Analysis of ionic solids with SNMS

Summary The application of plasma-based SNMS to salt samples was examined to reveal the analytical features of this technique for the analysis of environmental material, which is frequently composed of ionic compounds. 10¹⁶ s^–1 cm^–2 argon ions of 300–500 eV energy were used to sputter halides, sulfates, nitrates and carbonates. The mass spectra are dominated by atomic signals of all elements of the salt and contain additional minor signals of binary homo- and heteroelemental clusters. The latter, such as salt monomers of alkali halides, are useful for compound identification. In sputter equilibrium the atomic mass signals can be used for elemental quantitation with a matrix dependence of about ±30% for the detection factors of most elements. As the average elemental detection factors are shown to be governed by atomic ionization probabilities the erosion flux from salts comprises mainly atoms. Results indicate that thermal release of atoms as well as emission of clusters are the main cause of the matrix dependence. The relative yield of the clusters was found to be strongly enhanced with increasing mass difference of the combined ions. The sputter yield of NaCl was determined to be 3.3 atoms/projectile at 490 eV argon ion energy which results in a depth propagation of 5 nm/s for the given sputter conditions.