Electron microprobe analysis (EMPA) is a non-destructive method for determining the chemical composition of tiny amounts of solid materials. The electron microprobe can quantitatively analyze elements from fluorine (Z=9) to uranium (Z=92) at routine levels as low as 100 ppm. It was developed by R. Castaing in Paris as his 1950 Ph.D. dissertation. Commercial electron microprobes became available in the 1960s, and have become standard analytical tools.
Electron Probe Microanalysis (EPMA) is an elemental analysis technique which uses a focused beam of high energy electrons (5 - 30 KeV) to non-destructively ionize a solid specimen surface (including thin films and particles) for inducing emission of characteristic x-rays (0.1 - 15 KeV). The high-energy focused beam of electrons generates X-rays characteristic of the elements within a sample from volumes as small as a micrometer (10-6m) across. The resulting X-rays are diffracted by analyzing crystals (TAP, PET, LIF, ODPb, and PC) and counted using gas-flow and sealed proportional detectors. Chemical composition is determining by comparing the intensity of X-rays from standards (known composition) with those from unknown materials and correcting for the effects of absorption and fluorescence in the sample.
The electron microprobe is designed specifically for detecting and measuring characteristic X-rays. It uses an electron beam current from 10 to 200 nanoamps, roughly 1000 times greater than that in a scanning-electron microscope (SEM). These higher beam currents produce more X-rays from the sample and improve both the detection limits and accuracy of the resulting analysis. Analysis locations are selected using a transmitted-light optical microscope, which allows positioning accurate to about 1 micrometer, a feature not available on an SEM. The resulting data yield quantitative chemical information in a textural context. 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. Variations in chemical composition within a material, such as a mineral grain or metal, can be readily determined.
These quantitative procedures have been demonstrated to produce error distributions characterized by a standard deviation of less than 3% relative when the samples are in the ideal form of a metallographically polished bulk solid. Standards utilized in these analyses are in the form of pure elements or simple compounds (e.g., MgO or GaP). This analytical approach provides great versatility in the analysis of multi-element unknowns of virtually any composition, with the significant exception of light elements (atomic numbers less than 8). Detection limits are of the order of 100 ppm with wavelength dispersive spectrometry and 1000 ppm with energy dispersive spectrometry. Typical applications include metallurgical studies, failure analysis, thin film, particulate analysis, mineral analysis, ceramic analysis, and many others.
EPMA at the University of Texas at El Paso is a Cameca SX50 instrument. The microprobe has four wavelength dispersive detectors and a state-of-the-art Rontec solid state energy dispersive detector. The operating software of the SX50 is SX RAY N50 on Solaris 2, which is the revised SX100 software compatible to SX50. The upgrading of this operation system is from NSF grant No. EAR-0116660.
The electron microprobe offers several operational modes for chemical analysis. These are generally divided into qualitative and quantitative procedures. The qualitative analysis capabilities are briefly listed here :
Secondary electron imaging
- provides topographic images (surfaces, cracks, voids)
- magnification range from 63X to 10000X
- on-line gray and false-color images
- output in binary data file and TIFF image file
Backscatter electron imaging
- provides average atomic number contrast images and
topographic images
- magnification range from 63X to 10000X
- on-line gray and false-color images
- output in binary data file and TIFF image file
Qualitative x-ray imaging
- provides x-ray mapping of sample surfaces for up to 4
elements (WDS)
- on-line gray and false-color images
- output in binary data file and TIFF image file
Wavelength spectrometer scanning
- simultaneous wavelength scan acquisition on 4 spectrometers
- on-line database containing over 12,000 entries for peak
identification
- publication quality graphical output
Quantitative analysis generally involves the use of calibration standards and correction for deadtime, background, drift, matrix and interference effects in both the standards and the unknown sample.
Quantitative element analysis
- simultaneous analysis of 4 elements up to a maximum from O
to U elements
- large set of USNM, USGS, MAS and internal certified analytical
standards
- correction of deadtime, background, drift, matrix and
interference effects for fully quantitative analysis
- trace element procedure for low concentration elements
- full automation for unknown and standard data acquisition
- results in elemental and oxide weight percent, mole percent,
formula atoms and mineral end-members
- statistics on detection limits, sample homogeneity, and
analytical sensitivity
- draft and publication quality hard copy and ASCII file output
Quantitative x-ray imaging
- software provides automatic acquisition of rectangular and
irregular polygon areas for up to 10,000 analyses
- large area mapping up to 25mm and larger
- output in numerical ASCII file and publication quality contour
plot, 3-D surface plot, gray or false color TIFF
- image processing software
Contact information:
Dr. Minghua Ren
Geological Sciences Bldg. 106D
Ph. 915-747-5843
Fax. 915-747-5073
e-mail: ren@geo.utep.edu