508 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 24 



the case of five elements — all of light atomic weight — have practical methods 

 been long developed for concentrating the less abundant isotope for tracer 

 studies, namely, for H, C, N, 0, and S. For H, C, and S, convenient radio- 

 active species also exist; but successful tracing of N and O must depend in the 

 vast preponderance of cases on the rarer stable forms. For similar reasons, 

 if much tracing with Mg is ever to be developed, it would appear likely that 

 some extension in stable isotope tracing must be made. The same considera- 

 tions may well apply to He, Li, B, Ne, and probably Si. Aluminum is a 

 particularly unfortunate element from this point of view inasmuch as it exists 

 in only one stable form and has no known radioactive species of longer half- 

 life than 6.7 min. 



Recently the U. S. Atomic Energy Commission has announced the avail- 

 ability of isotopic mixtures for some 29 elements enriched with rarer species, 

 primarily by electromagnetic separation (and more recently, of a thirtieth — 

 He). These include isotopes of O and Mg. The extension of stable isotopic 

 tracing to heavier elements, for which isotopic separation is increasingly 

 difficult and for which convenient radioactive species are known, would seem 

 to depend upon the desirability of entirely eliminating the effects of the 

 radiation associated with the radioactive species of many of these elements 

 or, in some cases, upon the need for multiple labels of a given element. So 

 far there is little evidence that the former will prove a practical necessity; it 

 may well be, however, that even the slight amount of radiation accompanying 

 tracer doses will be shown, especially in the case of long-lived isotopes, to have 

 a deleterious effect, particularly from the genetic aspect, on living organisms. 

 Multiple labels for such elements as Ca and Fe may well prove of great value; 

 if so, stable species will very likely play a role. 



a. Metabolic Tracing. In the realm of metabolic tracing, the role, in 

 normal and pathological states, of elements and compounds normal to the 

 organism is unquestionably the most vital aspect. We deal here with both 

 essential and presumably nonessential elements. In the former category are 

 elements that range from those making up the major organic and inorganic 

 constituents of protoplasm (H, C, N, 0, Mg, F, S, and usually, K and Ca) to 

 those required by all living substance in minute quantities (Fe and possibly 

 also Mn and Zn — the so-called micronutrient, or trace, elements). The 

 need of certain organisms for additional macronutrient elements (Na and CI) 

 is well known; these, together with various micronutrient elements and others 

 "normal" to the protoplasm and protoplasmic products of various organisms 

 embrace the greater part of the periodic system. Many presumably unessen- 

 tial elements usually occur in exceedingly minute amounts. Some of these, 

 however, may eventually prove to be essential micronutrients. 



Actually or potentially useful isotopic labels are known for all elements 

 shown to be essential to living organisms (except possibly aluminum). 





