Influence of the cluster constituents' reactivity on the desorption/ionization process induced by neutral SO 2 clusters.

The influence of the chemical nature of the cluster constituents on the desorption/ionization process was investigated for desorption/ionization induced by neutral SO2 clusters (DINeC). The polar clusters act as a transient matrix in which the desorbed analyte molecules are dissolved during the desorption process. For drop-cast samples, the desorption/ionization efficiency was found to be largely independent of the pH value of the initial solution the samples were prepared from; positive ions were almost always dominant and no multiply charged negative ions were observed. The results were traced back to the interaction of SO2 with water present in the samples. Both H/D exchange experiments and surface charge measurements showed that SO2 from the cluster beam interacts with water on and in the sample forming sulfurous acid. The latter then acts as an efficient proton supply leading to an enhanced ionization efficiency. The results demonstrate the possibility to control the ionization efficiency when using reactive cluster constituents in desorption-based ionization methods such as DINeC and cluster-based secondary ion mass spectrometry.