Brief Introduction to Specimen Trimming

Step by step trimming procedure

September 22, 2014

Before ultrathin sectioning a sample with an ultramicrotome it has to be pre-prepared. For this pre-preparation, special attention must be paid to the sample size (size of the section), location of the sample (targeting) and accuracy of the block-face edges. This process is generally called trimming, wherein the sample is shaped mainly to a frustum of a pyramid.

Specimen trimming is needed prior to ultramicrotomy

For instance, to achieve the optimum sectioning results of biological embedded samples, the surrounding embedding material has to be entirely removed. Thus, only the sample is present in the block-face and a "homogeneous" material can be sectioned. Especially when sectioning with a glass knife, the embedding material would influence the section results due to the different compression behaviour. Additionally a further effect on the sectioning results is seen in the edges of the block-face. Only two exact parallel trimmed edges produce a straight ribbon of sections. Furthermore, the sharpness  of the  trimmed edge has a direct relationship to the section thickness which can be achieved. Finally, a rule of thumb should be mentioned: "The smaller the block-face is trimmed, the easier the sectioning performance will be."

Conventional trimming of embedded samples involves the use of razor blades and a great deal of skill by the ultramicrotomist. The shape of the block face and the straightness of the edges of the trimmed sample has a profound effect on the sectioning characteristics. Parallel edges top and bottom are a must. To achieve this quickly and safely is challenging and potentially hazardous with a razor blade especially on industrial materials. Important specimens can be destroyed by a minor inaccuracy in trimming.

Video "Trimming of Metals"

Fig. 1: Front face observation to determine the area of interest
Fig. 2: Perpendicular viewing for distance definition between sample and front face of the embedding material
Fig. 3: Observation of the sample during milling


Fig. 4: Liver sample trimmed (front face observation)
Fig. 5: Embedded Si sample (perpendicular view with distance definition graticule)
Fig. 6: Diamond miller used for embedded Si sample