295 



Beta Gamma 0.()()7().405 A' 



Chern. Cant. Radiochem. Cant. 



Intake Levels. Selenium is of agricultural importance due to its occur- 

 rence in crops in certain regions and its toxic effect on animals. Under 

 ordinary conditions the limit of toxicity to animals is about 3 to 4 ppm in 

 the diet. Grain from seleniferous areas may exceed this value, but the 

 mixed diet of humans usually does not. In small animals, daily ingestion 

 of 0.2 mg/kg body weight may cause disorders. The minimum lethal 

 dose, in milligrams per kilogram body weight, is about 3 for horses, 9 for 

 cattle, and 12 for pigs. About 10 ppm in the food of rats was fatal. 



Radioassay. Preparation Se-75-P will usually be the one of choice, 

 and gamma counting presents no particular problems. It has been 

 reported that animal tissues lost appreciable amounts of Se" merely as 

 a result of drying (Se-1). 



Chemistry. Tissue samples may be wet-ashed. However, unless 

 oxidizing conditions are maintained, the selenium may be reduced to 

 form volatile compounds. Even with excess HNO3, it has been reported 

 that losses occur if the digestion is carried out too quickly or at too high a 

 temperature. For chemical determination it is recommended that mer- 

 curic oxide be used together with H2SO4 and HNO3, followed by distilla- 

 tion as the bromide. The selenium in the distillate can be determined 

 volumetrically with hydriodic acid or colorimetrically with codeine. If 

 only radioassays are required, it would perhaps be best to make gamma 

 counts on the undigested tissue. 



Typical Methods. Sodium selenate was synthesized from the metal 

 irradiation unit and by analysis was found to contain 263 counts/min/Mg 

 of selenium (Se-2). Dogs were injected three times subcutaneously at 

 24-hr intervals, and samples of whole blood, plasma, and packed red 

 blood cells were measured with a Geiger tube. Various blood fractions 

 were prepared and wet-ashed with concentrated HNO3 and H2O2. 

 Digests were diluted to 10 ml, and 1-ml aliquots taken for counting. 

 Recovery studies were performed by carrying known amounts of radio- 

 selenium through all the experimental steps. In another study, radio- 

 selenium was used as follows to determine the life span of the duck red 

 blood cell (Se-3): Ducks weighing about 2 kg were injected intraperitone- 

 ally with 2.5 X 10" counts/min in 8 mg selenium in three doses. Red 

 blood cells from these animals were washed, reconstituted with isotonic 



