XVI. STABLE 1 S O T O 1' 10 .S A S TRACERS 585 



to eliminate accidental mistakes in both measurements and calcula- 

 tions. 



With molecular oxygen as the test gas, determinations of low 

 concentrations of O'^ and 0'^ could be calculated on the basis of meas- 

 urements on only t wo peaks by modifying equation (5) as follows : 



p (of 0'7) = — 32 - atom per cent 



and: 



P (of 0^*) = — wr: atom per cent 



2 34i2 + 1 ^ 



When 0^* is used as a tracer and carbon dioxide is the test gas, its 

 concentration may be found from : 



P (of O^*) = 44 atom per cent 



Z isK -j- 1 



since the contribution to the height of the 46 peak owing to C^^O^^O^^ 

 is a negligible factor. 



Before calculating low deuterium concentrations by an analogous 

 formula based on masses 2 and 3, a preliminary correction of the mass 

 3 reading owing to the presence of triatomic hydrogen molecules is 

 considered essential (see 3, p. 99). Intermediate concentrations of 

 hydrogen and deuterium offer additional difficulties owing to the 

 presence of several types of ion at both the mass 3 and 2 positions. 



The general equation for the concentration, P"*, of an isotopic 

 element, which may contribute two atoms with masses of either w or 

 n to the charged particle being measured, is: 



[]^p(n+. + .) _,_ p(2m + .)Ji00 



atom per cent {o) 



p(2n + s) I p(n + m + s) i p(2w + s) 



where each P on tlu; right of the equation represents the measured 

 ion current corresponding to the mass numbers given by the expo- 

 nents; s is the sum of the atomic mass numbers for any foreign atoms 

 common to all of the ionic particles. This equation therefore requires 

 measurements foi- all three mass numbers. For example, it is directly 

 applicable to determinations of N^^ for intermediate concentrations. 



Measurements of N^', and also of the oxygen isotopes, often must 

 be corrected for the presence of air that has leaked into the mass 



