Secondary ion mass spectrometry of sodium nitrate: Comparison of ReO₄^- and Cs⁺ primary ions

The use of perrhenate (rhenium tetroxide, ReO₄^-) as a bombarding particle was compared with Cs⁺ for its ability to generate molecular species from sodium nitrate. The purpose of the study was to quantitatively evaluate the enhancement in sputtering to be gained using a heavy, polyatomic primary particle. It was found that ReO₄^- is three to five times more efficient at generating ions such as Na₂NO₃⁺ and Na(NO₃)₂^-. The nitrate-bearing molecular ions were observed to decrease in intensity as primary ion dose increases; at the same time, nitrite-bearing ions were observed to increase. This observation is interpreted in terms of beam damage to the surface of the target. Disappearance cross sections (σ) using ReO₄^- bombardment were measured as 960 and 690Ų for Na₂NO₃⁺ and Na(NO₃)₂^-, respectively. σ values measured using Cs⁺ bombardment were slightly larger. These measurements show that for an equivalent area of the sample disrupted, ReO₄^- is more effective for the production of nitrate-bearing secondary ions, which increases the probability of completing a measurement before extensive beam damage occurs. Secondary ion energies were evaluated and shown to be comparable for the ReO₄^- and Cs⁺ bombardment experiments; for this reason, sample charging is not deemed to be a significant factor in these experiments.