Page 241 - Inorganic Mass Spectrometry - Fundamentals and Applications

Isotope ~ilutio~ Mass Spectrometry 22 7

Others make use of concentrations in terms of atoms, as does, for example,
Heumann [8,9]. Others use concentrations in terms of moles, as do Fassett and
Paulsen [ 101 and Dean [ 1 l]. Clearly each investigator selects the form with which
he or she feels most comfortable. Due caution is indicated to make sure of one’s
units of concentration before blindly using an equation from the literature.
There are several features to note about this equation. It important to rec-
is
ognize that it calls for weights that include all isotopes of the spike and sample and
not solely the weight of the isotopes used in Rm; the equation simplifies if these
latter are used, but most chemists think in terms of weight of the element (as in
parts per million) and not one its isotopes. The right side of the equation is unit-
of
less except the ~~/~~ term; the units used in this numerator and denominator thus
define the units of the concentration calculated. Even though the symbols assume
weights, volumes can of course be substituted. Typical units are micrograms per
gram (ppm) or micrograms per liter, but they can be whatever units the analysis
is
requires. Another point to note that the only isotopes that need be measured from
ratio;
the mixture of sample and spike are the two involved in the isotope dilution
none of the other isotopes in the mixture plays a role. The same is not the case for
the spike and the unspiked sample, however; the full isotopic composition of each
of these components is required to calculate their atomic weights. The amount of
laboratory work required is reduced when the isotopic composition of the sample
is known, as would usually be the case if it were the naturally occurring element,
for which the Inte~ational Union of Pure and Applied Chemistry (IUPAC) tabu-
lated values of isotopic composition [ 121 are invaluable; enriched isotopic spikes
usually have a certificate listing their isotopic compositions. In these relatively
is
common circumstances, the only analysis required that of the mixture.
Isotope dilution is applicable to any element for which an enriched isotope
is available. Figure 1.1 of Chapter 1 indicates which elements are amenable to iso-
tope dilution; in most cases the natural element has at least two stable isotopes, but
this is not necessarily the case. For example, 232Th, though radioactive (half-life
of 1.4 X lOlo years), is present in the earth’s crust; 230Th (half-life of ’7.5 X IO4
years), an isotope present in nature at such low levels as to negligible for most
be
applications, is used as a spike for isotope dilution purposes in the author’s labo-
is
ratory. Another common example the use of 233U (a synthetic isotope) as a spike
for uranium analyses. The only elements not amenable to the technique are those,
like cobalt and arsenic, that have only one stable isotope and all of whose ra-
dioactive isotopes have half-lives so short as to preclude their use.
In the laboratory, the isotope dilution procedure involves adding a known
amount of spike of known isotopic composition to a known amount of sample of
known isotopic composition; the mixture of spike and sample is equilibrated; the
ratio of the sample isotope to the spike isotope is then measured; and the resulting
Rm is inserted into the equation. For replicate analyses, this is the only parameter