Electron Probe X-Ray Microanalyzer (EPMA)

About the lab

Electron Probe X-ray Microanalyzer

The Electron Probe X-ray Microanalyzer (EPMA) is used to determine the chemical composition of solid materials on a microscopic scale, down to volumes of a few cubic micrometers. Smaller particles can also be analyzed, although at considerably lower levels of accuracy).

In this analytical technique a very narrow beam of high-energy electrons is focused at a selected point on the surface of a flat and highly polished sample. As the electrons penetrate the sample their energy is first released into and then re-emitted from the sample, via different processes giving rise to different types of signals, each of which carries information about some property of the sample.

X-ray signal

The X-ray signal carries information about the chemical composition of the microvolume where it was generated. By means of an appropriate detection system, it is possible to eventually obtain qualitative (elements present) and quantitative (weight %) analyses. The detection of the X-rays is accomplished by means of crystal spectrometers (WDS, wavelength dispersive spectrometry) and solid state Si(Li) detectors (EDS, energy dispersive spectrometry). A computer and specialized software are also needed to achieve that end, ultimately making it possible to generate large volumes of high quality data, either in a manual or in an automatic, unattended mode.

Other siganals

Other signals typically utilized in an electron microprobe are Secondary Electrons (SE) and Backscattered Electrons (BSE). The SE emission intensity is most strongly modulated by surface topography, that of BSE by compositional variations. Both signals are collected while the beam is being scanned on a very small portion of the sample surface. The reconstruction of the spatial distribution of the collected signal intensity generates two-dimensional images (maps) displaying surface relief (SE) and compositional contrast (BSE). “Chemical maps” of the sample surface can similarly be obtained using the X-ray signal.

Our microprobe is a Cameca SX50 instrument, with 3 wavelength dispersive spectrometers and diffracting crystals allowing analysis of elements from B to U with detection limits that can vary from a few ppm to several tens of ppm.

Analytical and automation software is the Enterprise version of the program (Probe for Windows) written by J. Donovan and marketed by Advanced Microbeam.

A Link (Oxford) Pentafet EDS detector and pulse processor, along with PC-based URSA software are used for qualitative analysis. The detector has a Be window, allowing for the detection of elements having atomic number 11 (Na) and higher.

Links

There are many web sites where you can read about EPMA, discussed in lesser or greater detail. Among those:

http://www.geology.wisc.edu/~jhonf/empa.html
http://jan.ucc.nau.edu/~wittke/Microprobe/Probe/html
http://epmalab.uoregon.edu/cameca.htm
http://epmalab.uoregon.edu/epmatext.htm

For links to other Cameca SX50 microprobe labs:

http://www.geology.wisc.edu/~johnf/sx50-users.html

Probe Availability