Page 246 - Inorganic Mass Spectrometry - Fundamentals and Applications

232 Smith

by
for most elements natural isotopic composition as specified International Union
of Pure and Appliced Chemistry (IUPAC) can be assumed. The only instances in
which this might not apply are those in which elements have isotopic compositions
in
that are not constant nature. Examples are fission products (such as neodymium),
in which small deviations from primordial composition might be encountered, and
the end products of naturally occurring decay chains. Lead, the end product of the
decay of uranium and thorium, is the outstanding example of the latter. 235U and
238U decay to 207Pb and 206Pb, respectively; 232Th decays to 208Pb. 204Pb thus the
is
only stable lead isotope that has no radioactive precursors. There are numerous
steps in each decay chain, mostly through short-lived nuclides. There was consid-
erable uranium, thorium, and lead present in the primordial earth, and the ratio of
the three elements varies widely with location.
This causes the isotopic composi-
tion of natural lead to display enormous variation; assuming any given composi-
tion is ill advised. Such variation in lead isotopes is only to be expected when one
recognizes that the half-life of 238U (4.5 X lo9 years) is about the age of the earth;
half the 238U present the earth’s formation has thus decayed 206Pb. These and
to
at
other na~rally occurring radioactive isotopes distort the isotopic compositions of
the
their decay products, thus providing basis of isotope geochronology. Examples
are cited in the next section.

By determining the amounts of a naturally occurring radioactive isotope and its
daughter in a given sample, geoscientists are able to calculate its age [6]. The fact
that the isotopes are naturally occurring means that half-lives are very long. Sev-
eral systems, including Rb-Sr, Sm-Nd, Lu-Hf, Re-Os, andTh-Pb, use isotopes with
half-lives in excess of 1O1O years. Many age-dating techniques use isotope dilution
to establish how much of each element is present. Plots of one ratio versus another,
~
~
~
called ~ ~ o allow determination ~ ~ ,
oof the specimen’s age. As one example, 176Lu
emits a beta particle, decaying to 176Hf with a half-life of 4.58 X 1Olo years. Fig-
ure 5.3 is the isochron for samples from Greenland; it establishes the date at which
Amitsog gneisses separated from the mantle as 3.59 * 0.33 X lo9 years ago [25];
this is very old, for the age of the earth is about 4.5 X lo9 years.
the
Studies such as this one allow geoscientists to investigate past and to get
some idea of geological conditions then. Because a small error in the measurement
of
of the ratio translates into a large uncertainty in the calculated age the sample,
isotope dilution mass spectrometry’s superior sensitivity and accuracy are impor-
tant in obt~ning meaningful results.
Environmental Applications
~ounting concern worldwide about emission of toxic substances into the envi-
ronment has driven elemental analysis to new levels of sensitivity and accuracy,