Biological preparations, for example in the form of plant specimens must be prepared before they can be inspected under a microscope, such as an electron microscope. The preparation requires subjecting the specimens to a fixation process so that the tissue will retain its structure without deterioration prior to and during the microscopic inspection. The fixation process uses a fixation substance such as an aldehyde that penetrates into the tissue of the specimen, thereby preventing a change in the specimen that could otherwise be caused by the subsequent embedding steps and by the vacuum prevailing in the specimen chamber of an electron microscope.
Conventional fixation substances include, among others, aldehyde, primarily glutaraldehyde, formaldehyde, or even the unsaturated aldehyde acrolein, whereby the fixation substances are frequently also used as mixtures of these formaldehydes.
Conventionally, fixation substances are used in the form of aqueous solutions for the so-called immersion fixation. Less frequently, fixation may also be accomplished by using these substances in the gas phase for performing a so-called fixation by fumigation.
A substantial problem has been encountered heretofore in conventional fixation processes with regard to the speed at which the fixation substance enters into the tissue and fixes the fine structure of the specimen prior to any possibility of a change in that fine tissue structure. Such changes must be avoided since they defeat the purpose of the microscopic inspection. Especially the preparation of plant specimens or rather their fixation poses two problems. On the one hand, above ground plant portions are enclosed by a substantially water impermeable outer skin, the so-called cuticula. On the other hand, air is present in the intercellular interstices in the leaves of plants, whereby the penetration of liquid, including fixation solutions into the leaf tissue is made more difficult.
In a book entitled "Basics of Histochemistry" (Grundlagen der Histochemie), by Hans Luppa, published by Vieweg Verlag Braunschweig, 1978, section 1, pages 42 and 43, a method has been described for fixing biologic preparations by using aldehyde fixation substances. This publication further describes on page 52 that trichloroacetic acid may be effective by hydrolysis on the heteropolar valence bindings to thereby loosen the hydration status and that trichloroacetic acid penetrates quickly into the tissue to thereby cause a type of preliminary fixation.
In connection with most of the conventionally used fixation methods, the plant specimens are first mechanically comminuted. Thereafter, different fixation sequential steps are employed, depending on the particular type of plant specimen. These conventional fixation steps require a relatively large volume throughput of fixation solution, the disposal of which subsequently causes a critical environmental problem. Further, conventional fixation methods require repeated manual intervention by the operator so that conventional methods are not suitable for an automatically performed fixation sequence. Such automatically performable sequence of fixation steps, however, is indispensable for experiments to be performed, particularly in a spacecraft in outer space or in areas that are difficult to access, for example, outside in the field.