Welcome to the online learning module for microanalysis. This module will provide you with background information and practical advice for the successful operation of a range of microanalytical methods, with emphasis on techniques that are commonly hosted on SEMs and TEMs. A basic understanding of the operation of the SEM and/or TEM is assumed in the module, so visit the SEM or TEM modules to refresh your knowledge if necessary.
What is Microanalysis?
Microanalysis is the identification of the chemical elements present either within or on the surface of an object, and additionally, how the atoms of the elements are arranged with respect to each other. Identification of the elements present may be qualitative or quantitative. Different isotopes of the elements, or their ratios, can also be measured. Microanalysis can also require working out the spatial relationships between the atoms in the object, i.e., its structure, particularly if it is crystalline. Some microanalytical techniques can provide information about crystal defects, chemical bonding or redox state.
In microanalysis, the analytical spot size ranges from about 100 μm to 100 nm, although many techniques are capable of generating maps of larger areas millimetres or centimetres across. The depth of the analysis is also important, and some techniques will analyse only the top few nanometers of the sample (surface techniques) while others will penetrate to depths of several microns. Smaller length-scales (<100 nm) that allow analysis at molecular or atomic resolution are part of nanotechnology or nanoanalysis.
In most microanalysis techniques, the sample can be analysed in situ, whereas for nanoanalysis small portions of the sample may need to be cut out for analysis, and for many bulk techniques the sample needs to be pulverised or dissolved before it can be analysed.
Most microanalysis techniques involve focussing a microbeam on the object to be analysed and measuring an output beam that results from the interaction of the input beam with the atoms and molecules making up the sample. The input beam may comprise light (including laser beams), X-rays and other electro-magnetic waves, electrons, protons or ions. The outputs that are measured also include light, X-rays, electrons and ions.
There are many techniques available for microanalysis. For example, see http://www.ammrf.org.au/techniquefinder/ or http://en.wikipedia.org/wiki/List_of_materials_analysis_methods.
Two of the most important considerations in microanalysis are spatial resolution, or the size of the object or particle to be analysed, and the detection limit of the technique.
The diagram below shows some of the most commonly used microanalytical techniques arranged according to their spatial resolutions and sensitivities.
Scanning electron microscopes (SEMs) and transmission electron microscopes (TEMS) can be fitted with a range of detectors that can give information about chemical composition and structure, as shown in the diagram. The best technique to use will depend on the required spatial resolution and depth resolution, whether qualitative or quantitative chemical analysis, or structural analysis, is needed, and the minimum detection limit.
In this module, some of the most common techniques used in combination with electron microscopy will be presented: