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How can specimens be prepared for observation?

Found in: Howto

Many every day objects can not be directly viewed under the compound microscope, because they simply do not meet the criteria. These objects must first be prepared.

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After all, you have to be able to mount the object on a slide and sometimes whole insects, to use an example, are too large and thick. These objects must first be prepared before they can be placed on a slide. There are several ways of preparation and there is no single way of doing this. It all depends very much on the specimen that you want to look at.

A microscopist has to be careful when preparing the specimens as the preparation steps might also damage the object. Here I would like to give an overview of the different possibilities for preparing a specimen.

  • Whole mounts: Many specimens can be observed without any preparation at all. These specimens can simply placed on the slide and immersed in a drop of water. Such specimens are referred to as “whole mounts”. The specimen must still be sufficiently small and transparent. Tiny insects and water samples with algae are possible examples. In the case of pond water samples, the organisms are still alive and it is possible to observe movement and other processes of life. In any case, one has to be careful that the heat of the microscope lamp does not damage the specimen! The weight of the cover glass might also squash the specimen, so one has to be careful here.
  • Microtoming: If the specimens are too large, then one has to invest a bit of preparation time to make it suitable for observation. One possibility is to cut the specimens into thin sections. This process is called microtoming (micro refers to small and tome to cut). Here the specimen is sliced into very thin sections using a special device. Many commercial slides use specimens which were microtomed. It is an advanced technique which requires treatment in a separate chapter. The object to be observed first has to be prepared. Water has to be removed and it has to be stabilized with appropriate chemicals. The object then has to be embedded in paraffin before it can be cut. Otherwise there is the danger that the microtome knife does too much damage to the specimen. Beginning microscopists can try to make thin sections using a sharp knife by hand, but one will not get results of the same quality.
  • Squashing: Squashing the specimen is another possibility of preparing thick and large specimens. A very small sample of the specimen is placed on the slide and a cover glass is placed on top. The specimen is then squashed with one’s thumb until a thin film forms between the slide and cover glass. Do not slide the cover glass over the specimen. The pressure should be applied vertically. Otherwise the shearing forces might damage the specimen. The squashing technique only works with specimens that are very soft, otherwise the cover glass might break and cause injury to your fingers. Soft bananas and other soft fruit can be prepared this way. Harder fruits can be made soft by boiling them first.
  • Tearing: Tearing the object is a further possibility. This has to be done carefully, otherwise the specimen is destroyed too much. The object is carefully taken apart with two dissecting needles. The objects have to be soft enough. The organs of insects, for example, can be processed this way. It goes without saying that a combination of the above techniques is often used for preparation.
  • Scratching: Some samples can also be obtained by scratching the surface. Take a knife and scratch over a freshly cut surface of a potato. This will remove starch grains for observation.
  • Polishing: Rock samples and other hard substances can be polished until they are very thin. This is something for experts and requires patience and experience. You also need the correct tools.
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