Scanning Electron Microscopy (SEM)
Sector: Extractive Industries
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 benefits of SEM over conventional microscopy include very high resolution and greater depth of field at magnifications from x200 to x1,000,000.
The James Hutton Institute SEM lab is accredited for the analysis of metal chips and debris.
- Carl Zeiss SIGMA VP Analytical Field Emission Scanning Electron Microscope
- Bruker QUANTAX 400 Energy Dispersive X-ray Spectrometer with Xflash 5030 Silicon Drift Detector
- Quorum Technologies PP2000T cryo-SEM preparation system
- Leitz Polarizing microscope equipped with a Spot ‘Insight’ digital camera (Diagnostic Instruments, Inc.
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.
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 displayed 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.
Wet or liquid sample are cryogenically frozen to -170oC. Following this, they can be analysed as solids using Secondary Electron Imaging, Backscattered Electron Imaging or X-ray Microanalysis.