XVI. STABLE ISOTOPES AS TRACERS 569 



the value of isotopic tracors. The usefulness of this procedure has 

 been amply demonstratecl by Rittenberg and T'oster {16), who made 

 quantitative analyses for a number of the amino acids resulting from 

 protein hydrolysis, and by others (see 20, p. 476). A central feature 

 is the fact that the isotope dilution method does not require the quan- 

 titative extraction of the components to be analyzed. It is only nec- 

 essary to provide a sample containing one or perhaps a few milligrams 

 of a test substance in a pure state. This circumstance is especially 

 important in dealing with amino acids, where it is i-elativel}^ easy to 

 obtain either quantitative fractions of impure compounds or non- 

 quantitative fractions of pure compounds, but exceedingly difficult 

 to obtain samples that are both quantitative and pure. However, 

 before any substance can be determined quantitatively, a pure syn- 

 thetic preparation of it containing an isotopic marker must be avail- 

 able. 



As an illustration of the method, let us suppose that we have a solu- 

 tion of hydrolyzed protein and that we wish to determine its glycine 

 content, X. A synthetic glycine sample of known weight, G, which 

 contains an N^^ concentration of Co atom per cent in excess of normal, 

 is added to the hydrolyzate. The synthetic and the natural glycine 

 form an inseparable mixture with a uniform distribution of the N^*- 

 tagged molecules. Therefore, the isolation of a small but pure frac- 

 tion of the total glycine will enable its diluted N'^ concentration, C 

 atom per cent excess, to be determined. Without resorting to a 

 quantitative extraction, the weight, X, of the original glycine can be 

 calculated from the following equation (/7, p. 116). 



(1) 



The correction factor Mx/Mq represents the ratio of the molecular 

 weights of the two glycine samples when the actual percentage of N** 

 present is considered. Most work to date has neglected this correc- 

 tion; though small, it is not always negligible in a quantitative sense. 



In practice the analysis of other amino acids is considerably^ more 

 complicated owing to the occurrence in synthetic preparations of both 

 optical isomers. But, even here, analysis is possible {2, p. 563) and 

 the isotope dilution method must be regarded as a generally applicable 

 procedure (see especially 22h). 



Since concentrations Co and C can be determined with an accu- 



