Sec. 15.1] THEORY OF TRACER METHODS 389 



the substance to be traced occurs normally in minute amounts. It is some- 

 times found, particularly in biological systems, that the addition of labeled 

 substance considerably in excess of the normal amount present leads to 

 processes that are not characteristic of the normal system. The possibility 

 that a labeling agent may not behave strictly as a tracer should always be 

 kept in mind in any novel application of tracers; caution should be exercised 

 in interpreting experimental results until it has been ascertained that the 

 tracer itself does not appreciably influence the system. 



The choice between stable and radioactive isotopic tracers is usually not 

 difficult to make; if a radioactive isotope of the element to be traced is avail- 

 able, it should be used. Radioactive isotopes are, from the technical point 

 of view, by far the easier to detect. A great variety of radiation measuring 

 instruments are available at reasonable cost. Samples are relatively easily 

 prepared, and measurements of specific activity are rapidly made. On 

 the other hand, with the exception of deuterium, stable isotopic tracers can 

 be measured only with the mass spectrometer. The preparation of samples 

 is tedious and in many instances extremely difficult. Both the sample prepa- 

 ration and actual measurement are, in addition, time-consuming and often 

 subject to many uncertainties. 



Perhaps equally important is the relative sensitivity of stable and radio- 

 active tracers. The dilution of a stable isotopic tracer in a system is limited 

 by the normal abundance of the isotope and inherent accuracy of the measure- 

 ments. Mass spectrometers usually have an accuracy of about 1 per cent, 

 or somewhat better under favorable conditions. If, for example, the normal 

 isotopic abundance is 1 per cent, as in the case of C 13 , the greatest possible 

 dilution factor is 10,000 and in practice is likely to be more nearly 1,000. 

 For radioactive isotopes, however, dilution factors greater than 10 6 are not 

 uncommon in routine practice. Quantities as large as 1 millicurie can some- 

 times be used in tracer experiments, and quantities as small as 10 -4 micro- 

 curies are always measurable. There remain finally the absolute quantities 

 of the two types of tracers which must be used. Whereas the mass of a 

 tracer quantity of radioactive isotope is negligible by ordinary standards, the 

 mass of a stable isotope necessary to label a substance is a considerable 

 fraction of the total amount of the element present. 



In several notable instances it is necessary to resort to stable isotopes. 

 In biological investigations the most important of these are, of course, oxygen 

 and nitrogen. There are no useful radioactive isotopes of these elements, 

 and the stable forms N 15 and O 18 must be used. 



In general, tracer applications are as extensive as the processes and systems 

 they are used to investigate. In the broadest sense, their applications can, 

 however, be divided into three categories: (1) quantitative determinations 

 of substances in complex systems, primarily by the method of isotope dilu- 



