Essentially everything is visible in the STEM, and impurities may well not wash off the grid. Most biologists have some assay for the biological activity of their specimens. However, many preparations contain biologically inert but physically visible material that may make it difficult to obtain good STEM data. Particulates from columns or gels are a problem. Additives for activity such as BSA, PEG, or trypsin are a problem.
Precipitations can concentrate the specimen, but also enrich contaminants.
At times, the particles of interest can be distinguished from contaminants. However, it is clearly preferable to eliminate contaminants when possible. The necessary concentration of a given specimen to give a good distribution of particles on a grid is somewhat empirical. However, for data taking it is very important. If the particles are too close together, they cannot be measured individually, but if they are too sparse, it is very difficult to obtain both particles and the control TMV in an image.