592 FRED M. UBER 



oxygen. Existing exchange evidence indicates that the use of oxygen 

 isotopes in the sulfate radical and in the nitro group may be permitted 

 under some circumstances, but the former does exchange with water 

 in concentrated acid solution. 



Exchange reactions of the sulfur present in various inorganic radi- 

 cals has also been studied {cf. 15). 



Nitrogen atoms attached to carbon appear to be molecularly 

 stable in biological systems as far as the amino acids are concerned. 

 In view of their great importance, this is very fortunate. Urea nitro- 

 gen under some circumstances may imdergo very slow exchange in 

 solution. Gaseous nitrogen has been shown not to exchange with 

 molecularly combined nitrogen in solutions of either sodium nitrate 

 or sodium nitrite. 



Carbon has been found to exchange rapidly between the carboxyl 

 group of sodium acetate and acetic anhydride {17, p. 159). In gen- 

 eral, one would expect stability of carbon-to-carbon bonds, but it 

 should not be assumed mthout experimental confirmation. 



7, Interfering Particles 



Two types of interference are recognized. One type results when 

 the test gas employed contains one or more chemical elements in addi- 

 tion to the specific one under investigation. The isotopic constitu- 

 ents of any accessor}^ elements bring about the interference. An ex- 

 ample occurs in the use of carbon dioxide for the analysis of C^^. 

 The peak at mass 45 results not onlj^ from C^^O^^O^^ but also from the 

 presence of C^^O^^O^^ ions (see Fig. 4). Thus the accessory oxygen 

 atoms with their possible masses of 16, 17, and 18 give rise to higher 

 measured ratios than can be ascribed to the excess concentration of 

 C^^ alone. Corrections for this type of interference can be made 

 readily if the concentrations of the offending atoms are known; an 

 illustration of the calculations involved has been given earlier. 



A second type of inteiference can be ascribed to the presence of 

 radicals of the test gas. These may be either singly or multiply 

 ionized. Reference may be made to the methane spectrum in Figure 

 1 as an illustration. The peak at mass 15 owing to CH3 radicals is 

 almost as high as the CH4 peak. Consequently in any analysis for 

 C^^, nearly half the C^^ atoms would contribute to the mass 16 peak 

 as C^^Hs ions, together with the C^^fj^ ions, rather than appear at 



