XVI. S T A B L 10 ISOTOPES AS TRACERS 565 



readily with the stable C^^ than with cither the radioactive C^* or 

 C'^ On the other hand, if studies in progress with radioactive car- 

 bon were to be supplemented by some investigations with isotopic 

 nitrogen, it would proliablj' be necessary to secure a mass spectrom- 

 eter so that N^^ could be used rather than the radioactive N^^. The 

 decisive factor here is the very short half-life of N^^, whi(^h is approxi- 

 matelj" ten minutes. It should also be borne in mind that shifting 

 from one type of equipment to another involves learning a diiTerent 

 technique and represents a loss of time unless it serves some useful 

 purpose. 



Some conception of the cost of equipment for measuring stable 

 isotopes mil be gained from a later section of this chapter; and for 

 radioactive isotope equipment, from a separate chapter. A mass 

 spectrometer will certainlj^ cost more initially than equipment for 

 radioactive isotope measurements. On the other hand a mass spec- 

 tometer is useful for the several isotopes of greatest interest to the 

 biologist without any additional modification or expense. Where 

 only deuterium experiments are contemplated, the concentrations 

 can be determined bj'^ density measurements on water so that the 

 cost of the apparatus is almost negligible (i, pp. 51 and 61). For 

 measurements on some of the perhaps most useful radioactive iso- 

 topes, namely, H^, C^'*, and S^^, special apparatus must be constructed, 

 owing to the very weak radiations emitted. Requirements with re- 

 spect to apparatus and technical skill for measuring the radioactive 

 isotopes characterized by more penetrating radiation are lower than 

 for the three elements mentioned above or for the operation of a mass 

 spectrometer. Both types of equipment may require the services of 

 an electronic expert when difficulties develop; and they will develop. 



A distinct advantage that attaches to the stable isotopes is their 

 stability. Where experiments must run over long intervals of time 

 or where they invohe time-consuming organic syntheses, it is com- 

 forting to know that the isotopes employed are not disintegrating. 

 For example, the rapid decay rate of N^^ and C" practically eliminates 

 their use for numerous experiments, although a few valuable studies 

 have been made with the latter. C^^ with a half-life of about 5000 

 years is essentially stable, while S^' (half-life S8 days) and IP (half- 

 life 11 years) have decay rates suitable for mf)st experiments. 



Although the radiation of energ}^ that attends the spontaneous 

 disintegration of radioactive isotopes constitutes a necessary event 

 from the standpoint of concentration measurements, it is conceivable 



