Geoscience Reference
In-Depth Information
Figure E.3
Eye-Dee Island and the principle of isotope dilution. Blue butterflies are released on Eye-Dee in
order to count its native population of red butterflies. If the number of foreign blue individuals
(spike) is known, the blue/red ratio in a sample gives the number of red individuals on the island.
does not affect the result (provided a minimum number of individuals is taken): once the
spike is added and has completely mixed with the sample, the mixing ratio can be obtained
from any fraction of the mixture, however incomplete. Isotope dilution does not require a
full recovery of the element to be analyzed. This absolute character of the method, offset
unfortunately by its painstaking nature, makes it an outstanding reference method.
Measurement of isotope compositions
This measurement principle is very different from those governing the methods described
above, as we seek to measure the ratio of two signals emitted simultaneously corresponding
to two different isotopes of the same element. Because the ratio of two simultaneous signals
is compared, the precision of isotopic measurements is far better than that of elemental
analysis, for which the intensity of the signal itself constitutes the measurement.
Figure E.4
is a diagram of a mass spectrometer. The different ion sources described in the following
correspond to different physical processes and to different applications:
1.
Gas bombardment
sources are based on the principle of spraying the sample gas with
electrons, which strip a peripheral electron from it. This type of source is used for
measuring the isotopic compositions of hydrogen (H
2
gas), carbon and oxygen (CO
2
gas), sulfur (SO
2
gas), and also the inert gases (He, Ne, Ar, Xe). The use of lasers for
in
