168 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



etc. Lens paper may be used as an auxiliary to promote even spreading, 

 but this introduces additional uncertainties. Burch (29) has described a 

 method in which about 0.3 ml of biologic fluid is spread upon a filter- 

 paper disk which is dried and then cemented to a metal disk. 



Usually a small volume of the liquid is added to the mount and evap- 

 orated on a hot plate or water bath or by infrared lamp. Care must be 

 taken to avoid mechanical loss by spluttering. However, the main prob- 

 lem is to get coverage of some uniform area. The source area may be 

 defined by a mark made with a wax pencil or by the use of lacquer. 

 There may be considerable creeping, especially with volatile liquids; this 

 may be avoided by the addition of a less volatile solvent before the final 

 addition to the mount. When cups are used, there is a tendency for 

 excessive deposition around the edges; this may be minimized by having 

 the cup supported on the hot plate by a hollow metal cylinder which 

 selectively heats the edges and causes the solids to be drawn to the center 

 of the mount. Devices are commercially available which provide a 

 slowly revolving turntable on which the mount is placed. The solution 

 is added slowly in a spiral line, and a continuous source of heat causes 

 rapid evaporation of solvent. 



The investigator can determine for himself whether standards prepared 

 by direct evaporation give sufficiently reproducible and comparable 

 results for the evaluation of biological samples prepared by another 

 method. For example, we have found that aliquots of Ca^^ solutions 

 directly evaporated in metal cups gave essentially the same counts with 

 about the same error as when the activity was precipitated as calcium 

 oxalate, counted, and corrected for self-absorption. Thus it was possible 

 to save time, since it was not necessary to carry standards routinely 

 through the same precipitation procedure as used for the samples. 



McKay and Zilversmit (30) have described a procedure for P^-, a 

 hard-beta emitter, which has considerable merit. Rectangular strips of 

 Reynolds household aluminum foil 21-^ by l^i in. are made into cups by 

 folding the edges manually or by using the sample press. This cup is 

 placed on a thermostatically controlled hot plate. Radioactive samples 

 in aqueous solutions are added to the aluminum cup in toto or dropwise. 

 When the radioactivity is present in alcohol or ether, the sample is always 

 added dropwise. In order to prevent creeping of the nonaqueous sol- 

 vents, a piece of filter paper l^i by 13-^ in. is placed in the cup before addi- 

 tion of the radioactive solvent. When the radioactivity is present in 

 dilute sodium hydroxide or hydrochloric acid, lead foil is used instead of 

 aluminum foil. When the sample is thoroughly dry, the edges are 

 unfolded, and the sample is covered with 3-in.-wide scotch tape. After 

 the corners are cut, the tape is folded over the edges to prevent escape of 

 the radioactive material. The sample is now ready for counting. When 



