ABSOLUTE DATING 



35 



ni 



2 V 



where m = atomic mass, e = ion electric charge, R = radius of 

 ion trajectory in the high-vacuum tube, and B = magnetic field 

 strength. The number of ions of different mass which, after describing 

 slightly divergent trajectories, fall on the collimator slit at the end 

 of the tube, is counted by the electronic set-up. 



Mass spectrometers are easiest to work with vapour mixtures. The 

 samples of lead or any other element of which the relative isotopic 

 composition must be ascertained, are introduced as a suitably ionized 

 gaseous chemical compound. They leak through a small orifice into 



High potential 



electrical field 



for ion acceleration 



electronic counting 

 and recording 

 system 



collimator sitt 



path followed 

 by ions 



magnetic 

 field 



J. 



Sample 

 preparation 



leak through which 

 -ionised atoms of sample 

 reach mass spectrometer tube 



High vacuum 

 pumps 



180* mass spectrometer 

 tube 



electric current 



and stabiliser. 



producing a 



magnetic field 



perpendicular 



to mass spectrometer 



tube 



Fig. 7. Diagram of a 180° gas-source mass spectrometer. 



