Technology - the preparational part

Unfortunately, most samples don’t match the criteria to be directly explored by SEM, without prior time-consuming preparation. Especially this is true for biological samples, which are usually non-conductive material and (even worse) –contain water. Non-conductive specimens must be sputtered with a very thin layer of conductive material (gold, gold/palladium, platinum, etc), to prevent the accumulation of static electric charges during their exposition to the high-energy electron beam. Additionally, sputtering is very useful to maximize signal output and to improve spatial resolution.

A specimen is normally required to be completely dry, since the specimen chamber is at high-vacuum. Hard, dry materials such as wood, bone, feathers or shells can be examined with little further treatment. But living cells, tissues and other soft-bodied organisms usually require chemical fixation to preserve and stabilize the structure. Then fixed samples need to be dehydrated very carefully, to minimize artifacts. Because air-drying causes collapse and shrinkage, the drying method-of-choice is usually critical point drying. The time consuming method replaces water with ethanol or acetone and replaces these solvents in turn with a transitional fluid, such as liquid carbon dioxide at high pressure. This ensures minimal change of the original state (living) and therefore is a key element of SEM images.

Specimen artifacts

As described above, most specimen artifacts result from inappropriate preparation (fixation and drying). To guarantee the best possible results, we have optimized protocols for many different specimens.