Streamlined Microanalysis in the Scanning Electron Microscope (SEM)

Instruments Used

Gatan Microscopy Suite® combined with EDAX’s Octane Elite EDS system and Gatan’s Monarc® detector to acquire x-ray signals and cathodoluminescence (CL) simultaneously.

Background

CL in the scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDS) are fundamental microanalysis methods in analytical sciences.

With EDS, an x-ray fluorescence spectrum uncovers the composition of a sample according to characteristic x-rays originating from the atomic structure of an element, while CL employs optical spectroscopy to uncover electronic and optical properties produced from the presence of defects or trace elements and the crystal structure.

A full investigation frequently involves characterization utilizing both the CL and EDS techniques. For example, in many gems, cultural heritage items and minerals, the CL signal presents the distribution of individual trace and minor elements at concentrations much lower than the sensitivity level of EDS.

Materials and Methods

Traditionally, it has been required to gather CL and EDS maps in sequence as the CL detector can block the x-ray signal.

Inconsistent analytical working distances frequently meant that the sample had to be moved between analysis methods, which results in increased electron dose, extended data collection times and complexity in image registration.

The efficient workflow utilizing the EDAX EDS detectors and Gatan Monarc CL detector allows the CL, SEM and EDS signals to be acquired at the same time within the Gatan Microscopy Suite software.

A shared analytical position can be used thanks to the proprietary collection mirror, maintaining excellent collection efficiency and a line of sight to the EDS detector.

An example dataset was taken from a kyanite (Al2SiO5) thin-section. Kyanite was selected because of its importance as a resource for the refractory market and the ambiguity over the role that trace and minor elements perform in establishing the refractoriness.

(left) Secondary electron image of kyanite thin-section; (center) EDS map revealing a predominantly coarse-grained quartz-kyanite segregation with small amounts of staurolite; (right) distribution of chromium in the kyanite phase extracted from cathodoluminescence data revealing significant intra-grain segregation indicative of multiple generations of formation—a similar map for titanium was also deduced but is not shown here for clarity.

Figure 1. (left) Secondary electron image of kyanite thin-section; (center) EDS map revealing a predominantly coarse-grained quartz-kyanite segregation with small amounts of staurolite; (right) distribution of chromium in the kyanite phase extracted from cathodoluminescence data revealing significant intra-grain segregation indicative of multiple generations of formation—a similar map for titanium was also deduced but is not shown here for clarity. Image Credit: EDAX

Summary

Results that could not be achieved using each technique alone were collected by capturing both CL and EDS signals. The distribution of Cr and Ti trace impurities inside kyanite and three separate mineral phases were revealed.

The methods of CL and EDS clearly work in harmony, and the feature of simultaneous acquisition provides a significant advantage for the analysis of specimens.

Credit

A special thanks to Dr. Giulia Degli-Alessandrini of The Open University for supplying the specimen utilized in this experiment.

This information has been sourced, reviewed and adapted from materials provided by EDAX.

For more information on this source, please visit EDAX.

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