302 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 9 



It is obviously important in measuring nitrogen to avoid air leaks, dis- 

 solved or adsorbed air in the solutions, in the sampling system, or in the 

 spectrometer. The presence of air contamination is made evident by the 

 oxygen peak at mass 32. If the ratio of the nitrogen 28 peak to the oxygen 

 32 peak for normal air has been determined previously for the spectrometer, 

 a correction for a small per cent of air contamination can be made readily 

 from the measured intensity of the O2 peak which, when multiplied by the 

 N/O ratio, gives the required mass 28 correction. 



9.17. Oxygen. The abundance of the oxygen isotopes in 18 -enhanced 

 samples may be determined from measurements of the oxygen molecule 

 masses 32, 33, 34, etc., or by introducing the oxygen as carbon dioxide and 

 measuring masses 44, 45, and 46. If the concentration of O 18 is a few per 

 cent or less, contributions by molecules of the form O l7 17 , 17 18 , and 

 Qi8Qi8 are entirely negligible and only masses 32, 33, and 34 are measured. 

 The atom per cent of O 18 is then 



% O 18 = 



100/ 



:u 



2 (/32 _J_ J33 _|_ /3 4) 



where P 2 = intensity of mass 32 

 I n = intensity of mass 33 

 I 3i = intensity of mass 34 



A similar analysis is applied for measurements of oxygen in the form of 

 carbon dioxide. Neglecting the contributions by C 13 16 17 and C 13 16 18 , 

 masses 44, 45, and 46 give the atom per cent of O 18 by the formula above. 

 For high concentrations of O 18 it may also be necessary to measure mass 47. 



Air contamination is important, but if its percentage is small its contribu- 

 tion to the measured peaks can be corrected. This may be done by measur- 

 ing the nitrogen 28 peak. If the ratio of the oxygen 32 mass to nitrogen 28 

 mass for normal air is known for the instrument, the appropriate corrections 

 are made as indicated in the last section for nitrogen. 



The oxygen atomic masses 16, 17, and 18 usually cannot be measured 

 accurately because of the background of water-vapor peaks. 



REFERENCES FOR CHAP. 9 



1. Aston, F. N.-.Phil. Mag., 38, 709 (1919). 



2. Aston, F. N.: "Isotopes," Edward Arnold & Co., London, 1924. 



3. Dempster, A. H.: Pkys. Rev., 11, 316 (1918). 



4. Herzog, R.: Z. Physik, 89, 786 (1934). 



5. Mattauch, J., and R. Herzog: Phys. Rev., 50, 617 (1936). 



6. Shaw, A. E., and W. Rall: Atomic Energy Commission Report MDDC-45,1946. 



7. Bainbridge, K. T., and E. B. Jordan: Phys. Rev., 50, 282 (1936). 



8. Bleakney, W., and J. A. Hipple: Phys. Rev., 53, 521 (1938). 



9. Nier, A. O., E. P. Nery, and M. G. Inghram: Rev. Set. Instruments, 18, 191 (1947). 



