MASS-SPECTROMETRY OF SECONDARY NEUTRALS AND IONS FOR CHEMICAL-ANALYSIS OF SALTS

Complete characterization of solid environmental samples requires sensitive chemical analysis, which includes quantitation of elements, identification of compounds and determination of their spatial distribution. In order to investigate the possible contribution to an analysis of microparticles collected from the atmosphere, plasma-based secondary neutral mass-spectrometry (SNMS) and SIMS were examined using salt samples as these are frequent components of such environmental materials. Chlorides, carbonates, nitrates and sulphates were bombarded with high fluxes of argon ions to meet the dynamic conditions for depth-resolved analysis. This first application of plasma-based SNMS to salts revealed that their neutral secondary emission mainly consists of those atoms or elements that the bombarded sample is composed of. Thus, mass analysis of the emitted atoms enables the identification of all elements and an example shows that elemental quantitation can be performed with a moderate matrix dependence. A small fraction of binary clusters within the total neutral emission offers compound-specific information. However, analysis of positive and negative secondary ions was found to be more appropriate for the recognition of compounds owing to the emission of characteristic polyatomic ions. Quantitation of the stoichiometry of complex sulphur anions was found to be rather less matrix-dependent using negative SIMS than SNMS. Although dynamic bombardment inevitably induces chemical reactions, the results demonstrate that depth-resolved analysis of salt-containing samples, including elemental quantitation and chemical speciation, can be performed using SNMS and SIMS.