EM Sample Preparation

Generally there will be little that you need to do to prepare your samples other than supply them to me. However for your background information the following brief outlines are provided.

Sample preparation procedures will depend on the type of sample that you have, but irrespective of your sample they are a number of constraints that your prepared sample must be meet in order to obtain a successful image. The three we will focus on are conductivity, stability and valid sampling.

Valid sampling

This is sometimes overlooked in sample preparation. The sample that you take to the EM must represent the initial sample in both the type of features and also the relative proportions of those features. This is easily achieved for dispersions in solution, but requires careful sampling techniques for solid samples. However on some occasions, finding out what the relative proportions of the sample features are, is irrelevant. On some samples identifying a particular component that is present is more important, on those occasions specifically sampling these components is the best option. Mostly the sub samples chosen are selected to represent both; the proportions of the features, and to specifically identify the features.

Conductivity and Coatings

In theory, in order to obtain an image the sample they must be conductive. This is not absolutely true but for practical purposes it is a good starting point. However, as most samples are not conductive they must be thinly coated with a conductive material, so that at least to the electron beam; which only penetrates micrometres into the sample, they will appear to be conductive.

SEM

Generally for an SEM this coat is either deposited as carbon by evaporation or as gold, gold-palladium, or platinum by sputter coating. Using these techniques, we are able to put a thin coat ( 5 to 40 nm thick ) over the sample, and as the coat is thin, at all but the highest magnifications it does not alter the appearance of the sample. When coating, flat smooth samples will present no problems, but rough convoluted surfaces will always have some areas which will be insufficiently coated, but no the less most surfaces can be covered to enable excellent image to be obtained. Generally samples will be coated with carbon when EDX analysis is required as the carbon minimizes extraneous x-ray peaks and minimizes x-ray absorbtion as the x-ray s leave the sample. Gold or platinum coats are used when imaging is required as they have higher secondary electron yields and a greater electron beam stopping power.

TEM

TEM samples are similarly coated with carbon to create conductivity. Carbon is used because it is a low atomic number material that asborbs the electron beam least, or sometimes shadowed with a thin metal coat, to create topographic contrast that is proportional to the coating thickness

Stability and Mounting

Samples must be stable so that they can be mounted successfully and also so that they do not move under the electron beam. Any sample movements after the sample is coated will break the thin conductive surface coat that is applied, which will in turn create sample charging problems when trying to obtain an image in the SEM. A combination of mechanical and chemical techniques are used to stabilize the sample, that would includes items such as; embeding resin, tape

Vacuum Compatibility

In most instances, the Enviornmental SEMs being the exception, the samples will ultimately be placed into a vacuum. This constraint means that the sample must be stable within that vacuum environment with vacuum levels of approximately 1E-5 torr. Additionally to enable the EM instrument to function correctly it must not have large amounts of absorbed gases or volatile materials in its structure that will be released at those high vacuum levels, this does include the presence of water.

For biological samples this often necessitates the use of dehydration techniques such as freeze drying, critical point drying and fixation or more recently using an electron microscope with a a cryogenic system.

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