ELECTKO.N MICKOSCOPY 



Fig. 8. Pre-shadowed carbon replica of a crys- 

 tal of the Southern Bean Mosaic Virus protein. 

 X26,600. (After Wyckoff and Labaw, by courtesy 

 of Nature) 



Fig. 9. Platinum/carbon replica of the surface 

 of a crystal of sodium faujasite showing very fine 

 steps about 10 A in height. X 50,000. 



simple and is carried out by the following 

 procedure. A V-shaped tungsten filament is 

 connected across a suitable transformer and 

 a short length of wire of shadowing metal is 

 wound on to it. The specimen is placed at 

 least 10 cm from this source and the appara- 

 tus pumped out. When a suitably low pres- 

 sure (better than 10"* mm Hg) has been 

 reached, the tungsten wire is heated, melt- 



ing the shadowing material which forms a 

 globule in the V of the filament. At the ap- 

 propriate temperature this globule of metal 

 will vaporize and be deposited onto the 

 specimen which has been tilted with respect 

 to the source at the required angle. The 

 amount of material needed is calculated so 

 that, for a mean atomic number of 50, no 



o 



more than 20 A of material is deposited on 

 the specimen. 



Unfortunately, these shadowing materials 

 generally form minute crystallites on the 

 specimen, which are visible in the electron 

 microscope at high magnification and there- 

 fore limit the resolution obtainable. The size 

 of the crystallites depends on the nature of 

 the material used, and the resolution of the 

 resulting electron micrograph will be limited 

 accordingly. Thus, if gold is used, crystallites 



o 



or granules more than 100 A across form, 

 giving a resolution of perhaps twice this 

 value. One of the best materials available is 

 platinum and, if this is used, the crystallites 

 are usually between 20 A and 30 A in diam- 

 eter and a greatly improved resolution is 

 obtained. Obviously it is necessary that the 

 shadowing material should have the finest 

 possible structure. Those most widely recom- 

 mended materials in this respect are plati- 

 num and uranium but they are difficult to 

 evaporate. Gold-palladium alloy, while hav- 

 ing a limited resolving power, can be evapo- 

 rated much more easily and is most often 

 used for routine work when a resolution of 

 not better than 50 A is required. Figure 8 

 shows a particularly striking example of 

 shadowcasting ; it is a palladium-shadowed 

 carbon replica of a plant A-irus protein 



o 



crystal (21). The molecules are 230 A in 

 diameter. 



A technique has recently been developed 

 for producing dense deposits which are non- 

 crystalline and do not granulate in the elec- 

 tron microscope (22). This technique in- 

 volves evaporating a mixtiu'e of platinum 

 metal and carbon in a manner similar to that 

 described for carbon. The resulting deposit 



236 



