Electron probe microanalysis (EPMA)

About this technique


Typically you would choose to do electron probe microanalysis (EPMA) when accurate quantified chemical compositions are required. It is commonly used to determine the compositions of mineral phases for geological samples but is also used in cases where quantified compositional information is required e.g. metal phases within welds, zoning or compositional variations across minerals or within mineral phases.

EPMA is a powerful analytical tool of electron microscopy, unique in its mode of non-destructive X-ray analysis of elements from boron to plutonium. It uses wavelength dispersive spectroscopy (WDS) to count X-ray peak intensities and their associated background counts with superior X-ray energy resolution than the commonly used energy dispersive spectroscopy (EDS). Certified standards of materials, minerals and metals are used in EPMA to calibrate the measurement of X-ray intensities for comparison with the sample of interest. The raw data is then processed with corrections associated with atomic number, X-ray absorption and fluorescence to produce highly accurate quantitative compositions of the sample.

Typical EPMA analyses of oxide minerals will acquire raw data of cation element concentrations and subsequently calculate oxygen concentration based on the valency of those cations analysed. Detection limits for cations can be as low as 100 ppm depending on the specific element of interest, the instrument, the acquisition times and calibration.

In contrast to the normalised standard-less X-ray analysis most commonly used in EDS on the scanning electron microscope, EPMA provides highly quantitative compositional data as it incorporates a fully calibrated analytical technique. EDS has X-ray energy resolution in the order of 120–150 eV whereas WDS used on the electron microprobe has X-ray energy resolutions in the order of 10 eV. This improved X-ray energy resolution allows you to resolve complex overlaps in the X-ray energy spectrum using the EPMA, which is not possible to resolve using EDS. An example would be in the analysis of rare-earth elements where common peak overlaps are observed with lead and sulphur.

For each analysis EMPA provides a sum total of element concentrations. The value of this sum provides additional information about the quality of the data, e.g. are all possible elements included in the analysis, is the mineral hydrated, what is the true the valency of each cation and are there possible overlaps in the X-ray spectrum?

Sample preparation for EPMA is critical for the quality of data. Since the X-ray intensities generated on the unknown sample are being compared to that of highly polished certified calibration standards it is important that the unknown sample be prepared and polished as well as the standards. Poor sample polish will increase X-ray scattering from the sample and result in poor analyses. Samples that are not easily prepared as flat polished samples can be difficult to analyse by EPMA. Due to the higher beam currents used in this technique, those samples that degrade easily under higher beam currents may not be possible to analyse.



Contact an expert

The University of Queensland
Mr Ron Rasch
T: 07 3365 7939
E: r.rasch@uq.edu.au

The University of Western Australia
Dr Malcolm Roberts
T: 08 6488 2770
E: malc.roberts@uwa.edu.au

The University of New South Wales
Dr Karen Privat
T: 02 9385 6468
E: k.privat@unsw.edu.au

SARF – The University of Adelaide
Mr Angus Netting
T: 08 8303 3134
E: angus.netting@adelaide.edu.au

James Cook University
Dr Kevin Blake
T: 07 4781 4864
E: kevin.blake@jcu.edu.au

University of Tasmania
Dr Karsten Goemann
T: 03 6226 2146
E: karsten.goemann@utas.edu.au