420 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



be submitted ready for analysis. Usually, solid samples of 0.5 g and liquid 

 samples of about 25 ml are employed. Solid samples should be shipped 

 in small glass vials closed with a plastic cap or rubber stopper, whereas 

 liquid samples should be sealed in glass ampoules or sample bottles. 



An example of the procedures employed may be cited from the studies 

 of Smales and Pate (13, 18, 19) on the determination of arsenic in sub- 

 microgram amounts in biological material. Up to 11 ml of licjuid samples 

 such as urine was placed in containers consisting of silica tubes sealed at 

 one end and joined at the other to a short length of capillary tubing; these 

 containers were irradiated unsealed. A solution containing 1 Mg/ml of As 

 was used as a standard and was irradiated in a similar tube at the same 

 time as the sample. Small volumes of liquid, up to 0.2 ml of blood or 

 urine, for example, were irradiated in small silica pipettes which were 

 sealed before irradiation. Standard solutions containing 1 Mg/ml of As 

 were treated in the same way. Solid samples such as tissue, hair, or nails 

 were irradiated after being sealed in short lengths of polythene tubing or 

 in bags made of polythene sheet. The standard was a mixture of alumina 

 containing 100 Mg/g of As which was prepared by grinding the two oxides 

 together. 



After irradiation and a waiting period of 1 to 24 hr, the w^eighed sample 

 or standard was transferred to a beaker, and 50 mg of carrier arsenic 

 added. Oxidation was carried out with hydrogen peroxide and nitric, 

 sulfuric, and perchloric acids. After oxidation, hydrochloric acid was 

 added, and the arsenic precipitated with ammonium hypophosphite. 

 The precipitate was collected, washed, and dissolved in hydrogen per- 

 oxide and hydrochloric acid solution. The solution was distilled under 

 oxidizing and reducing conditions, first from hydrochloric acid and hydro- 

 gen peroxide solution and then from hydrobromic acid solution. The dis- 

 tillates were combined and treated with ammonium hypophosphite to 

 precipitate the arsenic, which was collected, washed, dried, and weighed 

 to establish the chemical yield. The precipitate was counted with a 

 Geiger tube. The counts of all standards and samples were corrected 

 to a common basis of chemical yield, decay, and radioassay conditions. 

 The half-life and the beta energy were checked to ensure that only As^^ 

 was being measured. Calculations were made as follows: 



( 



Grams of As\ _ /grams of As\ corrected sample count 



in sample / I in standard y corrected standard count 



BIOLOGICAL APPLICATIONS 



Arsenic. The studies of Smales and Pate on arsenic, procedures for 

 which have been previously described, furnish an excellent example of 



