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.
Corona discharge Ion mobility spectrometry Explosive detection Atmospheric pressure chemical ionization