Page 33 - Inorganic Mass Spectrometry : Fundamentals and Applications

~he~al Ionization Mass Spectrometry 23

uncertainties in the concentrations of many elements [see, for example, Naka-
mura (81)l.
Elements are formed by three different mechanisms in stars, as elucidated
by Burbidge et al. [82]. In brief, these are the s process, involving the capture of
slow neutrons by nuclei; the r process, involving the capture of rapid neutrons; and
the p process, which leads to neutron-deficient nuclides that are generally less
abundant than those generated by the s and r processes. The relative abundances of
the various isotopes of a given element reveal which processes, or combinations
thereof, are involved in their creation in the nuclear reactions that power stars.
Measurements of isotopic and elemental abundances in stars thus serve to test
theories of stellar processes.
Mass spectrometry in general and thermal ionization in particular have
helped identify type 1 carbonaceous chondrites (C 1) as the extraterrestrial bodies
that best represent the composition of stars, gaseous nebulae, and other galactic
entities [so]. This is because, of all the meteorites that fall to Earth (known as the
poor man’s space probes), carbonaceous chondrites have undergone the least
metamo~hism and thus retain more of their original volatile element content.
There is excellent agreement between the composition of these chondrites and
measurements of the elemental composition of the solar photosphere and of many
other stars [83]. The distribution of even and odd mass nuclides in carbonaceous
chondrites forms a smooth curve, whereas such distributions in other meteorites
do not; a plot of this elemental distribution is given in Fig. 1.1 1. The fact that even-
proton elements are more abundant than odd-proton elements and have more
isotopes has long been known and is confirmed in this plot [84]. The most recent
of numerous tables of cosmic elemental abundances was published by Anders and
Grevasse in 1989 [85]. Cosmological application of mass spectrometry was
recently reviewed by De Laeter [ 801.

The nuclear area is one that has been heavily dependent upon isotope ratio mass
spectrometry performed by thermal ionization. Applications in this area are
among the major reasons for the continued push to analyze smaller and smaller
samples. There are two primary reasons for this: (1) maximum practicable reduc-
tion of the hazards associated with radioactivity and (2) presence of often only a
very small amount of the target element available. Areas addressed include evalu-
ation of uranium enrichment processes [%I, isotopic analysis of tr~s~anium
elements (all elements through einsteinium have been analyzed) [ 87, and envi-
ronmental monitoring for release of uranium and other actinides [88,89]. This last
area has received renewed emphasis in the wake of the Gulf War [90].
Isotopic analysis has been used extensively in addressing questions of
fund~ental physics. Walker et al. used the Dounreay reactor to refine values of