594 FRED M. UBER 



9. Over- All Accuracy 



For the type of biological investigation we have been discussing, 

 the accuracy obtainable in routine determinations of isotopic concen- 

 tration is usually quite adequate. There seems to be universal agree- 

 ment among the workers in this field that such determinations can 

 be made readily within an over-all accuracy limit of 1%. There is 

 reason to believe that carefully conducted measurements can reduce 

 this limit by a factor of two, or perhaps, with extreme care, by a factor 

 of four. It is very important to realize the limits of accuracy of an 

 experimental method, even though an investigation may not necessi- 

 tate pushing a method to its ultimate limit. Wherever possible, it is 

 preferable to operate at a comfortably safe distance inside the extreme 

 limits of accuracy. Although the significance of experimental results 

 in each case must be determined eventually by an adequate statistical 

 evaluation of the data, one would prefer to know in advance that there 

 existed a sufficient margin of safety so that the experimental conclu- 

 sions would not be open to question. 



E. ACCOMPLISHMENTS WITH INDIVIDUAL ELEMENTS 



1. Deuterium and Heavy Water 



Since heavy hydrogen was the first stable isotope to be isolated in 

 quantity, a voluminous literature on its biological applications is to be 

 expected. The behavior of numerous organisms subjected to heavy 

 water of various concentrations was observed in early experiments 

 and the deuterium label was traced to a great variety of body constit- 

 uents {24)- Much useful information is contained in papers pre- 

 sented at a 1938 sj^mposium on deuterium (15). A very important 

 group of researches using deuterium as a label concerns fatty acid 

 metabolism and the general behavior of fats in the animal body. It 

 happens that deuterium may be stably bound to carbon in the higher 

 fatty acids and by rather simple synthetic methods (see 17, p. 139). 

 Two excellent resumes of this work by the principal investigators 

 {23,24) stress the complex relationships and interconversions continu- 

 ally in progress among this important group of body constituents. 

 The deuterium label has also been used with several amino acids {24). 

 The transmethylation reaction involving methionine deserves special 

 mention; so does the discovery of the inert nature of lysine, which is 



