Metallic Evaporation and the Diameter of Tobacco Mosaic Virus with the Electron Microscope

Abstract

Tobacco mosaic virus has been used as a known test object for the intercomparison of several methods of small particle measurements, all involving metallic evaporation. Various characteristics of these methods, which determine their accuracy, were investigated. Measurement of groups and the derivation of a mean size therefrom is primarily limited in accuracy only by the exactness of juxtaposition of the successive particles in the group. The measurement of the images of single particles is subject to the relatively large error associated with the uncertainty in the relationship between the true particle edge and the image of the edge which has received a coating of metal. The measurement of shadow lengths reduces this error by the factor 2 tanα/2. A symmetrical shadow method is described which reduces the error of substrate tilt, and is insensitive to amount of metal deposited. An average over the four best methods gave a value 150.0±3.6A agreeing closely with the x-ray determination.