Distortion in dimensions produced by shadowing for electron microscopy

Abstract

SYNOPSISChanges in dimensions of some spherical and cylindrical particles due to metal shadowing for electron microscopy have been investigated. Monodisperse polystyrene latex, colloidal gold particles, Bushy Stunt and Tobacco Mosaic Virus, normal human adult haemoglobin and DNA molecules were used as experimental material. Particles were deposited on carbon coated platinum grids and shadowed with platinum vapour using a fixed shadowing geometry. With the exception of TMV, haemoglobin and DNA, distortion was measured on individual particles by taking micrographs of the same particles before and after shadowing. Distortions were measured on a large number of particles both parallel and perpendicular to the shadow direction. On the smaller particles, the distortions in the two directions were estimated from the heights, deduced from shadow length, and measurement of the widths parallel and perpendicular to the shadow. The distortions were plotted against particle diameters. The distortion parallel to the shadow was found to be greater than that measured perpendicular to the shadow and both were found to increase up to a particle diameter of about 200 Å and thereafter became practically constant. The use of the calibration curves in the measurement of the dimensions of small biological particles has been discussed using the example of normal human foetal haemoglobin and egg albumin molecules.