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.
- 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.
- The X-ray signal carries information about the chemical composition of the micro-volume 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 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 displaying surface relief (SE) and compositional contrast (BSE).
- Our microprobe is a JEOL JXA8230 instrument, with 5 wavelength dispersive spectrometers and various combinations of different diffracting crystals:
|Crystal||2d (A)||Range of elements|
|TAP||25.757||F ~ P (Ka )|
|LIF, LIFH||4.027||Sc ~ Rb (Ka)|
|PETJ, PETH||8.742||Si ~ Mn (Ka)|
|LDE1||60||C ~ F (Ka)|
|LDE2||95||B ~ N (Ka)|
Detection Limits depends on the element of interest and its matrix, (e.g. 6 ppm for Fe, <30 ppm for Se, etc.). Detection limit in weight percent for a single analysis line with a confidence of 99% (assumes 3 standard deviations above the background, JEOL).
Where : ZAF is the ZAF correction factor for the sample matrix
I is the raw count rate on the analytical standard and unknown samples
Cstd is the pre-determined concentration in the standard
Spatial resolution varies from ~3 um in diameter for small particles to mm2 level for raster areas.
Penetrating depth can be varied for film/coating analyses, varying from submicron to over 10 um depending on the element of interest and its matrix.
Liquid nitrogen free, high count-rate silicon drift detector (SDD) EDS spectrometer.