Quantitative analysis of ionic solids by secondary neutral mass spectrometry

Abstract

Using an argon high frequency plasma for excitation and postionization, secondary neutral mass spectrometry was performed to examine the ability of the technique for depth-resolved quantitative analysis of ionic solids. An argon energy Ep=300–500 eV was applied to bombard all alkali halides, some sulfates, Li2SiO3, Li4SiO4, NaNO3, and Na2CO3. The mass spectra obtained were found to be dominated by atomic signals including all elements of the measured salt. Averaged relative sensitivity factors for 20 elements calculated from these signals show close conformity to atomic ionization probabilities. Thus, the majority of sputtered neutrals from salts consists of atoms and these dominate also the whole sputter flux. All measured sensitivity factors lie within a range of 400 being the sensitivity ratio of cesium to fluorine. A moderate matrix dependence of mostly less than 40% is probably due to coemitted thermal and/or nonatomic species. A minor fraction of the neutral sputter flux (<1% total signal intensity) consists of binary molecular clusters, of which the heteroelemental could be useful for compound identification. Erosion rates were determined to be 0.3±0.1 nm/s for five different salts under 0.6 mA/cm2 primary current. The corresponding sputter yields are given.