickly coated with a newly-formed layer of 2–4nm in the presence of concentrated Cr(VI) (e.g., >100mg/L). The passivation structure is stable over a wide range of pH unless pH is low enough to dissolve the passivation layer. X-ray photoelectron spectroscopy (XPS) depth profiling reconfirms that the composition of the newly-formed surface layer consists of Fe(III)–Cr(III) (oxy)hydroxides with Cr(VI) adsorbed on the outside surface. The insoluble and insulating Fe(III)–Cr(III) (oxy)hydroxide layer can completely cover the nZVI surface above the critical Cr loading and shield the electron transfer. Thus, the fast passivation of nZVI in high Cr(VI) solution is detrimental to the performance of nZVI for Cr(VI) treatment and remediation.

Microstructural vs compositional preservation and pseudomorphic replacement of muscovite in deformed metapelites from the Longmen Shan (Sichuan, China)

Pressure-temperature-deformation (P-T-ε) paths for metamorphic rocks that experienced a complex deformation history commonly rely on the link between successive metamorphic assemblages and the different microstructures preserved at the sample scale. However, with changing P-T conditions, metamorphic minerals in a specific microstructure can chemically re-equilibrate. The direct link between deformation phases and mineral compositions for thermobarometry purposes can therefore be distorted. This study focuses on a series of garnet-biotite metapelites from the Longmen Shan (Sichuan, China) that preserve muscovite of different chemistry in distinct microstructures. To quantify the degree of re-equilibration of muscovite, a microstructural study was coupled with high-resolution chemical mapping. The chemical evolution of muscovite was modeled along a P-T loop previously constrained with phase equilibria calculations, semi-empirical and empirical thermobarometry. Results show that metapelites experienced a three-stage metamorphic history: (1) heating and burial up to 11±2kbar, 530±20°C, (2) minor decompression and heating up to 6.5±1kbar, 575±10°C and (3) decompression and cooling down to 4±1kbar, 380–450°C. Muscovite partially re-equilibrated by pseudomorphic replacement during the three metamorphic stages, although it is mainly observed in prograde microstructures preceding the pressure peak. The main factors controlling the re-equilibration are the intensity of the deformation and the fluid availability during metamorphism. The P-T conditions of metamorphic assemblages thus reflect pulses of fluids release that enhanced mineral resorption and local replacement. Such micro-chemical behavior is probably relatively common in metapelites and questions the reliability of the P-T predictions based on relict phase chemistry, apparently preserved in microstructures that might have been affected by later re-equilibration.

Structure and local chemical properties of boron-terminated tetravacancies in hexagonal boron nitride.

Imaging and spectroscopy performed in a low-voltage scanning transmission electron microscope are used to characterize the structure and chemical properties of boron-terminated tetravacancies in hexagonal boron nitride. We confirm earlier theoretical predictions about the structure of these defects and identify new features in the electron energy-loss spectra of B atoms using high resolution chemical maps, highlighting differences between these areas and pristine sample regions. We correlate our experimental data with calculations which help explain our observations.

Tip-Enhanced Raman Imaging: An Emergent Tool for Probing Biology at the Nanoscale

Typically limited by the diffraction of light, most optical spectroscopy methods cannot provide the spatial resolution necessary to characterize specimens at the nanoscale. An emerging exception to this rule is tip-enhanced Raman spectroscopy (TERS), which overcomes the diffraction limit through electromagnetic field localization at the end of a sharp metallic tip. As demonstrated by the Zenobi group in this issue of ACS Nano, TER imaging is an analytical technique capable of providing high-resolution chemical maps of biological samples. In this Perspective, we highlight recent advances and future applications of TER imaging as a technique for interrogating biology at the nanoscale.

Evaluation of ionization yields under gallium bombardment

AbstractThe Cameca NanoSIMS 50 is known to provide high resolution chemical mapping for negatively charged secondary ions (50–100 nm). Nevertheless, concerning positive secondary ions, the resolution is only in the 250‐nm range. In order to obtain a better lateral resolution for positive ions, a Liquid Metal Ion Gun (LMIG, Canion 31+, Orsay‐Physics, Fuveau, France) filled with gallium (Ga+) has been installed on the Cameca IMS‐6F. In the case of this study, we have measured the useful yields in inorganic samples. Limitations in the secondary ion signal intensity, which is inherent to the Gallium bombardment, can be partly balanced by using oxygen flooding, as a method to enhance the ion yield. Examples of applications are presented to demonstrate that the IMS‐6F equipped with a Gallium LMIG fulfills the requirements to be a complementary technique to the NanoSIMS. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Optical fibre SERS sensors

Surface-enhanced Raman scattering (SERS) has established itself as an important analytical technique. However, efforts to transfer the technology from the laboratory to the production line, clinic or field have been frustrated by the lack of robust affordable substrates and the complexity of interfacing between sample and spectrometer. Prompted by the success of optical fibre systems for implementing normal Raman scattering spectroscopy in remote locations and biomedical applications, attention has now shifted to the development of SERS-active optical fibres. Other workers have attempted to develop SERS probes with extended interaction lengths and both far-field and near-field SERS imaging techniques for high-resolution chemical mapping of surfaces. This review discusses the development of these technologies and presents the current state of the art. Although recent developments show great promise, some outstanding challenges and opportunities remain to be addressed.

Hybrid Tomography of Nanostructures in the Electron Microscope

AbstractA variety of tomographic experiments and modes for electron tomography of nanostructures are introduced, derived from the general concepts of quantitative computed tomography, binarised geometric tomography, including shape-from-silhouette, and spectroscopic chemical mapping. Our emphasis is on working out concepts of combining at least two of these tomography modes in order to share their respective advantages and improve the overall reconstruction quality. In this work, the following three hybrid modes are presented: (i) ADF-STEM tomography and EDX tomography into high-resolution 3D chemical mapping, (ii) geometric tomography and lattice-resolved backprojection into HREM-tomography for convex bodies, and (iii) geometric tomography and e-beam nanosculpting into “tomographic nanofabrication”.

At-sea high-resolution chemical mapping: extreme barium depletion in North Pacific surface water

We have modified an inductively coupled plasma mass spectrometer (ICPMS) for use at sea, which permits mapping of dissolved Ba at high temporal and spatial resolution. While evaluating this system off the Pacific coast of Baja California, we discovered Ba depletion greater than 60% in surface waters over tens of kilometers. Observed Ba abundance is among the lowest reported in any ocean water. No corresponding depletion was seen in Sr, which strongly suggests that acantharians are not the primary source of biogenic Ba in these surface waters.

The development of information storage materials — How microscopy can help?

The response of giant magnetoresistance (GMR) devices depends critically on the film microstructure, with parameters such as layer thickness and interfacial abruptness being crucial. This paper presents results obtained using high resolution electron microscopy (HREM), chemical mapping and atom probe microanalysis. Local variations in the magnetic properties are induced by the microstructure and also when the films are patterned to form small elements. These lead to changes in the magnetization reversal mechanism. Some results of the studies of the magnetization reversal carried out using in situ in Lorentz transmission electron microscopy (LTEM) magnetizing experiments are also included.

Evolution of Crystallizing Interstitial Liquid in an Arc-Related Cumulate Determined by LA ICP-MS Mapping of a Large Amphibole Oikocryst

High-resolution chemical mapping of a large, single, oikocrystic INTRODUCTION amphibole grain from the Eriksberg gabbro, by laser ablation The most commonly studied cumulus rocks are those inductively coupled plasma mass spectrometry (LA ICP-MS), formed by crystallization of relatively dry basaltic liquids documents the progressive evolution of interstitial liquid in a hydrous (e.g. Parsons, 1986; Cawthorn, 1996). These include most basaltic system. The amphibole has a nearly constant mg-number layered intrusions, ophiolites, oceanic cumulates, and and only minor variations in most major elements, in part because most cumulates recovered from the Moon and in metof the resorption of olivine buffering the liquid composition. However, eorite collections. Studies of these cumulates have innon-buffered trace elements including Sc, V, Nb, Zr, Th, U, and cluded efforts to understand how the crystals are the rare earth elements (REE) show at least an order of magnitude segregated from the liquid (cumulus processes), as well variation. For example, V varies from 1000 ppm, Zr as how the resulting pile of crystals plus liquid evolves from 4·3 to >1000 ppm, Nb from 0·14 to 12·3 ppm, and the during slow cooling (postcumulus processes). By comREE patterns range from bowed up with a negative Eu anomaly parison, very little effort has been directed toward underto bowed down with a positive Eu anomaly—all within a single standing these processes in the crystallization of relatively crystal. The distribution of amphibole compositions indicates that hydrous basaltic liquids such as those associated with arc the interstitial liquid was not uniformly distributed as crystallization magmatism. The presence of a significant amount of proceeded. Rather, the compositional variations reflect progressively water in the liquid is especially important for the postmore channelized flow of interstitial liquid during compaction. When cumulus evolution of a crystal pile, as it will stabilize the interstitial liquid evolved so that the crystallizing amphibole had hydrous minerals and cause vapor saturation while a >300 ppm V, vapor saturation was reached and the behavior of the significant amount of interstitial liquid remains. incompatible trace elements changed markedly. The final amphibole to The importance of fractional crystallization in procrystallize is enriched in soluble incompatible elements and depleted ducing compositional variability in crystallizing silicate in Zr and Nb—a relationship that is consistent with crystallization liquids has been well established since Bowen’s exfrom a fluid. Where compaction returns such evolved liquid or fluid perimental work (Bowen, 1928). During perfect fractional to an overlying magma reservoir, resulting fractionation trends will crystallization, the solids are isolated from the liquid before the liquid evolves appreciably and thus they proshow the relative depletions in Ti, Nb, and Zr seen in arc-related vide an instantaneous record of the liquid evolution. basalts. Incorporation of liquid in a crystal pile can change both the composition of the solids and the evolution of the supernatant liquid (initially the parental liquid). If the

Microstructure evolution during solid state reactions of Nb/Al multilayers

The microstructure produced during investigations of reactive formation of NbAl 3 from Nb/Al multilayer films was found to be in general agreement with that employed in the two-stage kinetic model of Coffey et al. [ Appl. Phys. Lett. 55 (1989) 852]. Upon completion of the reaction, however, recrystallization and rapid grain coarsening led to significant changes in film microstructure and deviations from the predictions of the model. In an effort to clarify the picture of reactive phase formation emerging from these studies, several complementary analyses were employed to characterize the reaction. First, thermal data obtained from constant-heating-rate differential scanning calorimetry experiments were used to determine the annealing conditions for ex situ cross sectional transmission electron microscopy studies of the evolving film microstructure. Next, high-resolution chemical mapping was employed to track the penetration of the product phase into the Al grain boundaries. It was found, for example, that the NbAl 3 layer was 8–10 nm thick when the first stage of reaction was completed. It was also found that during stage two, the NbAl 3 layer consisted of columnar, somewhat faceted grains and that substantial recrystallization occurred after the termination of the reaction. These observations suggested refinements of the structure type classification scheme for reacted films proposed by Barmak et al. [ J. Electron. Mater. 26 (1997) 1009]. Lastly, the impact of bilayer thickness on the product microstructure was investigated using a novel sample architecture in which several bilayers with different periods comprise one multilayer. Investigations of this multi-period, multilayer sample correlated with those on single-period multilayers, indicating that the reaction reached completion at lower temperatures as the bilayer thickness was reduced.

Quantitative Chemical Mapping with Low-Voltage EDS Spectrum Imaging and Multivariate Statistical Analysis (MSA)

One of the advantages of performing X-ray microanalysis at low (≤5 kV) operating voltages is that high spatial resolution (≤250 nm) chemical maps of the specimen can be acquired. Spectrum imaging, where a full spectrum is acquired for each pixel in a two-dimensional array, provides the most comprehensive method of characterization, as long as the sampling density (pixel size) is sufficiently smaller than the spatial resolution. Multivariate statistical analysis (MSA) methods are effective in reducing the large (typically ∼10 MByte) raw spectrum images to the modest (typically ∼100 kByte) data files that contain all of the statistically significant information of interest about the specimen. Preliminary analysis of a cross section of a computer chip from a major semiconductor company was previously performed using limited spectrum imaging capabilities available with the 4pi X-ray mapping module, which allowed for simultaneous acquisition of only 48 channels. MSA of images acquired with only a 960 eV portion of the spectrum containing the Al-K, Si-K and W-M lines showed that excellent contrast between the Si- and W-rich regions of the specimen could be achieved in spite of the strong overlap between Si-Kα and W-Mα (∼34 eV separation).