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Onium compounds (or onium salts) are saltlike compounds containing a complex cation, in which the central atom (the term "central atom" is not entirely precise, but is often used) is of an element with nonmetallic character (nitrogen, phosphorus, oxygen, sulfur, chlorine, bromine, iodine, etc.). The onium cations in these cases are named "ammonium", "phosphonium", "oxonium", "sulfonium", "chloronium", "bromonium" and "iodonium", respectively. Different definitions for onium compounds are given in the literature. According to the nomenclature rules of IUPAC (1), polyatomic cations formed by adding more protons to monatomic anions than are required to give a neutral unit have the ending -onium. Substituted derivatives may be formed from the names of the basic cations, e. g. hydroxylammonium, tetramethylstibonium, dimethyloxonium (CH 3 ) 2 OH + .

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Electrothermal atomic absorption spectrometry is applied to investigate the release of the selenium and tin atoms from pyrocoated graphite support. The Coats and Redfern and Sturgeon approaches are used for estimation of the apparent activation energies of atom release. The addition of tungsten and palladium modifiers leads to enhancement of the activation energies and changes the bind of the vaporizing species for analytes