Training a graduate Sedimentary Petrography class on SEM-EDS

Today I collaborated with Dr. Kristina Butler, a sedimentologist in the UT Dallas SESS department, to train her Advanced Sedimentary Petrography class on Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) techniques. We discussed a few topics that I think are important to point out here on the blog.

Can thin sections be used in a SEM?

• The short answer is yes, thin sections can be used in a SEM. The main stipulation to this being that you will need to have your thin sections prepared without a cover slip. You will want to coat your thin section with Au or C to provide a conductive layer so that you get a clear image and reduce charging. If you have a cover slip attached to the thin section then you will not be able to coat the sample directly. Instead, you will coat the cover slip giving you no visual of the sample. In addition, a coverslip will attenuate EDS analysis and give a nasty Si peak that drowns out other peaks in the spectra.

Courtesy of struers.com

Can you apply a live coordinate system to the sample through the SEM?

• While there may be some higher end SEMs that have this feature, most entry-to-mid level SEMs will not have this feature.

What is the difference between backscattered electron (BSE) and secondary electron (SE)?

• When the electron beam hits the target sample is produces both elastic and inelastic collisions between electrons and the atoms within the sample. Elastic scattering changes the trajectory of the incoming beam electrons when they interact with a target sample without significant change in their kinetic energy. Larger atoms with a higher atomic number have a higher probability of producing elastic interactions because of their greater cross-sectional area. This is why they carry chemically sensitive information allowing users to spot minerals with heavier elements that appear brighter. Secondary electrons are low-energy electrons ejected from the surface of the sample itself and are the result of inelastic interactions. These allow for higher resolution of the samples texture and topography.

BSE v.s. SE - Courtesy of nanoscience.com

Demonstrating how rock/mineral samples are prepared for SEM analysis.