270 MOLYBDENUM 



Molybdenum 99 (2.85 days) Beta 0.5, 1.22 Gamma 0.141 



Cat. No. Sp. Act. Form Cost Chern. Cont. Radiochem. Cont. 



Mo-99-I 7mc/g IVI0O3 $12/46 mc — 10^«mc Tc^^™ (6 hr) daughter 



Mica W. Int. C. Scint. C. Rf/mc 50 % Self-abs. 



1.3 X 10-4 1.4 X lO-" 8.7 X 10-3 j g 155 (^aic.) 



Critical Org. Body Air Water Effect. T^ 



Bone 50 MC 2 X IQ-^ 14 2.8 days 



Intake Levels. Molybdenum is almost universally found in plant and 

 animal materials. There is suggestive evidence that it is essential for 

 animals in trace amounts as a constituent of xanthine oxidase (Mo-1). 

 The normal dietary level is probably less than 1 ppm, and most plants 

 contain of the order of 0.5 ppm on a dry basis. In some areas from 5 to 

 200 ppm may be present in the plants, which are then usually toxic for 

 ruminants, especially if the copper levels are low. Molybdenum is 

 definitely essential for plants, and 0.1 ppm in the nutrient solution has 

 been found to stimulate the growth of lettuce, whereas 0.01 ppm has been 

 found essential for the growth of tomato seedlings (Mo-2). Soluble 

 molybdenum compounds were fatal when fed to rats and guinea pigs at 

 1200 to 6000 mg/kg but much less so at 120 to 600 mg/kg. Levels of 

 400 to 800 mg/kg administered intraperitoneally to guinea pigs were 

 highl}^ toxic. Chronic toxicity largely depends on the amount of copper 

 in the diet, and with low copper as little as 80 ppm has caused toxic 

 symptoms in rats (Mo-3). 



Radioassay. The technetium-99m daughter will usually not interfere, 

 especially if hard-beta or gamma counting is used, or if about 60 hr is 

 allowed to lapse between the taking of the sample and the measurement 

 so as to allow attainment of equilibrium. Gamma counting, although 

 somewhat less sensitive, offers the advantage that self-absorption correc- 

 tions will not be necessary. Solution counting is also convenient. The 

 specific activity available is not particularly high, and there may be some 

 difficulties in reducing the dosage to physiological levels. 



Chemistry. Biological samples may be dry-ashed, but the temperature 

 should not exceed 450°C in order to avoid losses. However, wet-ashing is 

 usually preferred, and a recent modification of a widely used procedure 

 has been described (Mo-4). The dry, ground tissue is digested with sul- 

 furic, nitric, and perchloric acids, and after boiling to remove the last 

 traces of perchloric acid, it is diluted with distilled water and boiled again. 

 The solution is neutralized to methyl orange with ammonia^ acidified with 

 HCl, and treated with sodium fluoride, an iron solution, and then potas- 

 sium thiocyanate. After addition of stannous chloride the molybdenum 

 complex is extracted into isoamyl alcohol for colorimetric evaluation. 



