Scanning electron microscope and microprobe

Description:

In 2021, the CRPG acquired a JEOL JXA 8230 electron microprobe. This machine is dedicated to quantitative chemical mapping. The electron gun uses a tungsten filament, and the accelerating voltage can be varied between 0.2 and 30 kV, depending on the required use.

This microprobe is equipped with 5 wavelength dispersive spectrometers (WDS), each with 2–4 crystal monochromators and a P-10 or Xenon X-ray counter. This configuration allows precise, in-situ quantitative analyses of all elements from Boron to Uranium. Furthermore, an energy dispersive spectrometer (EDS) allows the rapid identification of elements and can be used in conjunction with the other spectrometers for simultaneous analyses of elements present at very different concentrations.

Secondary electron (SE) and backscattered electron (BSE) detectors are used to acquire images at magnifications from 40× to ~100,000×.

BSE images reveal compositional contrasts based on the molecular mass (average Z) of the sample at the scanning level. A panchromatic cathodoluminescence (CL) detector is also attached to the microprobe, which efficiently reveals zoning defects in crystals (for example in olivine, quartz, zircon, apatite…).

The volume of interaction between the beam and the sample depends on the analytical conditions, but is estimated to be 1 µm³ for classical analyses of silicates; this volume is reduced at lower accelerating voltages and higher sample densities. That said, if very mobile (e.g. alkaline) elements are being analysed, the beam can be defocused to 20 µm in diameter.

Detection limits also depend on the analytical conditions (they are higher for lighter elements, i.e. from Boron to Fluorine) and are generally between 10 and 200 ppm.

The wide range of analysable elements is made possible by the CRPG’s extensive catalogue of very well characterised standards.

Analytical samples must be solid and preferentially in perfectly polished thick or thin sections (ideally 1” in diameter). For non-conductive samples (natural rocks, experimental silicate samples…), a carbon coat 20–30 nm thick is necessary to evacuate incident electrons.

Analyses:

• Point-by-point quantitative analyses: all elements present are quantified to determine the composition of the sample at the analytical point (~1–20 µm in diameter).
• Qualitative and quantitative elemental mapping: the spatial (2D) distribution of selected elements is obtained by scanning the surface and is represented in single- or multi-elemental maps. Chemical maps spanning multiple centimetres can be obtained in our facility.