Mechanisms for negative reactant ion formation in an atmospheric pressure corona discharge

Abstract

In an effort to better understand the formation of negative reactant ions in air produced by an atmospheric pressure corona discharge source, the neutral vapors generated by the corona were introduced in varying amounts into the ionization region of an ion mobility spectrometer/mass spectrometer containing a ⁶³Ni ionization source. With no discharge gas the predominant ions were O₂ ⁻, however, upon the introduction of low levels of discharge gas the NO₂ ⁻ ion quickly became the dominant species. As the amount of discharge gas increased the appearance of CO₃ ⁻ was observed followed by the appearance of NO₃ ⁻. At very high levels, NO₃ ⁻ species became effectively the only ion present and appeared as two peaks in the IMS spectrum, NO₃ ⁻ and the NO₃ ⁻·HNO₃ adduct, with separate mobilities. Since explosive compounds typically ionize in the presence of negative reactant ions, the ionization of an explosive, RDX, was examined in order to investigate the ionization properties with these three primary ions. It was found that RDX forms a strong adduct with both NO₂ ⁻ and NO₃ ⁻ with reduced mobility values of 1.49 and 1.44 cm²V⁻¹ s⁻¹, respectively. No adduct was observed for RDX with CO₃ ⁻ although this adduct has been observed with a corona discharge mass spectrometer. It is believed that this adduct, although formed, does not have a sufficiently long lifetime (greater than 10 ms) to be observed in an ion mobility spectrometer.