only, and from a single sample injection. A typical spectrum is shown in Figure 8.16.
As with ICP-AES (p. 297 et seq.), interfacing with separation procedures may be possible and the
superior sensitivity possessed by ICP-MS means that it is often the more effective of the two techniques
in this respect. A particular interference associated with ICP-MS is due to the formation of ions by the
argon plasma gas. Obviously argon ions themselves will interfere directly, e.g.^40 Ar+ at mass number 40
and at 80. In addition polyatomic ions can also be formed between argon and some matrix
elements such as C, O and N. For example,^40 Ar^12 C+ will interfere at mass 52,^40 Ar^14 N+ at mass 54, and
(^40) Ar (^16) O+ at 56. These problems have proved rather intractable until recently, but currently progress is
being made in overcoming interferences from polyatomic species.
Instrumentation
The layout of an ICP-MS is shown schematically in Figure 8.17 and comprises three essential parts; the
ICP torch, the interface and the mass spectrometer. The ICP torch differs little from that discussed
earlier and the mass spectrometer is very similar to those used for organic mass spectrometry and
discussed in Chapter 9. Typically a quadrupole instrument would be used. The construction of the
interface is shown in Figure 8.18 and is based on the use of a pair of water-cooled cones which divert a
portion of the sample stream into the ion optics of the mass spectrometer whence the mass spectrum is
produced by standard mass spectrometer operation. Some modern instruments also incorporate a so-
called de-clustering interface' located behind the skimmer cone. This is essentially an enclosed
hexapole ion bridge into which small amounts of helium or xenon can be released to reduce the mean
free path of the ions passing through the interface. The consequent collisions reduce the ion energy
spread and in so doing improve the resolution of the instrument. In addition, and perhaps more
importantly, the collisions break up molecular species before they reach the mass analyser and thus
enable the production of a spectrum free of argon interferences. Figure 8.19 illustrates this effect.
Applications
ICP-MS may be applied to the determination of elements across the whole of the periodic table from
lithium to the actinides. With certain exceptions, limits of detection are of the same order or better than
those for graphite furnace-AA or ICP-AES. Table 8.4 makes a comprehensive comparison.
A particular advantage of ICP-MS derives from its ability to display a complete mass spectrum at one
time. Combined with sample introduction by laser ablation it constitutes a very powerful tool for first-
look analysis, e.g. in geological prospecting or ecological surveys. ICP-MS is applicable to the whole
range of areas where minor or trace elements are to be determined.