EXPERIMENTAL MATERIALS AND METHODS 



25 



by labelling one ion with a beta-emitting isotope, the other with a 

 gamma-emitter, and measuring radioactivity before and after 

 insertion of a tinfoil or glass screen between the sample and the 

 mica window. From such measurements, the intensity of both 

 gamma and beta radiation can be determined. Alternatively, 

 combinations of short-lived and long-Hved isotopes can be employed, 

 two measurements of radioactivity being made, one before, and one 

 after, the short-lived isotope has decayed to an appreciable extent 



Table 6. 

 a list of radioactive isotopes useful in the study of salt absorption 



(Davies and Wilkins, 1951). Apart from the convenience, accuracy 

 and sensitivity of radioactivity determinations, the technique has 

 the advantage that movements of ions between tissues and media 

 can be detected and quantitatively determined, even when there is 

 no net change of concentration (see Chapter 3 p. 32). The major 

 disadvantages of radiochemical techniques are that expensive 

 equipment is required, and health hazards exist which necessitate 

 fastidious cleanliness and other precautions. For further information 

 about the use of radioactive tracers in experiments with biological 

 materials see, for example, Kamen (1947), Comar (1955) or Glover 

 (1956). 



