Electron Probe Microanalysis instrumentation

Electron Probe X-Ray Microanalysis (EPMA) is a spatially resolved, quantitative elemental analysis technique based on the generation of characteristic X rays by a focused beam of energetic electrons. EPMA is used to measure the concentrations of elements (beryllium to the actinides) at levels as low as 100 parts per million (ppm) and to determine lateral distributions by mapping. The modern EPMA instrument consists of several key components ... 

Figure 5.27. A summary of the relationship between the spatial resolution and the MMF for X-ray microanalysis in several electron-probe instruments (references given). The two shaded areas represent the ranges of future EPMA microanalysis predicted by Newbury et al. (1999) and the future AEM microanalysis estimated by Williams et al. (2002). (Reproduced by permission of Williams et al., 2002.)...

Microscopes. There are two basic modes of operation for X-ray analysis in a modern-day AEMs with a static (or flood) beam and with a rastered beam. This instrument is essentially a conventional TEM with either (a) scanning coils to raster and focus the beam or (b) an extra NminiN (or objective pre-field) condenser lens to provide a small (nm-sized) cross-over of a static beam at the objective plane. Some AEM configurations contain both scanning coils and a third condenser lens whilst others may have only one of these. In either condition, a small-sized electron probe can be obtained as a static or a rastered beam. The basic electron-optical principles which provide nanometer-sized beams for microanalysis are similar to those for electron microdiffraction which are well described by Spence and Carpenter [19]. 

The earliest commercially available X-ray spectrometer appeared on the market in 1938. Ten years later, Friedman and Birks built the prototype of the first commercial X-ray secondary emission instrument. Somewhat later, Castaing and Guinier built the first electron probe microanalyzer using a focused electron beam to induce X-ray emission of the elements present in microscopic samples using electron probe X-ray microanalysis (EPXMA). The use of protons in PIXE for chemical analysis was first demonstrated by Sterk in 1964. From the 1960s until the present day, the use of large-particle accelerators (synchrotron rings) has resulted in a dramatic... 

Because the mechanical requirements of the ED detection geometry are moderate in comparison to the WD equivalent, in the more recently developed analogs or variants of the XRE method solid-state detectors are almost exclusively employed. Such related methods are electron probe X-ray microanalysis (EPXMA) and (microscopic) proton-induced X-ray emission (/r-PIXE). It should be noted, however, that many EPXMA instruments are also equipped with WD detectors (e.g., for low-Z element analysis). Important variants of the conventional XRF technique that have achieved... 

This technique encompasses a large scope of instruments ranging from mobile spectrometers to high resolution spectrometers. On-line analysers and probes that can be fitted to scanning electron microscopes (SEMs) allow instant analyses to be performed (i.e. microanalysis by X-ray emission). 

Linked with its qualities, assessed above, as an imaging and structural tool, the STEM assumes prime importance when considered as a microanalytical instrument. As pointed out in the introduction, the interaction of the fine probe in STEM with, potentially, only a small volume of the sample suggests the possibility of microanalysis on a scale hitherto unattainable. Two main areas will be considered here -the emission of characteristic A -rays by the sample, and the loss of energy from the primary beam in traversing the latter. Ideally, a fully equipped analytical electron microscope will utilize both techniques, since, as a result of the relative positions of A"-ray detector and the energy loss spectrometer in the electron optical column, simultaneous measurements are possible. However, for the sake of convenience we will consider the methods separately. 

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