3.2 Atomic Emission Spectrometry Theory
The operation of an ICP-AES system relies upon the same interaction of molecules with electromagnetic radiation that was presented in Chapter 1. The two emission systems, FAES and ICP-AES, differ in the way atomic species are created and excited. Because of the relatively low temperatures (~2000-2500 C) in a flame-based system, not all of the atoms or elements present in the sample are excited, particularly if they exist in a polyatomic compound. Some elements readily form non-emitting and refractory oxides that result in an underestimation of their concentration. In plasma-based systems the temperature is considerably hotter (~6000 to 10 000 K) that results in more effective excitation of atoms (generally greater then 90%) of approximately 60 elements including some nonmetals. This intense heat prevents polyatomic species from forming, thus increasing the detection limits for many elements. Atoms are excited, and in many cases ionized, by the intense heat of the plasma, and the emission of a photon occurs via resonance fluorescence (normal valance electron relaxation by photon emission). While plasma-based systems eliminate many problems, they are not free of interferences due to the excitation and subsequent emission of spectral lines for every element in the sample as well as the Ar added to facilitate plasma generation. The spectral overlay that results from these possible emissions is overcome in modern instruments with specialized sequential monochromators (Section 3.4.4). ICP-AES, compared to FAAS/FAES, offers high selectivity between elements, high sensitivity, a large dynamic range, especially as compared to FAAS that is limited by Beer’s law, lower detection limits, multi-element detection, and fewer matrix interferences.
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