GENERAL PROCEDURES FOR RADIOASSAY 



171 



thick at ends 



Each end- 



tube 



tubing are commercially available, but the tubing has to be machined to 

 fit. The 4-iii. plastic tubes are cut to a taper on each end so that a tight 

 inside fit is made when the tube is pressed into the cup. Worn tubes may 

 be reversed and the other end used. This apparatus is accommodated by 

 the regular 50-mI centrifuge-tube carriers. The plastic tube is sufficiently 

 resilient so that a reasonably tight fit is obtained. To ensure that no 

 leakage occurs, water is placed between the plastic tube and the carrier; 

 this serves to equalize the centrifugal pressure. The cups and precipitate 

 can readily be brought to constant 

 weight by drying. Since the cups are 

 inexpensive and used only once, the 

 samples can be kept for reference pur- 

 poses as long as necessary. The 

 method has given good reproducibility. 



Electroplating. Electroplating is an 

 excellent method for the preparation 

 .of uniform thin films of many of the 

 metals, and a general discussion has 

 been presented by Dunn (9). The 

 chemical literature should be con- 

 sulted for the specific conditions re- 

 cjuired for each element. Many types 

 of electrolysis cells have been de- 

 scribed. However, a commercial unit 

 is available for general isotope work. 

 This apparatus consists of a leak-proof 

 cell formed by a glass cylinder, a rub- 

 ber gasket, and a flat metal disk which 

 serves as the cathode and bottom of 

 the vessel. The unit is easily assem- 

 bled or disassembled. Platinum wire is usually employed as the anode, 

 and copper disks are often satisfactory for the cathodic deposition of 

 the film. The copper surface must be thoroughly cleaned for satisfactory 

 film formation; this can be done by degreasing in organic solvents, fol- 

 lowed by final cleaning in cold dilute HCl or HNO3. The power supply 

 is conveniently furnished from ordinary 6-volt storage batteries, although 

 commercial power supplies are available for this purpose and are conven- 

 ient for the investigation of various voltage-current relationships. 



In general, the biological material must be ashed prior to the electro- 

 plating procedure. The ash solution will usually contain large amounts 

 of sodium, potassium, and phosphate, etc., compared with the element t(; 

 be determined. If less than 10 mg of ash is used, it may not be necessary 

 to employ a separation procedure before electroplating. Usually 1 to 



l^etal counting cup, 

 area 4.75 cm'^ 

 inside diameter, ^'/^^ 



Fig. 5-5. Plastic-tube-aiul-cup appa- 

 ratus for collection of precipitates by 

 centrifugation. 



