For biological samples there are a few methods available that can be used depending on the type of sample and analysis needed.
For small biological samples, i.e. nerve cells, fly brains, micro plankton, etc. for analysis under TEM or STEM, a thin slice of the sample is needed to allow for electron transmission. The samples are first fixed, next they have to be dehydrated to remove any water from within the samples, and then placed in resin, which is polymerised by baking overnight to produce hard resin blocks. To produce thin sections of the sample, the resin embedded samples are cut down and sectioned on either of the microtomes available (Leica EM UC 7 Ultramicrotome or Reichert-Jung Ultracut E), when a slice with an area of interest is found, it is placed on a copper grid ready for analysis in the TEM or STEM.
For samples that do not need to be sectioned, there are a few different methods of sample prep that can be done.
Larger samples such as insects, hair, pollen etc. can simply be air dried to remove water, before being coated, ready for analysis.
Other samples may require more preparation: most biological samples especially microbes and fungus will need to be fixed before drying can occur. Once fixed, the samples are first dehydrated with alcohol, samples are then dried by replacing the alcohol with liquid CO2. The samples are then heated and put at pressure so that the liquid CO2 becomes a gas and is removed.
Another drying process available is freeze drying. Samples are cryo-fixed using liquid nitrogen and then placed in a vacuum to remove the water from within the sample via sublimation.
The samples are then ready to be coated.
Some material samples may only need a small amount of prep. For looking at the texture, structure, and some basic chemical analysis (Energy Dispersive Spectroscopy- EDS), any metallic or conductive samples will not need any preparation before SEM analysis. Non-conductive samples will need coating before analysis.
If the crystal structure of the sample, or more in-depth chemical analysis (EDS) is required, then a planar surface is needed, and therefore more preparation is required. Samples will be cut down to size and a cross section surface created. For small samples they can be compression mounted, or, for more porous or fragile samples they can be embedded in resin. Larger samples may not need mounting and can be left as is. The samples are then ground and polished down using progressively smaller grit sand papers and polishing cloths until a scratch free surface is achieved. If needed the samples can then be coated.
At PEMC there is capacity for a final vibratory polishing step on a Vibromet. This final polishing step uses chemical and mechanical polishing to produce a high quality, stress free surface that is required for Energy Back Scatter Diffraction (EBSD) analysis.
The lab has JEOL IB-19520CCP cooling cross sectional polisher. This instrument allows for a much finer polish to be achieved when manual methods are not enough or a sample is too small to be able to be sectioned by hand. It works by using an ion beam to clean the surface and precisely mill away at the sample.
The CCP can be used in two polishing modes: planar polish using a rotational holder to polish a larger area on the sample surface, and cross sectional polish using the standard holder to polish a cross section of up to 3mm thick. The polisher also has the capability to conduct polishing at low temperature conditions, using a liquid nitrogen cooled cold finger.
Any samples that aren't stable outside of evacuated conditions can be transferred to either of our JEOL FEG-SEMs under argon gas using a vacuum transfer system, reducing the risk of surface tarnish and contamination after polishing.
