ABSTRACT
In ICP-MS, the sample, which is usually in liquid form, is delivered into the sample introduction system, comprising a spray chamber and 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 and then into a 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 wavelengthspecific photons of light is the fundamental basis of atomic emission. However, there is also enough energy in the plasma to remove one or more electrons 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 on the order of 4-12 eV). It is the generation, transportation, and detection of significant numbers of positively charged ions that gives ICPMS its characteristic ultratrace detection capabilities. It is important to mention that although ICP-MS is predominantly used for the detection of positive ions, negative ions are also produced in the plasma. However, because the extraction and transportation of negative ions is different from that of positive ions, most commercial instruments are not designed to measure them. The process of the generation of positively charged ions in the plasma is conceptually shown in greater detail in Figure 2.1.
