Hobbies And Interests
Electron Microprobe Analysis
Electron microprobe analysis (EMPA), developed by R. Castaing in Paris in 1950, is used for assessing the chemical composition of tiny amounts of solid materials without destroying them. An electron microprobe is based on the principle that if a solid material is bombarded by an accelerated and focused electron beam, the incident electron beam has sufficient energy to liberate matter and energy from the sample. The microbeam instrument uses a high-energy focused beam of electrons. This beam generates X-rays that are characteristic of the element within a sample as small as 3 micrometers across. The X-rays produced are diffracted by analyzing crystals and counted using gas-flow and sealed proportional detectors. Scientists then determine chemical composition by comparing the intensity of X-rays from known compositions with those from unknown materials, and correcting for the effects of absorption and fluorescence in the sample.
Applications
EPMA is the ideal choice for analyzing individual phases in igneous and metamorphic minerals, for materials that are small in size or valuable or unique (e.g., volcanic glass, meteorite matrix, archeological artifacts). Of great interest when analyzing geological materials are secondary and back-scattered electrons, which are useful for imaging a surface or obtaining an average composition of the material.
Setup and Technique
To analyze solid materials using EPMA, flat, polished sections need to be prepared. In an electron microprobe, the focal point on the specimen is bombarded by a narrow beam of electrons, exciting secondary X-rays. The X-ray spectrum for each element consists of a small number of specific wavelengths. The electron microprobe consists of an electron gun and a system of electromagnetic lenses for producing a focused electron beam, scanning coils that allow the beam to raster across an area of the specimen, a specimen stage with X-Y-Z movement, a detection system of solid-state detectors near the specimen and/or wavelength spectrometers and, often, a light microscope for viewing the specimen. For detecting and quantifying the spectrum of secondary X-rays the specimen emits, two methods are used: wavelength detection (WDS), using a diffracting crystal to isolate the characteristic X-ray peaks, and energy detection (EDS), using a solid-state detector that differentiates between the energies of incoming photons.
Advantages
The primary advantage of EPMA is the ability to acquire precise, quantitative elemental analyses at spot sizes as small as several micrometers. The electron optics of an EPMA setup allow higher resolution images to be obtained than are seen using visible-light optics. EPMA analysis is nondestructive, so X-rays generated by electron interactions do not lead to volume loss of the sample. Thus, it is possible to reanalyze the same materials. The spatial scale of analysis, along with the ability to create detailed images, makes it possible to analyze geological materials in situ and resolve complex chemical variation within single phases.