Preparation of samples for electron probe micro-analysis
A member of the analytical centre lapping a sample to produce a smooth surface. The process can also be automated.
Some sample preparation is required for most quantitative analytical work using an electron microprobe. Using its on-site saws, laps and polishing equipment, the Museum’s analysis centre produces 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.
- Onto 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 resin blocks are parallel.
- Lapping to produce a smooth surface. This may be performed by hand or on an automated jig using fine (600 grade) abrasive - usually silicon carbide.
- Polishing - carried out in steps on a special cloth with a slurry of very fine alumina, or diamond, ranging in size from 6 to 0.3 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 excess charge produced by the electron beam. Usually carbon is used and the film is typically 30 nanometres thick.