Page 32 - Inorganic Mass Spectrometry : Fundamentals and Applications

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One area of major importance is in geological applications. There are about 25
radionuclides whose half-lives are long enough that they are still present in the
Earth’s crust, having been there at the Earth’s formation; these range from 235U
with a half-life of 7.1 x 108 years to 209Bi with a half-life in excess of 1019 years
in addition about 30 radionuclides whose half-lives are too short
e survived from the primordial Earth that are present because they
are decay products of uranium and thorium; the most abundant is 2341J, whose
half-life is 2.45 x 105 years and which comprises 55 ppm of naturally occurring
uranium [72]. Radioactive decay is a statistical process whose rate is measured by
the nuclide’s half-life. A radionuclide and one of its stable daughters constitute a
geological clock from which the age of the formation in question can be deter-
mined. Some of the common methods involve performing mass analysis using
thermal ionization. Among these are Rb-Sr, IJ-Pb, and Nd-Sm. The interested
reader is referred to the excellent book by Faure, which covers application of
isotope ratio measurements to all aspects of geology [2].
Deviation of an isotopic ratio in a mineral from its normal value can give
insight into climatic conditions of past ages during which the mineral deposits
formed. Because isotopic fractionation is a function of the mass of the isotopes,
lighter elements are more sensitive probes than heavier elements in this area.
Boron is an example; even though its two isotopes differ by only 1 mass unit (mk
10 vs. 1 l), the 10% difference is quite high. This is in a sense a two-edged sword,
for the very reason it is a sensitive geological probe makes isotopic fractionation
of the elemental ions at mlz 10 and 11 under thermal ionization conditions
unacceptably high and difficult to control. For this reason, a molecular metaborate
ion is usually monitored; Cs2B02+ (m/z 308,309) is a comon choice [78]. For
exam$e, the loBPIB ratio was used to study the origins of zoned deposits that
crystallized from ancient* lakes [79]. vestigators were able to identify borate
deposits o~ginating via several di chanisms in a commercially important
source of boron.

lo lic
i
Among the questions of importance to cosmology are the elemental composition
of stars and other galactic matter and the isotopic compositions of those elements.
Investigations of this type have covered sev&al decades and represent a nice col-
laboration between theoretical astrophysicists 8 and mass spectrometrists [SO].
Thermal ionization has played a role in analysis, both isotopic and, through
isotope dilution, ofmconcentration, of many of the elements and helped resolve
some ?of the anomalies that were present in the results ,of eaxly work. Isotope
dilution is inherently a precise method of quantifi~ation md was able to reduce