Mechamism of Plasma Ashing and Its Applications to Analytical Chemistry

Abstract

Oxygen under low pressure is electronically excited and dissociated to atomic oxygen which reacts with solid samples and gently decomposes organic matrices to leave inorganic ash materials. This so-called plasma ashing is therefore featured by low temperature incineration together with high recovery of inorganic constituents. It has been revealed that the initial oxidation products from the contact of atomic oxygen with the alkyl groups of the organic samples are mostly carbonyl compounds and, in the case of reaction with unsaturated groups, epoxide compounds are also obtained. A fine capillary thermometer placed on the surface of the sample measured temperatures of 120-150°C during the plasma ashing, but the actual temperature at the oxidation sites were expected to be 200-300°C, temperatures derived from a comparative study for heat polymerization of orthophosphate during the plasma ashing and a furnace heating of known temperatures. The plasma conditions relative to oxidation rates, pretreatment of samples of different types and some applications to analytical chemistry are described and discussed.