文档介绍：Chapter 11
Isotope Dilution
The isotope dilution method consists of mixing a natural sample with an
artificial spike and measuring the isotopic ratios of the mixture using
mass spectrometry, providing very precise quantitative determination of
the concentrations of elements in trace quantities. A spike is a solution
that contains a known concentration of the element, artificially enriched
in one of its minor isotopes. For example, natural Rb samples have
% 87Rb and % 8sRb (Fig. ). Rb spikes are made by
artificially enrich the minor isotope a solution with 90% 87Rb
and 10% 8sRb (Fig. 1 ) is a Rb spike. Of course, a solution with
% 87Rb and % 85Rb is a better Rb spike. When the known
quantities (mass) of the sample and spike are mixed, the resulting
positions can be used to calculate the concentration of the
element in the sample.
The spike in most cases is single spike, but it can be double spike.
Single spike method provides the information of the concentration of the
sample, whereas the double-spike method can yield the concentration of
the sample and the mass fractionation factor that can be used to calculate
the true isotopic ratios in the sample. Take Pb isotope as an example.
Natural Pb samples have % 201Pb, % '06Pb, % '"Pb and
% "'Pb. A single spike is made by artificially concentrating one of
the minor Pb isotopes, for example 'OJPb. A double spike is made by
artificially concentrating two minor isotopes. For example, we can
concentrate '04Pb and '&Pb isotopes to make '04Pb- "'Pb double spike or
concentrate '04Pb and '07Pb to make 2wPb-'07Pbdouble spike.
The theory of isotope dilution is a fine example that a better
understanding of the theory can improve experimental measurements.
223
224 Quantitative Geochemistry
100
90 Natural Sample A
80 85Rb
n
8 70
Y
Q
0 60
C
Q 50
0
C 40
3 "Rb
P
U 30 n
20
10