Elemental mapping

Elemental mapping is a microscopic technique where the distribution of elements over the surface of a sample is determined. It requires the use of an electron microprobe, using Energy-Dispersive X-Ray Spectroscopy or Wavelength-Dispersive X-Ray Spectroscopy

How it works

A beam of electrons is fired at a sample. The size of this beam determines the trade off between resolution and scan time.

The beam causes elements to emit X-rays at a characteristic frequency which can be detected by the electron microprobe.

The technique contrasts with the simpler and quicker back-scatter electron microscopy (BSE) which measures the electron reflectivity of a surface; this gives an indication of its composition but does not make it possible to determine the distribution of individual elements.

Uses

Mineralogy

The technique is most commonly used by mineralogists; the change in elemental composition from the centre to the edge of a mineral can yield information about the history of the crystal's formation, including the temperature, pressure, and chemistry of the surrounding medium.

Palaeontology

In exceptionally preserved fossils, such as those of the Burgess shale, soft parts of organisms may be preserved. Since these fossils are often compressed into a 2D fil, it can be difficult to deduce what features were what: a famous example is that of triangular extensions in Opabinia, which were interpreted as either legs or extensions of the gut. Elemental mapping showed that they had a similar composition to the gut, favouring the second interpretation.

For more information about element abundance in the Burgess shale, see Burgess_shale_type_preservation#elemental mapping