7 - Chemical analysis in the transmission electron microscope

Summary

Introduction

Quantitative chemical analysis using the characteristic x-rays emitted by the specimen in the transmission electron microscope (TEM) is a natural extension of the standard electron microprobe, which is probably familiar to most mineralogists and geologists. However, because the TEM specimens are in the form of a thin foil, quantitative chemical information can be obtained from much smaller volumes of specimen and with relatively minor corrections compared with those necessary with the standard electron microprobe. The TEM technique has already made a significant contribution to mineralogical research, and its use is increasing rapidly. In this chapter, the essential features of x-ray chemical analysis in the TEM are summarized, together with some brief comments on the complementary technique of electron energy loss spectroscopy (EELS). A detailed account of the theory and practice of x-ray microanalysis and EELS has been given by Williams (1984) and is essential reading for anyone proposing to use these techniques.

When the high-energy electron beam in an electron microscope interacts with the specimen, the electrons may be deflected by the potential field of the atoms without loss of energy (i.e., the electrons are elastically scattered), or the electrons may be inelastically scattered (i.e., with loss of energy). The inelastically scattered electrons provide a means of detecting and determining the concentration of the atomic species present in a specimen. There are several mechanisms by which inelastically scattered electrons lose their energy.