Sec. 9.16] AfASS SPECTROGRAPHS 301 



occluded gases, the quartz tube is heated to 1100°C in a gas-oxygen flame 

 until the barium carbonate is completely dissociated. A detailed discussion 

 of carbon-isotope chemistry will be found in reference 25. 



Determination of the ratio C 13 /C 14 is obtained directly from the mass 

 peaks 44 and 45 corresponding to C 12 16 16 and C 13 16 16 + C 12 16 17 , 

 respectively. The contribution of C 12 16 17 to mass 45 is usually negligible. 

 Masses 12 and 13 can also be used, but the beam intensities are considerably 

 smaller than for the molecular ions. They are not, however, difficult to 

 measure, and more important, they are not affected by the oxygen isotopes. 

 In terms of the ratio R of the intensity of C 12 (mass 12 or 44) to C 13 (mass 13 

 or 45) the atom per cent concentration of C 13 is 



%C 13 = 100 



R + l 



where R = C 12 reading/C 13 reading 



9.16. Nitrogen. The relative abundance of the nitrogen isotopes 14 and 15 

 in a sample substance is determined from measurements of the nitrogen 

 molecule masses of 28, 29, and 30. The preparation of the nitrogen from the 

 organic sample material requires first its reduction to ammonia and then 

 oxidation of the ammonia to free N2 by hypobromite [26]. The ammonia 

 obtained from the sample substance is contained in 0.05iV hydrochloric acid 

 which is boiled down to drive off dissolved gases and then transferred to a 

 nitrogen generating system. With the system evacuated, hypobromite is 

 added to oxidize the ammonia according to the reaction 



2NH 3 + 3NaOBr -+ N 2 + 3H 2 + 3NaBr 



The nitrogen is toeplered into a small sample bottle equipped with a stopcock 

 and transferred to the spectrometer sampling system. 



For N 15 concentrations of the order of 1 per cent and less, only masses 

 28 (N 14 N 14 ) and 29 (N 14 N 15 ) need be measured. The probability for forma- 

 tion of N 15 N 15 is negligibly small. The atom per cent of N 15 in the sample is 



100/ 29 100 



c/„ TV 15 = ■ = — 



/o x> 2/28 + /29 2R + 1 



where 7 28 = ion current of mass 28 (N 14 N 14 ) ( 

 I 29 = ion current of mass 29 (N 14 N 15 ) ' 

 R = p*/pf> 



When the concentration of N 15 is greater than several per cent, it is necessary 

 to measure masses 28, 29, and 30 since the contribution of N 15 N 15 becomes 

 appreciable. The atom per cent of N 15 is then 



/o ^ - 2 (/28 + /29 _j_ J30) 



