Analytical procedure (EPMA)


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by Pierre Lanari (last update Nov. 2018)

If you want to acquire high-quality X-ray maps for quantitative analysis, we recommend you to follow the procedure given in Lanari et al. (2018).

The analytical session is classically divided into two parts: (1) semi-quantitative compositional mapping via X-ray maps and (2) in-situ spot analyses of mineral compositions used to calibrate the X-ray maps. The section below briefly outlines some important aspects of this analytical procedure along with recommended electron microprobe operating conditions.

Microprobe elemental mapping (calibration)

  • Wavelength dispersive mode
  • 15 KeV accelerating voltage
  • 100 nA specimen current
  • 200-300 ms dwell time per pixel for optimal precision.
  • 1-10 μm resolution (size of pixel)

The acquisition is typically realized in two passes that includes 8-10 elements depending on the number of spectrometers available and the elements Na and K must be acquired during the first pass.

Be careful while defining the image resolution. Keep in mind that the pixels at the boundaries between two mineral phases will be mixing pixels, they represents mixed composition between the two phases. The smallest analyzed object/mineral grain must have the size of 5-10 pixels. We recommend using the smaller beam size possible to reduce the mixing effect; always use a beam size smaller than the pixel size!

Additional details and examples are given in the XMapTools’ user-guide

Microprobe point analyses (calibration)

  • Wavelength dispersive mode
  • 15 KeV accelerating voltage
  • 10-20 nA specimen current
  • ~40 s dwell time including background measurement.

Several analytical transects consisting of about 40 point analyses each are required and these transects should cross-cut the main mineral grain boundaries. We recommend using at least one horizontal and one vertical profile for which the position is well known. A total of 10-20 point analyses per mineral phase is required for an optimal standardization (bad analyses will be rejected during the processing). The step size between two point analyses in transects should be a factor of the pixel resolution.

As the thin-section can move between the mapping and the point analyses, be careful to locate the profiles in a BSE image with the best possible precision, i.e. select the first and last point of each profile/transect on recognisable features and go across different areas of the mineral assemblage.

It is also possible to add (in addition) some point analyses to sample all the phases.

Analytical standardization