Wetting agents for biological electron microscopy

SUMMARY When preparing specimens for shadowing it is frequently necessary to add a wetting agent to aqueous suspensions to enable them to spread on hydrophobic support films. However high‐resolution platinum/carbon shadowing is capable of visualizing wetting agent molecules, so the effect of added wetting agent on the final appearance of the specimen must be considered. To facilitate interpretation of the specimen fine detail the background should be smooth and flat. This is achieved by using the concentration of wetting agent which produces a close‐packed monolayer of wetting agent molecules after drying. Bacitracin is a better wetting agent for use when shadowing than the commonly‐used bovine serum albumin (BSA). Because its molecules are smaller than those of BSA, the size of the background granularity is smaller, which facilitates interpretation of the specimen, and the layer of wetting agent molecules is thinner, so small specimen details are less likely to be buried and rendered invisible to the shadowing process. These points are described in detail and illustrated with the help of models where necessary.