Scientist Solutions - EPMA (electron probe microanalysis)

THIS MAY BE LONG BUT IT WILL GIVE AN OVERVIEW. i could not attach it as it contained pictorial material

In electron probe microanalysis (EPMA) an electron beam accelerated at 10-30 keV is microfocused on a small area (10 nm - 1 m in diameter), and specimen microvolumes are excited by scanning the beam over the target. The inner-shell electrons of target atoms are ionized, and upon relaxation they emit either x-rays from one-stage transition, as described before, or undergo less-radiation two-stage transitions producing Auger electrons. X-ray emission decreases in favor of Auger emission as the atomic number decreases, and this is the main reason why Auger electron spectroscopy is more sensitive in light-element analysis. When x-ray emission occurs, the emitted x-rays can be detected by WDS or EDS, providing information about the elemental composition of material microregions. The depth of analysis depends on the accelerating voltage and the target properties. In EDS the depth is approximately 1 m, while in WDS, which usually operates at higher accelerating voltage, the depth may exceed 5 m. The lateral resolution is somewhat larger due to diffuse scattering of the incident electrons. The detection limits are 0.1% by weight for EDS and 0.01% by weight for WDS. EPMA requires conductive specimens to avoid electrostatic charging. For the study of insulators conductive coatings should be applied.
Modern EPMA systems are coupled to scanning (SEM), conventional transmission (CTEM) or scanning transmission (STEM) electron microscopes. Thermionic SEMs can be equipped with both WDS and EDS. Low-pressure and environmental SEMs, CTEMs and STEMs are mostly compatible with EDS. EPMA offers the following analysis modes: (a) point analysis, where a spectrum at a selected point is taken; (b) line scan analysis, where the elemental gradients along a linear direction are measured, and (c) area scan analysis, where the elemental distribution in the form of elemental mapping is traced. Imaging and analysis in EPMA is performed by detecting various electron signals emerging from secondary emitted electrons, backscattered electrons and characteristic x rays. However, since the sample volume contributing to these signals is different, care should be taken for the correct interpretation of the results. Secondary electrons, providing the conventional SEM images, originate from the uppermost few nanometers. Backscattered electron images, especially the inelastically scattered fraction, originate from a depth of hundreds of nanometers, while characteristic x-rays originate from a depth of a micron or more. Since the atomic number contrast of backscattered detectors is stronger than that achieved with the secondary electron detector, backscattered images are preferred for assigning elemental distributions, especially in multiple-phase specimens. The use of backscattered images in most important when analyzing features with sharp edges (e.g., interfaces), because detection of the forward scattered electrons by the secondary electron detector produces image flaring. This distortion is minimized in backscattered electron images, and clear interfaces are obtained.
As in XRF spectrometry, WDS demonstrate superior resolution than EDS, better sensitivity for low element analysis (Z < 10), lower detection limits by a factor of 10-100; superior elemental distribution images (elemental mapping) will be obtained. However, WDS are more sensitive to topographical features due to beam defocusing, need higher vacuum and higher beam currents than EDS, and are considered more destructive to organic materials. For quantitative analysis of specimens with unknown composition, EDS offer the advantage of rapid multi-element detection. Nevertheless, WDS are the spectrometers of choice for resolving overlapping peaks or for the quantitative analysis of major and minor constituents. Both spectrometer types are sensitive for elements with Z  5. The most common artifacts in qualitative analysis by EDS are the sum peaks, the escape peaks and the internal fluorescence peaks. Sum peaks are recorded at the low-energy region from major spectral components due to coincident detection. Escape peaks are produced from the photon detection process in the [Si (Li)] detector, where sometimes Si is in an excited state emitting x-rays at ~ 1.74 keV. Internal fluorescence peaks may arise from the thin Si layer in front of the detector. Care should be taken for peaks over 15 keV, which usually appear at low intensity, because the efficiency of the detector at the high-energy region is reduced, since some x-rays can penetrate through the detector. In WDS analysis the main problem is the production of a series of peaks from the diffraction orders of the same element which appear at different angles (harmonic peaks). In the low-energy region (< 2 keV) peak shifting may occur due the chemical state of the element in the sample.
Quantitative analysis in EPMA is based on the fundamental observation that x-ray intensities are directly related to the elemental concentrations. Usually it is performed by data reduction computer programs for spectral data correction. Baseline, dead time, atomic number (Z), absorption (A) and fluorescence (F) correction algorithms are generally used. Standardless analysis is considered a semiquantitative procedure due to matrix effect interferences, although in some cases acceptable results may be obtained. The use of standards minimizes the matrix effects, resulting in low detection limits.
The most important advantage of EPMA is elemental mapping, which provides qualitative and quantitative information on the area distribution of the elements in a specimen (Figure 3.6). Two types of elemental mapping exist. In conventional or analog dot-mapping the x-ray signal from the spectrometer is synchronized with the image display, so that an elemental distribution image of modulated brightness is recorded. The concentration variations are denoted by differences in the area density of the dots. Due to limitations in the spatial resolution of the x-rays emitted, the maximum magnification for dot-mapping should not exceed 2000X, otherwise overlapping of pixels may occur. For WDS the low-magnification limit is X400 or more, dependent on the specimen topography, to avoid defocusing artifacts. WDS are much more sensitive in dot-mapping than EDS. The latter are not reliable for elements with concentrations < 5% by weight. In digital or quantitative mapping the true x-ray counts are retained for each image pixel, and thus quantitative analysis can be performed at any point. Grey-scale or color coding may be given to show concentration variations.
The application of EPMA for problem solving should be used under several restrictions. EPMA should not be applied for bulk analysis, since scanning a large area to obtain an average quantitative composition results in a very high error due to inhomogeneity of the region. In such applications the specimen should be scanned at many sites or analyzed by bulk analysis techniques such as XRF. Similar problems are faced when analyzing material sections. One section should not be considered representative of the material structure. There are several material requirements for EPMA, regarding surface roughness and conductivity. The specimen should be relatively flat without deep grooves or scratches, especially when WDS analysis is used. Polishing should be performed mechanically and not by any chemical means that may affect the chemical composition of the specimen. Any polishing artifacts like smearing, phase extraction or interlocking should be considered in the analysis. Specimens containing water and organic matter, like biological tissues, are very sensitive to dehydration and radiation damage, either during analysis or during beam exposure prior to analysis when searching the region of interest. Fixation and controlled dehydration has been recommended to avoid artifacts. Nevertheless, conventional methods (e.g., glutaraldehyde, osmium tetroxide, ethanol solutions and critical point drying) may alter elemental distributions. The same concern applies for demineralization procedures routinely used in TEM studies. Cryogenic procedures, like quench-freezing or freeze-drying, offer several advantages for imaging and analysis, since no side interactions are involved. EDS are preferentially used for tissue analysis to avoid high count rates and sample currents, although the high background levels in EDS reduce minor constituent detection capacity, rendering EPMA as a semiquantitative elemental analysis method.
The thickness and the quality of the conductive coating applied on insulators to avoid electrostatic charging are of primary importance for both imaging and analysis. A thin layer of graphite should be used in EPMA to avoid any interferences of the atomic number of the coating with the analysis. In some modern SEMs equipped with variable-pressure modes, imaging of the specimens is performed at low vacuum in a hydrated state, without the need for coating. However, the excitation energies are insufficient for EPMA. Analysis by EDS only is feasible, but at higher acceleration voltages, which can cause beam damage on the specimens. A major problem in EPMA is the study of soft tissue-biomaterial interactions due to structural incompatibility, which creates interfacial artifacts. A typical example is the measurement of fluoride uptake by enamel bonded to a glass-ionomer cement. The fluoride uptake has been documented by WDS/EPMA studies using line scan analysis perpendicular to the interface. Nevertheless, in a recent study employing advanced microanalytical techniques, it was demonstrated that there was no fluoride uptake by enamel and that the fluoride probed at the enamel surface was attributed to attached glass-ionomer cement microfragments. Apparently excessive dehydration of the glass-ionomer cement under high vacuum produced excessive material cracking and interfacial debonding, leaving adherent microfragments on the enamel, which in the EPMA analysis were considered as phases that diffused into enamel. In such complicated cases multi-technique characterization involving high resolution interfacial imaging and complementary microanalytical techniques is required. EPMA coupled to the SEM is the most frequently used elemental microanalysis method in dental materials research.