An initial test of a method for the estimation of real mean particle size from shadowed samples

During shadowing, a "cap" of metal develops on small particles. This cap increases apparent particle with (measured normal to the shadowing direction) by an extent which cannot be predetermined. The extent of this increase in particle size (here defined as the "cap," X) is estimated in the present method by using opposite (180 degrees sample rotation) bidirectional shadowing. It is argued that the bidirectional cap is the sum of the two unidirectional caps, and therefore that X = 2A - (B + C), where A is the mean bidirectionally shadowed particle size, and B and C are the two mean unidirectionally shadowed particle sizes. As a validation of the method, the mean diameter of air-dried ferritin was estimated and the results appear to confirm the hypothesis (mean diameter by present method, 10.7 +/- 0.2 nm; mean diameter by previous methods, 10.89 nm).