Scanning Electron Microscopy

Left: Secondary electron image of soil that has been freeze fractured. Right: Back scatter electron image of the image on the left. (J. Devonshire)

The scanning electron microscope works by raster-scanning a narrow beam of electrons across the surface of the sample, which then produces secondary electrons (formed by ionisation), backscattered electrons (elastically scattered) and X-rays.

The secondary electrons form a high resolution topographical surface image with good depth of field due to the narrow beam of electrons. The backscattered electrons form an image that indicates the distribution of different elements. This can suggest the elemental composition in soil samples and can be used in biology to image immunogold labelling.

Characteristic elemental x-rays are also detected allowing the localisation if specific elements and their distribution. This technique is called elemental dispersive spectroscopy (EDS).

Sample preparation for scanning electron microscopy is either by dehydration and critical point drying or by cryo-SEM of fully hydrated samples. Cryo-SEM is the preferred method to preserve the sample in the state closest to real life. All samples need to be coated for better imaging. Gold-palladium, chromium or carbon (for EDS) are available in Bioimaging.

Left: Image of plant leaf with selected points for elemental dispersive spectroscopy (EDS) analysis. Right: Elemental spectrum of 'Spectrum 1' from the image on the left. The sample is coated with gold accounting for the high Au peak.