Analytical procedure (EPMA)


In order to acquire high-quality X-ray maps for quantitative analysis, it is recommended to follow the procedure provided in De Andrade et al. (2006) and Lanari et al. (2014). There are two parts required in this microprobe data acquisition procedure that involves (1) qualitative elemental mapping and (2) in-situ spot analyses of mineral compositions used in standardization of 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 of the image resolution and always remember that the pixels at the boundaries between two mineral phases represents mixed composition pixels. The smallest analyzed object/mineral grain must have the size of 5-10 pixels. Use a beam size of < 5 μm and that is always < pixel size!

Microprobe point analyses (calibration)

  • Wavelength dispersive mode
  • 15 KeV accelerating voltage
  • 10 nA specimen current
  • 20-40 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 (e.g. transects across symplectites, etc.) A total of 20 point analyses per mineral phase is recommended for a good standardization (bad analyses will be removed during the processing). The step size between two point analyses in transects should be greater than 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 a starting point and an end point for each profile/transect across different areas of the mineral assemblage.

Analytical standardization