1. What is an Electron Microprobe?

2. Wavelength Dispersive Spectroscopy?

3. Do you have an Energy Dispersive Spectrometer (EDS) on your Microprobe?

4. What is Backscattered Electron Imaging?

5. What is Secondary Electron Imaging?

What is an Electron Microprobe?

Electron Probe Microanalysis (EPMA) is an elemental analysis technique which uses a focused beam of high energy electrons (5 - 30 KeV) to non-destructively analyze a solid specimen surface (including thin films and particles) for inducing emission of characteristic x-rays (0.1 - 15 KeV). The spatial resolution of x-ray microanalysis of thick specimens is limited to a volume with dimensions of approximately 1 micrometer due to electron scattering effects. This volume may be larger for low energy emission lines that can still be excited by lower energy electrons that have been highly scattered a significant distance from the impinging beam on the specimen surface. Quantitative matrix (interelement) correction procedures based on first principle physical models provide great flexibility and accuracy in analyzing unknown samples of arbitrary composition. Spatial distribution of elemental constituents can be visualized quantitatively by digital composition maps and displayed in gray scale or false color. Detection limits are of the order of 100 ppm (0.01 wt%) with wavelength dispersive spectrometry and 1000 ppm (0.1 wt%) with energy dispersive spectrometry. Typical applications include metallurgical studies, failure analysis, thin film, particulate analysis, mineral analysis, ceramic analysis, Plastics analysis, forensics and many others.

A microprobe is basically an electron microscope that has been optimized to produce a very stable electron beam that remains focused on one spot rather than scanning over a sample as the more common SEMs (Scanning Electron Microscopes) do. The microprobe is used to determine the chemical composition of solid materials - from minerals and glasses in rocks and meteorites, to metals, and even composite materials and plastics and polymers. It is able to determine the chemical composition 1) because it produces a very stable, highly focused electron beam; 2) because most high-energy electrons that penetrate materials interact with the atoms in the materials to generate small amounts of X-rays; and 3) because every element has a known, characteristic set of X-rays. By counting the number of characteristic X-rays produced from electrons impacting suitable standard materials as well as unknowns, it is therefore possible to both qualitatively and quantitatively determine the chemical makeup of a sample. The probe does this through Wavelength-dispersive and Energy-dispersive analyses....

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Wavelength Dispersive Spectroscopy?

The main function of the electron microprobe is to produce highly accurate and precise quantitative chemical analyses of areas as small as 1-2 microns (1-2 thousandths of a millimeter) in diameter using wavelength dispersive spectrometry (WDS). Our microprobe is optimized to analyse elements in the range of Boron (Z=5) through Uranium (Z=92). Our probe can also analyze Beryllium (Z=4) under optimal conditions. Detection limits for common elements using WDS are in the range of 100-500 ppm, although the instrument can be set up to detect and analyze elements with concentrations in the single ppm range (given very long count times). Our new SAMx automation system allows us to obtain about 25-30 15 element analyses per hour once standardization is completed.

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Do you have an Energy Dispersive Detector (EDS) on your Microprobe?

We can also produce qualitative analyses using our energy-dispersive (EDS) detector. EDS rapidly (~5 sec) provides information about the chemical characteristics of a sample (what's there and  an idea of how much). EDS does not provide highly accurate concentration data and sometimes has problems analyzing samples containing elements with overlapping elemental peaks.

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What is Backscattered Electron  Imaging?

Since the microprobe also functions as a Scanning Electron Microscope (SEM), we can generate highly magnified images of our samples. Backscattered electrons have higher energies than secondaries, and are produced when electrons from the primary beam are "bounced" back out of the sample by elastic collisions with atoms. The number of electrons a given atom will backscatter is proportional to its mean atomic number. Materials composed of larger, heavier atoms will backscatter more electrons, producing brighter gray tones in the images than less dense materials (differences in average atomic mass of 0.1 amu can be resolved). Backscattered electrons thus produce an image that is related to material composition, providing both spatial and chemical information.

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What is Secondary Electron Imaging?

Secondary electrons are low energy electrons emitted from very near the sample surface. This signal provides an image of the sample topography, and hence, external morphology. This can be applied not only to simple characterization of a sample material but also to a variety of other applications including component failure analysis or the determination of chemical stability of materials indicated by growth or dissolution features. The example at left shows some pollen at 1000X magnification..

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