Analytical Methods:Electron Microscopy

SASSA Home Page &#8658; Analytical Methods Home Page &#8658; Specialist Techniques &#8658; Electron Microscopy

Electron Microscopy
Electron microscopes use a beam of electrons, rather than light which is used in optical microscopes, to examine the surface and internal structure of materials.

Questions
In archaeology electron microscopes are frequently used to study the microstructure of artefacts. However, electron microscopes are also used occasionally in geoarchaeology for analysis of particle and artefact surfaces to identify tell-tale trends of water or wind-blown transport, to quantify weathering, and to study post-depositional crystal growths. They also allow examination of microscopic crystalline bodies in soils such as faecal and earthworm granules.

Case study examples of the use of electron microscopy in geoarchaeology are:
 * The palaeohydrology of Iron Age sites in the Mun River Valley, NE Thailand.

Samples and sample preparation
Sample sizes for electron microscopes are determined by the size of the chamber and stages available, but are generally in the order of a few mm's or cm's diameter. In some some cases samples may need to be dried before analysis this can be damaging, particularly to organic samples. However, alternative methods are usually available and should be discussed with the instrument operator prior to submission of samples for analysis. Samples will need to be stabilised on the stage usually involving a special sticky pad. Many samples also need to be coated with a thin conductive coating to prevent the build up of electrical charge on its surface. Gold is usually used, but for more sensitive samples carbon may also be used.

For electron microscopy there is no need to section and polish samples unless element-analysis is required. Sectioning and polishing should not be carried out is the sample is to be imaged in a scanning electron microscope.

Electron microscope theory and equipment
Electron microscopes basically work in the same way as optical microscopes only the use a focussed beam of highly energetic electrons rather than light.

All electron microscopes consist of an electron source, which generated the electrons. An electrical potential is applied to accelerate the electrons towards the sample and a series of lenses are used to focus the beam. The detector analyses the way in which the electrons have interacted with the sample.

There are two main types of electron microscopes:
 * Scanning electron microscopes (SEM)
 * Transmission electron microscopes (TEM)

Scanning electron microscopes
In SEM the electron beam scans over the surface of the sample. Some of the electrons hitting the sample are backscattered (BSE), becuase the generation of BSEs depends on the atomin number of the atom it interacted with BSE detectors can be used to highlight areas of different composition in a sample. In other cases beam electrons interacting with a sample impart some energy to the atoms they interact with causing the atoms to release secondary electons (SE). The SEs provide information about the surface topography of the sample and are usually used in imaging.

With special detectors SEMs may also be able to detect the X-rays and auger electrons that are produced as a result of these interactions and which can provide information about the elemental composition of the sample.

Spatial resolution in a SEM is usually in the order of a few nanometers.

Transmission electron microscopes
The spatial resolution in a transmission elecron microscope may be as good as only a few atomic diameters (&Aring;). In TEM the electron beam is transmitted through very thin samples and the way in which the transmitted electrons are scattered is used to build a picture of the inner structure of the sample. This technique is less well used in geoarchaeology.

Data
Electron microscopy produces images of the sample, interpretation of these images is a specialist task.

Related techniques

 * Micromorphology
 * Micro-analytical techniques

 &larr; Back to Specialist Techniques