GENERAL PROCEDURES FOR RADIOASSAY 



169 



counting is done manually, it is put around a cylindrical thyrode Geiger 

 tube and held in place with a wire test-tube clamp. An automatic sample 

 changer is also described to handle this form of sample. 



In these operations the accuracy of pipetting is a major factor. Fol- 

 lowing is a procedure found satisfactory for the use of micropipettes to 

 handle volumes from 0.01 to 0.5 ml: A simple pipette filler is assembled 

 from a 1-ml syringe fitted with a rubber stopper which is bored to accom- 

 modate the pipette (Fig. 5-3). The pipette is filled with the test solution 

 and brought to the mark, the tip wiped with cleaning tissue, and the solu- 

 tion delivered onto the mount. The 

 pipette is filled with water, which is 

 added to the mount. This procedure 

 is repeated twice. The pipette is 

 rinsed once or twice each wuth water, 

 alcohol, and acetone, is then dried with 

 an air current, and is ready for the next 

 sample. Macropipettes and larger 

 volumes of solution should be used for 

 regular dilutions, and the ordinary 

 analytical use of the pipette is pre- 

 scribed. Pipetting in a radio-chemi- 

 cal laboratory is never done by mouth, 

 and although commercial pipetters are 

 available, a simple device described 

 by Hood (31) has been most satisfac- 

 tory for routine use (Fig. 5-3). The 

 pipetter consists of a 10- to 30-ml 

 rubber bulb into which is inserted the shank of a No. 18-23 hypodermic 

 needle that has been broken off to % Q-\n. length. A rubber disk is pierced 

 by the needle and in use serves to make a seal on the pipette. The needle 

 is inserted into the bore of the pipette, and the solution drawn up by 

 suction using the bulb. The pipetter is removed, and the meniscus level 

 thereafter controlled by finger in the usual way. 



Filtration. It is often necessary to prepare soHd counting samples from 

 suspensions, for instance, BaCOa for assay of C^^ or CaC204 for assay of 

 Ca^^. Comar et al. (32) have reviewed the various methods that have 

 been suggested for this purpose. Many filtration devices have been 

 developed for the purpose of quantitative collection of precipitates for 

 radioassay. Techniques involving the use of filter paper suffer from the 

 general disadvantages of variable texture, difficulty in reproducing con- 

 stant weights, tendency of the paper and precipitate to buckle, and 

 necessity for careful handling of the final sample to avoid damaging the 

 precipitate. Although some of the methods eliminate certain of these 



Fig. 5-3. Simple pipette fillers. 



