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Sample preparation methods are similar to those used for FAAS and ICP-AES. However, nitric acid is
favoured for sample digestion since the other mineral acids contain elements which cause spectroscopic
inteferences.

Because of its capability for rapid multielement analysis, ICP-MS is particularly suited to sample
introduction methods which give rise to transient signals. For example, electrothermal vaporization,
flow injection and chromatographic methods can be interfaced and many elements monitored in a single
run (see Chapter 7).

A major attraction is the ability to perform isotope ratio measurements, e.g. in many geological
applications to determine the age of rocks, and isotope dilution analysis. The latter in particular is
gaining-popularity as a highly accurate, precise and hence traceable, method of analysis, so it is
worthwhile describing these techniques in more detail.

5.7.1 Isotope Ratio Analysis

Isotope ratio measurements are performed whenever the exact ratio, or abundance, of two or more
isotopes of an element must be known. For example, the isotopic ratios of lead are known to vary
around the world, so it is possible to determine the source of lead in paint, bullets and petrol by
knowing the isotopic abundances of the four lead isotopes 204, 206, 207, 208. Another example is the
use of stable isotopes as metabolic tracers, where an animal is both fed and injected with an element
having artificially enriched isotopes and the fractional absorption of the element can be accurately
determined.

In order to perform the isotope ratio experiment correctly it is necessary to compensate for a number of
biases in the instrumentation. Quadrupole mass spectrometers and their associated ion optics do not
transmit ions of different mass equally. In other words, if an elemental solution composed of two
isotopes with an exactly 1:1 ratio is analysed using ICP-MS, then a 1:1 isotopic ratio will not
necessarily be observed. In practice, transmission through the quadrupole increases up to the mid-mass
range (ca m/z 120), then levels off or decreases gradually up to m/z 255. This so-called mass bias will
differ depending on mass, with the greatest effects occurring at low mass, the least effect in the mid-
mass range and intermediate effects at high mass, as shown in Fig. 5.10. Even very small mass biases
can have deleterious effects on the accuracy of isotope ratio determinations, so a correction must always
be made using an isotopic standard of known composition, as shown in the equation