Electron Microprobe Notes: Introduction

1.1. Introduction to Microbeam Techniques

A large number of microbeam techniques (those that use sharply focused incident beams of particles or energy) have been developed for the analysis of geological materials. The table below lists the more familiar techniques.

Incident Beam	Technique	Secondary Effect or Measured Signal
White light	Reflected light microscopy Polarized light microscopy UV-IR microspectrometry	Reflected light Transmitted light Transmitted light
Continuous laser light	Micro-Raman spectrometry Selected-area gas release mass spectrometry	Scattered light Heat & Released gas
Pulsed laser light	Laser-ablation inductively-coupled plasma mass spectrometry (Laser ICP-MS)	Volatilized material
X-rays	X-ray microscopy Micro X-ray fluorescence analysis (Micro XRF)	Transmitted X-rays Characteristic X-rays
Electrons	Transmission electron microscopy (TEM) Atomic number imaging and analysis Scanning electron microscopy (SEM) Electron microprobe analysis (EMPA) Cathodoluminescence microscopy Auger microprobe analysis	Transmitted electrons Backscattered electrons (BSE) Secondary electrons Characteristic X-rays Visible light Auger electrons
High-energy protons	Proton-excited X-ray emission analysis (PIXE)	Characteristic X-rays
Low-energy ions	Ion microprobe analysis (IMP)	Sputtered secondary ions

Most techniques are non-destructive, but several (in italics above) result in sample damage (pitting). In laser ICP-MS, incident pulsed laser light volatilizes material from the sample, and in IMP (SHRIMP = Sensitive High Resolution Ion MicroProbe) low-energy ions ablates the sample leaving holes.

Nondestructive techniques are more numerous. Many use characteristic X-rays to analyze the sample material. Micro XRF uses an incident X-ray beam to produce X-rays; EMPA, scanning electron microscopy (SEM), and cathodoluminescence microscopy utilize a focused electron beam to produce characteristic X-rays, secondary electrons, and visible light, respectively, from the sample. PIXE uses a focused beam of high-energy protons to produce characteristic X-rays from the sample.

This course will focus on electron microprobe (EMP) analysis, which is probably the most commonly applied micro-analytical technique for materials analysis. In addition to characteristic X-rays, most microprobes are equipped to analyze other effects produced by electron bombardment of a sample, including backscattered electrons, secondary electrons, and cathodoluminescence. Thus the study of EMP analysis will incidentally cover many of the other analytical techniques. In addition, this course will briefly cover X-ray fluorescence (XRF) analysis, a non-microbeam technique very similar to EMP analysis, because it is a very commonly utilized for materials analysis.