Cameca SX100 electron microprobe

Electron probe microanalysis (EPMA)

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True-colour cathodoluminescence image taken with the Cameca SX100. Fluorite (blue-violet) is shown associated with calcite (yellow-orange) in a carbonatite from India.

Key facts

Technique: electron probe microanalysis (EPMA)

Quantitative analyses obtained for: elements with an atomic number greater than five

Typical detection limits: in the order of 0.02 weight per cent

The wavelength dispersive X-ray (WDX) electron microprobe is capable of unattended overnight elemental analysis of materials in polished sections or blocks. Images from the sample surface can also be obtained.

Analysis is achieved by focusing a beam of electrons onto the sample surface, which produces X-rays with wavelengths or energies characteristic of the elements present.

Detectors

  • Five wavelength dispersive X-ray (WDX) spectrometers
  • Bruker AXS 4010 XFlash silicon drift energy dispersive X-ray (EDX) detector
  • secondary electron detector
  • backscattered electron detector
  • panchromatic cathodoluminescence detector

Applications

  • In addition to point analysis, the Cameca SX100 can be used to obtain X-ray maps that show the spatial distribution of elements in selected areas of the sample surface.
  • By using a very large aperture and defocused beam at high current, the image obtained on the charge-coupled device (CCD) optical camera can be displayed and recorded as a true-colour cathodoluminescence image. 
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An artificially coloured backscattered electron (BSE) image and three X-ray maps showing the distribution of tin (Sn), niobium (Nb), and scandium (Sc) in a small grain of tin- and scandium-bearing titanite in a sample of rhyolite from north Wales. The grain displays sector zoning. Scale bar = 100µm.

 

Sample preparation

Samples need to be polished, either as circular blocks or thin sections, and carbon coated. Samples that need to be prepared (cut, mounted and polished) should be presented to the preparation laboratory at least four weeks prior to the Cameca SX100 booking.

The Museum’s Imaging and Analysis Centre uses on-site saws, laps and polishing equipment to produce a range of thin and polished sections, blocks and wafers of palaeontological, mineralogical, biological and industrial materials such as concretes.

Stages of sample preparation
  • Cutting with a saw and trimming to a size that can be mounted.
  • Mounting in one of two ways:
    • Into a resin block: the sample is placed in a circular mould and impregnated with a resin. This option allows for additional imaging with an optical microscope using reflected light.
    • On to a glass slide: the sample is affixed to a standard-size glass slide with resin for further lapping and polishing. This method allows imaging to be achieved using both transmitted and reflected light microscopy.
  • Removal of excess sample and grinding to ensure the top and base of the resin blocks are parallel.
  • Lapping to produce a smooth surface. This may be performed by hand or on an automated jig using a fine (600 grade) abrasive, usually silicon carbide.
  • Polishing is carried out in steps on a special cloth with a slurry of very fine alumina or diamond ranging in size from 0.3-6 microns.
Carbon coating

Prior to analysis many samples - particularly those containing minerals - need to be coated with a thin film of conducting media to dissipate the excess charge produced by the electron beam. A film of carbon around 30 nanometres thick is usually used.

Museum lead

John Spratt, Electron Probe Microanalyst

Submitting a sample

Materials should be:

  • dry
  • stable under a vacuum
  • capable of withstanding temperatures of up to 80°C

Unsuitable samples include:

  • wet materials
  • those with high vapour pressures
  • gels
  • organic materials
  • materials that decompose at low temperatures
Sample sizes that can be accommodated

Polished blocks:

  • 25mm diameter (maximum of six)
  • 30mm diameter (one position only)
  • 32mm diameter (one position only)
  • 40mm diameter (two positions only)

Polished thin sections:

  • 48mm x 26mm (up to four sections)
  • 48mm x up to 80mm
  • 25mm diameter