Electron Microscopy is an extremely versatile tool which allows the study of both morphology and material composition from virtually all areas of science and technology.
The EM section has a Scanning Electron Microscope (SEM) fitted with an Energy Dispersive Spectrometer (EDS) for elemental analysis. High resolution Secondary Electron Imaging, Backscattered Electron Imaging, X-Ray microanalysis and X-Ray mapping are all possible. In addition, the SEM is equipped with a cryogenic stage allowing for the preservation of fluid phases in the samples, and analysis in situ if necessary.
Typical samples analysed by the EM Section include:
- Biological (plants, fungi, bacteria, insects)
- Geological (mineralogy, grain size, mineral relationships, porosity)
- Fibres (glass, asbestos, natural, man-made)
- Powders and Dust
Secondary Electron Imaging
A beam of electrons is scanned across the surface of the specimen; images are built up from low energy secondary electrons which reflect the topography of the sample. The benefits of SEM over conventional microscopy include very high resolution and greater depth of field at magnifications from x20 to x100,000.
Backscattered Electron Imaging
Backscattered electrons (BSE) provide an extremely useful signal for imaging in scanning electron microscopy as they respond to composition (atomic number or compositional contrast) and to local specimen surface inclination (topographic or shape contrast). BSE images obtained from flat polished surfaces reveal compositional changes due to variations in the average atomic number across the specimen.
The electron beam is finely focused onto the specimen resulting in characteristic X-rays being produced from a microvolume (approximately 1µm3) of the sample. These X-rays are detected by an Energy Dispersive Spectrometer (EDS) and the results plotted as a spectrum. Each element has its own ‘fingerprint’ of peaks which allows both a qualitative and quantitative determination of the elements present in the selected region of the sample. EDS is an essential tool in geological applications combining elemental composition and morphology to identify minerals. It also has uses with biological specimens for example, localising elements such as calcium, potassium or phosphorus.
Digital elemental distribution maps can be collected simultaneously with electron image acquisition thus giving a visual representation of the chemical distribution in the sample. X-ray mapping is performed using Position-tagged Spectrometry (PTS), a method whereby X-ray photons generated by the scanning electron beam are tagged with the position of their origin. From a PTS file, data can be extracted to form images, elemental maps and spectra. In PTS files a full spectrum is stored at each pixel, therefore additional elements can be mapped after the initial acquisition of the data.