SELECTED PAPERS 



tron, since the half life time of this isotope is only 2 1 minutes, restrict- 

 ing its applicability to experiments of a duration of about two hours. 



In this respect the stable C 13 is, of course, quite preferable, but alas 

 the isolation of this isotope obviously still asks for a very cumbersome 

 procedure. This may be inferred from the fact that both Werkman and 

 Wood and their schools have been supplied with C 13 by the physicist 

 of Minneapolis Professor Nier and that this scientist recently stated 

 that he holds the world record for C 13 -production amounting to 5 

 grams annually! And when reading the hopeful statements in scien- 

 tific journals that this year the Sun Oil Corporation of Philadelphia 

 will start the commercial production of C 13 it is not generally realized 

 that the annual production by this firm is estimated to be 500 grams! 

 With these facts before our eyes it will be wise to be not too optimistic 

 regarding a world-wide application of carbon isotopes in a near future. 



In addition also the following should not be neglected. Publications 

 on metabolic studies with the aid of the stable carbon isotope are as a 

 rule very attractive for the reader, the results reported usually being 

 simple and clear-cut. There is, however, a great danger that one will 

 be insufficiently impressed by the enormous amount of experimental 

 work which has been required in order to obtain these results. In 

 many cases it has been necessary to search a great number of com- 

 pounds on a possible excess content of labelled carbon, with only a 

 restricted number of positive results. Especially when an attempt is 

 made to arrive at a complete balance of labelled carbon added and 

 recovered many very careful analyses with the mass-spectrometer are 

 indispensable. And if finally it becomes necessary to fix the location 

 of the heavy carbon atoms in the molecule of a metabolic product, 

 various purely chemical degradation reactions of this product have to 

 be studied before the mass-spectrometer can again give the desired 

 answer. To this should be added the necessity of carefully recovering 

 in all this work the precious isotope ! 



All this asks for a more or less specialized staff of workers and, 

 therefore, tends to limit the immediate universal introduction of the 

 isotope method in the study of metabolism. It is, therefore most wel- 

 come that in the last years in the United States quite another line of 

 attack in the study of metabolic processes has been discovered which 

 seems to be full of promise and to which I shall return later on. 



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