XVI. STABLE ISOTOPES AS TRACERS 57 1 



tainly find biological application. Typical mass spectra for a few 

 common carbon compounds are shown in Figure 1. Note the charac- 

 teristic variation in the relative concentrations of different ionized 

 fragments, especially for the isomers of butane. Although automatic 

 mass spectrometers for this purpose are admittedly complicated and 

 expensive, there is a possibility in some cases that access may be had 

 to one of the many instruments of this type already in operation at 

 industrial research laboratories (see list in 1, p. 48). 



It should be pointed out that quantitative analyses of small com- 

 plex gas samples usually must rest on a previous calibration for each 

 of the various components of the sample. This necessary complica- 

 tion, often readily surmountable with low molecular petroleum sam- 

 ples, appears formidable for biological compounds of high molecular 

 weight. To be analyzed in the usual mass spectrometer (see Fig. 3), 

 all liquid compounds should be appreciably volatile at room tempera- 

 ture. In specialized types of gas analysis, where one may be inter- 

 ested in determining the concentration of only one or two constituents 

 of a complex sample, the problem is much more amenable to analysis. 

 For example, an impurity of even one part in a million can be de- 

 tected in commercial cylinders of compressed oxygen. Another 

 related application of this technique is the location of leaks in metal 

 vacuum systems. For this latter purpose it is customary to use a 

 very much simplified form of mass spectrometer {8,12) and to probe 

 suspected areas of the apparatus for leaks with helium. In this way 

 leaks may be localized quickly without undesirable contamination 

 and vexatious delays. 



C. DEMANDS ON EQUIPMENT AND TECHNICAL SKILL 



1. Availability of Concentrated Isotopes 



Earlier workers in this field were forced to concentrate their own 

 isotopes or to beg supplies from others who did. Several laboratory 

 methods for the concentration of isotopes have been carefully worked 

 out and are described in great detail in the literature (see 1,3,6). 

 Should commercial supplies not be available, it is quite feasible to set 

 up the necessary equipment and to concentrate a particular isotope 

 in the laboratory. Usually this is a very involved undertaking. It 

 should not be attempted with a viewpoint to obtaining isotopes at a 



