170 



ISOTOPIC TRACERS 



Accelerating 

 electrodes 



yXuU 



Cathode ray 

 oscilloscope 



n r 



ion collector 



Gas inlet 



Vacuum 



Fig, 12-3. A time-of-flight mass spectrometer. Gas is ad- 

 mitted, ionized by a stream of electrons, and accelerated 

 toward the ion collector by the accelerating electrodes. 

 Current on the ion collector is registered on the oscillo- 

 scope. 



The other basic type of mass spectrometer (Fig. 12-4) separates ions 

 of different mass in a magnetic field. The tube has a 60°, 90°, or 180° 

 bend surrounded by the poles of a large magnet. As the ions traverse the 

 tube they are deflected by the magnetic field through an angle dependent 

 on their momentum. The accelerating voltage and the magnetic field can 

 be adjusted to focus ions of a selected mass on the target electrode. By 

 varying the accelerating voltage or the magnetic field or both, the whole 

 spectrum of ions can be swept across the target. Current in this electrode 

 will vary according to the number of ions, just as in the time-of-flight 

 instrument. If the "mass spectrum" is scanned automatically, the instru- 

 ment will record the relative amount of each component in the mixture. 



The actual operations required in a tracer experiment using stable 

 isotopes become somewhat complex. We might perform a tracer experi- 

 ment in which N^^ is used to follow the production of ammonia (NHs)- 

 N^^Hs has a mass of 17, but N^'^Hs has a mass of 18, and the two could 

 be separated on the mass spectrometer. Ordinary water also has a mass 

 of 18, however, and H2O would obscure the N^'^^a. In this case it would 

 be preferable to convert the nitrogen to some other form with unambigu- 

 ous mass numbers. If we measure C^-Qo (Mass 44) and C^^02 (Mass 

 45), it becomes necessary to correct for O^' (C^'O^'^O^': Mass 45). Ex- 

 cept for these technical details which must be considered, the mass spec- 

 trometer permits tracer experiments with stable isotopes, experiments 

 which can be just as effective as radioisotope experiments. 



