Other Instrumental Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 311

Figure 12.1 Major components of an ICP-MS system ( # Agilent Technologies, Inc., 2005,
Reproduced with permission, Courtesy of Agilent Technologies Inc.)
(separating ions), (g) dynode detector (counting ions), and (h) data handling and
system controller. It should be noted that variations exist among different
manufacturers. The quadrupole-based mass spectrometer, as depicted in this figure,
and the electron multiplier-based detector are the most commonly used.
To understand the hardware of ICP-MS, one should first recognize the
importance of an interface between ICP and MS. The ICP torch, used to atomize and
ionize materials (Chapter 9), requires a high operational temperature (6000 K),
whereas the mass spectrometer is operated at room temperature and requires the
vacuum condition to avoid collisions with any gas molecules before ions can reach
the detector. This task is accomplished through an interface between ICP and MS,
and the use of vacuum pumps to remove nearly all gas molecules in the space
between the interface and the detector. Second, one should note that in ICP-MS, ions
are directly counted rather than measured on the basis of light emission. Hence, there
will be no optical device (e.g., optical lens, prisms) in ICP-MS and devices of
differing mechanisms are needed.
Ions from the ICP torch are first focused with an electrical field. This is
analogous to the optical lens used in ICP-OES to bend the light beams of various
wavelengths. These ions of various m/z values are then separated in a quadrupole
system (Fig. 12.2), which consists of four metal rods approximately 20 cm in length
and 1 cm in diameter. The quadrupole separates ions by setting up the correct
combination of voltages and radio frequencies to guide ions of selected m/z between
the four rods. The ions that do not match the selected m/z pass through spaces
between the rods and are ejected from the quadrupole. With this regard, the
quadrupole is also referred to as a mass filter. The mass filter is equivalent to a
monochromator in ICP-OES used to separate out the wavelengths of the light
emitted by elemental ions.