Page 136 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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entropy) and adiabatic (i.e. there is no transfer of energy as heat), resulting in a supersonic expansion
accompanied by a fall in temperature. This supersonic expansion takes the form of a cone with a shock-
wave stucture at its base called a Mach disc. The region within the expansion cone is called the 'zone
of silence', which is representative of the ion species to be found in the ICP, i.e. the ionization
conditions have been 'frozen'.
The skimmer cone is another metal cone, the tip of which has an orifice approximately 0.7 mm in
diameter, that protrudes into the 'zone of silence', and is axially in-line with the sampling orifice as
shown in Fig. 5.3. The ions from the 'zone of silence' pass through the orifice in the skimmer cone, into
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a second intermediate vacuum chamber held at < 10 atm, as an ion beam The ion beam can then be
focused by means of a series of ion lenses, which deflect the ions along a narrow path and focus them
on to the entrance to the mass analyser.

Q. How do sample introduction systems used for ICP-MS compare with those used for ICP-AES?
Name one important difference.

Q. What are the ionization mechanisms that occur in the ICP?

Q. What aspect of the interface allows the atmospheric pressure ICP to be interfaced with a vacuum
mass spectrometer?

5.4 Mass Analysis
Mass analysis is simply a method of separating ions of different mass-to-charge ratio (m/z). However,
since the ions of interest are almost exclusively singly charged, then m /z is equivalent to mass for
practical purposes. There are two types of mass analyser commonly employed for ICP-MS, namely the
quadrupole and the magnetic sector.

5.4.1 Quadrupole Mass Analysis

Quadrupoles are comprised of four metal rods, ideally of hyperbolic cross section, arranged as shown
in Fig. 5.4. A combination of radiofrequency (RF) and direct current (DC) voltages are applied to
each pair of rods, which creates an electric field within the region bounded by the rods. Depending on
the RF/DC ratio, the electric field between the rods will allow ions in a narrow m/z range to pass,
typically 0.8 m/z — just how narrow will depend on a number of factors which influence the resolution.
Hence, by changing the RF/DC ratio in a controlled manner, the quadrupole can be