2 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



such as P''^ and Na-\ However, in many cases the elements are com- 

 prised of several isotopes, such as Cu'^^-Cu^^ or O^'^-O^'-O^^. 



The atoms of certain elements are known to undergo spontaneous dis- 

 integration with the emission of atomic particles (radiation) and to leave 

 behind a system lighter than before and possessing physical and chemical 

 properties different from those of the parent element. Such unstable 

 atoms are described as radioactive, and the prefix radio- is used to desig- 

 nate this behavior. In other words, radioisotopes differ from stable 

 isotopes in that the protons and neutrons of the nuclei of the former are 

 in an unstable arrangement and therefore undergo spontaneous disinte- 

 gration, a process that cannot be altered in any way known to man. The 

 adjective labeled is used to describe an element, compound, or organism 

 that contains an altered isotopic content. The asterisk is also employed 

 to designate an element labeled with the radioisotope (e.g., labeled phos- 

 phorus, radiophosphorus, P'^'-, and P* are used here interchangeably 

 unless otherwise noted). 



In addition to the natural radioisotopes of the heavy elements of atomic 

 number greater than 81, the following lighter radioelements are known to 

 occur naturally: K^o, Rb", In^S La^^^^ Sm^^', Lu^'^, Re^". Of these, 

 only K^" rnay be of importance in biological radioisotope studies because 

 of its presence in tissues. The major contribution to biological research, 

 of course, has resulted from the availability of the artificially produced 

 radioisotopes. 



The above discussion has been primarily to establish terminology, and 

 the general references (1 to 14) may be consulted for background informa- 

 tion on the subjects of atomic theory, structure of the atom, and produc- 

 tion of radioisotopes. Such information is of collateral interest but not 

 essential for the biological use of radioisotopes. 



In consideration of the principles of application, it is necessary only to 

 recognize that (a) before decay the radioisotope will behave in the system 

 similarly to its stable counterpart, and (6) the radiation that it emits 

 upon decay can be conveniently measured, thus providing an estimate of 

 the amount of radioisotope present. There are situations, as outlined 

 in Chap. 2, where these assumptions do not hold, but they are of sufficient 

 general validity to serve as a basis for the present discussion. An arbi- 

 trary classification of principles in increasing order of complexity has been 

 employed for purposes of clarity. It will be apparent that data derived 

 from more complex experiments would automatically provide information 

 of a qualitative or locational nature as discussed in the first two sections 

 below. The inclusion of experimental details has been purposely min- 

 imized at this time so that there will be no distraction from the fundamen- 

 tals involved. Although the subject matter has been primarily limited 

 to radioisotopes, it will be recognized that the principles will also apply to 



