Mass spectrometric study of negative ions extracted from point to plane negative corona discharge in ambient air at atmospheric pressure

In this paper, we report the detection and mass analysis of negative ions formed in a negative corona discharge using ambient air at atmospheric pressure. Using a mass spectrometer capable of operating at atmospheric pressure with high sensitivity and excellent mass resolution we have detected many anions that were not observable in earlier studies. Practically all the ions detected were in the form of clusters, mostly containing water, although some clusters containing HNO3 molecules were observed. In contrast to previous reports a relatively high abundance of O2−·(H2O)n clusters was observed especially when the gap between the two electrodes is small. At larger separations CO3−·(H2O)n, O3−·(H2O)n, NO3−·(H2O)n and HCO3−·(H2O)n cluster were detected with yields between 2 and 10%. Previous work has reported negative ion signal with an amu of 125. In contrast to previous suggestions that this represents OH−·(H2O)6 we show that it is a cluster of NO3−·HNO3. Such results demonstrate conclusively that in any atmospheric pressure discharge most of the ion chemistry involves clusters and therefore models and simulation using unimolecular ions in the description of such discharges are liable to serious misinterpretation.