Page 11 - Inorganic Mass Spectrometry : Fundamentals and Applications

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Thermal (or surface-the two terms are synonymous in this context) ionization
has a long history in the measurement of isotope ratios. It has seen widespread use
in the determi~ation of atomic weights [l], age dating in geological applications
[2], and in various nuclear applications [3]. Quantitative results can be obtained
through the technique of isotope dilution (see Chapter 5). In this guise, it has
entered fields previously closed to it, including the environment [4,5] and nutrition
[6,7]. Its primary attractions are its high sensitivity for many elements and its good
precision and accuracy. This topic has been the subject of several recent review
articles [8,9].
In thermal ionization, ions are produced by the interaction of analyte species
with a heated surface, which is usually a metal. No other source of energy is used.
Most commonly, singly charged metal ions are monitored in the analysis, but this
is not invariably so. If negative ions are monitored, an oxide species is often a
better choice than the metal ion. Significant work in the last few years has led to
new appreciation of this hitherto neglected area of research [lo].
Not all elements are amenable to thermal ionization, positive or negative. It
is obvious that elements whose natural state is a gas cannot be addressed by this
technique. In addition, some elements are too volatile; others have a first ioniza-
tion potential too high, and a few, such as mercury, display both characteristics.
Generally speaking, solid elements with first ionization potentials below about 7.5
eV can be analyzed through measurement of positive ion beams. Use of silica gel
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