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ring, whereas with the second it was not. These experiments indicate 

 that glycine is used in the biosynthesis of protoporphyrin, but that the 

 carboxyl [COOH] carbon is removed during the synthesis. Although 

 this example may seem very simple, it has not been possible to determine 

 this role of glycine by any other method than tracer techniques. N 15 

 experiments have also shown that glycine is a precursor of protopor- 

 phyrin. 



A rather different application of C 14 is carbon dating. In this, the 

 radioactivity of a sample of wood or other organic material is determined. 

 If it is part of a recently living system, the carbon atoms are in dynamic 

 equilibrium with the C0 2 of the atmosphere which produces about 

 10 counts/min/gm of carbon due to C 14 . In material which has been 

 nonliving for a long time, the C 14 gradually disintegrates and is not 

 replaced. Thus, a piece of ivory about 5,760 years old would be 

 expected to have half as much C 14 per gram of carbon as recent material. 

 A piece of wood about 11,500 years old should have one-quarter as 

 much (that is, 2.5 counts/min/gm of carbon). The radioactivity due to 

 the C 14 is small and weak. Nonetheless, with suitable precautions to 

 eliminate counts due to cosmic rays, other disintegrations, background 

 radioactivity in the room, and electronic noise, it is possible to measure 

 the C 14 content very accurately. When it has been possible to compare 

 the date computed from C 14 measurements with that known from other 

 sources, these two have agreed within experimental error. For carbon- 

 containing objects, between 2,000 and 50,000 years old, the C 14 date 

 can be computed more precisely than any other type of date. For many 

 objects of significance to archeology, anthropology, and evolution, C 14 

 dates are the only ones possible. 



4. I' 31 



Stable, naturally occurring iodine consists primarily of isotope I 127 . 

 Many radioactive isotopes of iodine are known; among these are I 128 , 

 I 129 , I 130 , and I 181 . The isotope I 128 has a half-life of 25 minutes, and 

 I 130 a half-life of 12 hours. Both have been used for biological studies 

 but are too short-lived for most experiments. The isotope I 129 has a half- 

 life of more than 10 7 years; this is too long-lived to be useful as a tracer. 

 However, I 131 has a half-life of 8.0 days which is a very convenient length. 

 It implies a much higher disintegration rate per gram atom than C 14 . 

 In the matter of a few months, the I 131 is almost completely disintegrated 

 and it no longer represents a health hazard. Owing to its short half- 

 life, the counts obtained must always be interpreted in terms of the 

 fraction of the original sample not yet disintegrated. 



In disintegration, I 131 emits many particles; these include negative 



