ABSTRACT
In inductively coupled plasma mass spectrometry the sample, which is usually in liquid form, is pumped into the sample introduction system, comprising a spray chamber and a nebulizer. It emerges as an aerosol, where it eventually finds its way via a sample injector into the base of the plasma. As it travels through the different heating zones of the plasma torch, it is dried, vaporized, atomized, and ionized. During this time, the sample is transformed from a liquid aerosol to solid particles, then into gas. When it finally arrives at the analytical zone of the plasma, at approximately 6000-7000 K, it exists as ground state atoms and ions, representing the elemental composition of the sample. The excitation of the outer electron of a ground state atom to produce wavelength-specific photons of light is the fundamental basis of atomic emission. However, there is also enough energy in the plasma to remove an electron from its orbital to generate a free ion. The energy available in an argon plasma is ~15.8 eV, which is high enough to ionize most of the elements in the periodic table (the majority have first ionization potentials in the order of 4-12 eV). It is the generation, transportation, and detection of significant numbers of positively charged ions that give ICP-MS its characteristic ultra trace detection capabilities. It is also important to mention that although ICP-MS is predominantly used for the detection of positive ions, negative ions (e.g., halogens) are also produced in the plasma. However, because the extraction and the transportation of negative ions are different from that of
FIGURE 2.1 Generation of positively charged ions in the plasma.
