XVI. STABLE ISOTOPES AS TRACERS 589 



The different mass peaks in the ionization current curve of Figure 

 4 must be sufficiently well separated to avoid appreciable interference 

 with each other. This separation is a function of the slit widths of a 

 mass spectrometer, but it decreases with increasing mass of the ions. 

 For cases of insufficient resolution, and particularly where peaks dif- 

 fer markedly in intensity, an estimated correction may need to be 

 applied to the intensity of the weaker peak, thus decreasing the ac- 

 curacy. 



3. Natural Variations in Isotope Concentrations 



Generally accepted values for the natural concentrations of the 

 isotopes considered in detail in this chapter are listed in Table I. 

 Agreement between published values for some of these absolute ratios 

 has not been very good. The small discrepancies probably result 

 largely from differences in particle discrimination among the various 

 mass spectrometers. In addition, it has been demonstrated that 

 there is a natural variation in the concentration of isotopes depending 

 on their past history {2, p. 561; 17, p. 8). 



"Normal" C^^ concentrations have been found to vary as much 

 as 5% between carbon obtained from limestone and carbon obtained 

 from plant cells. Similarly 0'^ from carbonate ores has a concentra- 

 tion about 4% higher than that found in water generally, but deu- 

 terium from widely different sources is constant to within a few 

 thousandths of 1%. There has been little work on variations in the 

 natural occurrence of N^^, but some analyses made on amino acids 

 isolated from the protein of mammals have indicated a slight excess 

 varying from 0.000 to 0.006 atom per cent N^^ over that found in air. 

 In one study with arginine, an excess of 0.008 atom per cent N^^ was 

 found. No information concerning natural concentration variations 

 for the several isotopes of sulfur is known to the writer. It may well 

 be expected that very small variations in concentration will occur in 

 some biochemical reactions. "Wliile a systematic study of such varia- 

 tions could be of interest in itself, perhaps its most useful function 

 would consist of indicating probable limits of accuracy for investiga- 

 tions employing isotopes as tracers. Most past experiments have 

 had such a large margin of safety with respect to concentration 

 changes that the conchisions have not been jeopardized o\ving to these 

 slight natural isotopic variations. While selective behavior of living 

 organisms toward low concentrations of isotopic compounds is ab- 



