Flame emission compared with plasma sources

Plasmas compare favourably with both the chemical combustion flame and the electrothermal atomiser with respect to the efficiency of the excitation of elements. The higher temperatures obtained in the plasma result in increased sensitivity, and a large number of elements can be efficiently determined. Common plasma sources are essentially He MIP, Ar MIP and Ar ICP. Helium has a much higher ionisation potential than argon (24.5 eV vs. 15.8 eV), and thus is a more efficient ionisation source for many nonmetals, thereby resulting in improved sensitivity. Both ICPs and He MIPs are utilised as emission detectors for GC. Plasma-source mass spectrometry offers selective detection with excellent sensitivity. When coupled to chromatographic techniques such as GC, SFC or HPLC, it provides a method for elemental speciation. Plasma-source detection in GC is dominated by GC-MIP-AES... 

The number of excited atoms at typical flame temperatures (ca 2200-3200 K) is very low indeed compared with the number of ground-state atoms, even for easily excited lines. For difficult-to-excite lines (e.g. Zn 213.9 nm), it can be shown that only about one excited atom will exist at any given time in an air-propane flame when aspirating a 1 mg 1 zinc solution. This is one reason why flames are poor sources for atomic emission spectrometry, but are well suited to atomic absorption spectrometry, i.e. most of the atoms are in the ground state. As will be seen, the typical temperatures obtainable in plasma sources are of the order of 8000 K, at which there is a much high ratio of excited-to ground-state atoms, and hence a much greater intensity of atomic emission. 

What are some of the advantages of plasma. sources compared with flame sources for emission spectrometry ... 

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