36 MEASURING TIME IN GEOLOGY 



the high vacuum of the instrument proper, at which time they are 

 accelerated by the high-potential electric field. 



The basic construction of a mass spectrometer is schematized in 

 Fig. 7. A mass spectrometer consists of three units. At the beginning 

 there is the system for handling the samples. This is purely a chemical 

 apparatus, in which the samples are purified, enriched and converted 

 into the gaseous compound suitable for mass spectrometry. Then 

 follows the actual instrument tube, which is often small compared 

 with the accessory parts. At the end of the tube is the ponderous 

 electronic equipment which counts the separate ions that either fall 

 on a number of collimator slots, or are plied over a single slot at the 

 end of the high-vacuum tube by suitable variations of the magnetic 

 field strength. These counts are amplified and recorded in such a 

 way that the mass spectrometer gives an answer that does not require 

 any further processing. 



Isotope dilution 



In contrast with the cruder pre-war methods, which were preva- 

 lently chemical, it follows that mass spectrometry uses the physical 

 differences of the nuclei of the various isotopes directly to determine 

 their ratios. It is nowadays often possible to dispense completely 

 with chemical analyses of relative or absolute abundance of parent 

 and daughter elements in a radioactive decay series. The technique 

 used is that of isotope dilution, which was originally introduced to 

 geological problems in the rubidium — strontium dating. In this meth- 

 od the sample is mixed with various small amounts of the same 

 element of known isotope ratios. The variations in isotope ratio 

 resulting in the mixture and recorded in the mass spectrometer 

 permit a calculation of the isotopic ratio of the original sample. 

 Samples of elements with uniform isotopic composition are cur- 

 rently produced by atomic reactors, and as such are available for 

 absolute geologic dating. 



The use of isotope dilution in mass spectrometry has been a major 

 stride foru'ard in absolute dating. Instead of the clumsy chemical 

 techniques, the quantitative analyses of parent and daughter elements, 

 in which even the smallest samples are composed of a very great 

 number of atoms, we now only need a couple of mass spectrometric 

 analyses. The samples may now be very much smaller than in chem- 



