PEMC offers a wide variety of sample preparation techniques and facilities, for many different kinds of material for both scanning and transmission electron microscopy investigations.
In the centre there are a range of optical microscope for macro-imaging, for use by itself or in conjunction with electron microscopy.
Almost anything can be looked at under an electron microscope (with the exception of any liquid that cannot be frozen and living specimens cannot be placed in a microscope due to the vacuum in the chamber). Typically samples need to be conductive for analysis, so any non-metallic, or naturally non-conductive samples will need to be sputter coated with a conductive material. There is the capacity to coat samples in: gold, chrome, gold-palladium and carbon (for chemical analysis).
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. they 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, using chemical and mechanical polishing to produce a high quality, stress free surface that is required for Energy Back Scatter Diffraction (EBSD) analysis.
For more information on how PEMC can help with SEM related sample preparation or more, please get in touch with a technician.
