62 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



containing a single radioactive species. Special precautions must bs 

 taken with unprocessed irradiation units. 



A serious and widespread difficulty of this type resulted from the use 

 of P^- from pile bombardment of Na2HP04. In 1940 Libby (5) showed 

 that orthophosphates underwent disruption as a result of neutron bom- 

 bardment, and found only about 50 per cent of the P^'- atoms as ortho- 

 phosphate in the product. It was suggested that the rest was probably 

 in the form of phosphite. Thomas and Nicholas (6) in 1949 and Fried 

 and MacKenzie (7) in 1950 confirmed this behavior for pile-bombarded 

 orthophosphate, and the latter workers were thereby able to explain some 

 anomalous fertilizer results. Borland, MacKenzie, and Hill (8) further 

 characterized this phenomenon and showed that higher temperatures in 

 the pile caused a lower degree of contamination. The seriousness of this 

 behavior is not so much concerned with studies on orthophosphates as 

 such, since these materials can be obtained in pure form by bombardment 

 of sulfur and processing. Rather, it is important that certain naturally 

 occurring fertilizers, particularly rock phosphate, be labeled so that the 

 phosphorus availability can be determined directly. MacKenzie and 

 Borland (9) have investigated the possibilities of removing the contamina- 

 tion from neutron-irradiated rock phosphates and have suggested a sat- 

 isfactory procedure which consists in heating at 450°C in steam at atmos- 

 pheric pressure for 168 hr. 



Radiocolloids. It has long been known that some carrier-free tracers 

 in solution behave like colloids rather than true solutes. Under such con- 

 ditions these substances have been called radiocolloids. A literature 

 review on radiocolloids has been compiled by Schweitzer and Jackson 

 (10), and Wahl and Bonner (11) have presented tables on the radiocolloid 

 formation of various radioisotopes. Radiocolloids may be detected by 

 such procedures as dialysis, ultrafiltration, diffusion, electrophoresis, 

 adsorption, and autoradiograms. The following elements are known to 

 form radiocolloids under appropriate conditions: barium, beryllium, 

 bismuth, cerium, lanthanum, lead, magnesium, niobium, plutonium, 

 polonium, protactinium, scandium, thorium, tin, titanium, yttrium, and 

 zirconium. Some of the factors tending to promote colloid formation are 

 as follows: (a) use of solvents in which the tracer tends to hydrolyze or 

 form an insoluble compound, (6) presence of foreign particles in the solu- 

 tion, (c) presence of certain electrolytes, and {d) increased age of solution 

 (11). 



It would be expected that radiocolloids might behave differently in a 

 biological system than do ions of the same radioisotope. Dobson et al. 

 (12) have shown that the distribution of radiocolloids of zirconium in 

 animals was entirely dependent on particle size. The localization of 

 radiocolloids in the reticulo-endothelial system, especially the bone mar- 



