CHROMIUM 237 



Intake Levels. Chromium is not an important naturally occurring 

 constituent of biological materials, although traces do occur in soils. A 

 primary interest has been in the use of Cr^^ for the determination of red- 

 cell volume (C'r-I). The main advantage over other labels is that the 

 Cr^'-labeled cells retain their activity for as long as a day or more. 

 Human blood has been reported to contain about 14 to 20 ^g/lOO g 

 (Cr-2). About 340 ppm chromium as potassium dichromate in food was 

 about the limit of tolerance for rats, and a daily intake of 10 mg was fatal 

 for dogs in 3 months. CrCls was about as toxic as chromate. 



Radioassay. For most animal work the high-specific-activity material 

 will be needed to reduce the mass administered. Even higher specific 

 activities than that listed can be obtained commercially from the bom- 

 bardment of samples that have been enriched in Cr''". However, these 

 may not be necessary if sensitive measurements are made. Although the 

 internal counter gives the highest sensitivity, the self-absorption consid- 

 erations with such a soft-gamma radiation become of extreme importance, 

 and therefore scintillation counting may often be the method of choice. 



Chemistry. Dry-ashing yields low recoveries presumably on account 

 of the sublimation of CrCls-GH-iO at 83°C, whereas wet digestion is sat- 

 isfactory. Chromium has been estimated chemically in tissue by acid 

 digestion, separation from extraneous ions by virtue of differential sol- 

 ubility properties in different valence states, and photometric evaluation 

 of the color produced with diphenylcarbazide (Cr-2, Cr-3). 



Typical Methods. Red cells from 50 ml of human blood were incu- 

 bated with a solution containing 40 to 200 mc Cr^^ in 336 to 2800 /xg Cr as 

 Na2Cr04 (Cr-4). The red cells were washed, reconstituted, and injected 

 into the patient, from whom blood samples were taken 10 min thereafter. 

 The whole blood was centrifuged, and the packed red cells diluted to four 

 times their initial volume and inverted ten times to ensure complete 

 hemolysis. A 0.2-ml alicjuot was pipetted onto a planchet, 0.8 ml H2O 

 added, and the hemolysates dried for counting with either a gamma tube 

 or an internal proportional counter. In this procedure the self-absorption 

 was found to be uniform between samples, and therefore this method was 



