ISOTOPIC TRACERS 161 



sequences of intermediates could be involved. A series of tracer experi- 

 ments can delineate the chemical pathway. As a less extensive modifica- 

 tion, a tracer can tell us whether a particular reaction occurs at all. If 

 we provide cells with the suspected substrate of a reaction labeled with 

 a tracer and then allow time for the reaction to occur, we should be 

 able to recover the product of the reaction, now labeled with the 

 tracer. 



The tracer experiment 



The isotopic tracer experiment consists of substituting an unusual or 

 uncommon isotope of an element for the more abundant form. For ex- 

 ample, radioactive carbon (C^'*) can be substituted for ordinary carbon 

 (C^^) and will go through the same chemical reactions. For the purposes 

 of tracing, it is not at all necessary that every atom should be the unusual 

 isotope. A small fraction of the tracer isotope serves to label the whole 

 amount of a compound. The tracer experiment, then, requires an ele- 

 ment or compound in which small fractions of the molecules contain 

 the unusual isotope of an element. The other chief requirement in the 

 experiment is some method for detecting the uncommon isotope before, 

 during, and after the process being investigated. The fraction of the 

 atoms which must be labeled depends upon the ability of the detecting 

 method to distinguish the tracer atoms. 



"Isotope" means literally "same place" and refers to atoms that occur 

 at the same place in the periodic table; therefore, they contain the 

 same number of protons and electrons, which determine the chemical 

 activity, but, because they contain different numbers of neutrons, they 

 differ in mass. The hydrogen series illustrates the point reasonably well. 

 Ordinary hydrogen is ordinary hydrogen because it makes up about 

 99.98 per cent of the naturally occurring hydrogen. It is the simplest pos- 

 sible atom, consisting of one proton and one electron. About 0.02 per 

 cent of the naturally occurring hydrogen atoms contain a neutron in 

 addition to the proton and electron. This neutron doubles the mass of 

 the atom without appreciably changing its chemical properties. A third 

 type, present as a trace in nature but manufactured artificially in reactors, 

 contains a second neutron and therefore has three times the mass of 

 ordinary hydrogen. These three isotopes of hydrogen are designated 

 iH\ iH^ iH^. The subscript 1 indicates the atomic number and thus the 

 chemical identity of the element. The superscript (1, 2, 3) indicates the 



