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 63 Ni ionization source. With no discharge gas the predominant ions were O 2 − , however, upon the introduction of low levels of discharge gas the NO 2 − ion quickly became the dominant species. As the amount of discharge gas increased the appearance of CO 3 − was observed followed by the appearance of NO 3 − . At very high levels, NO 3 − species became effectively the only ion present and appeared as two peaks in the IMS spectrum, NO 3 − and the NO 3 − ·HNO 3 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 NO 2 − and NO 3 − with reduced mobility values of 1.49 and 1.44 cm 2 V −1 s −1 , respectively. No adduct was observed for RDX with CO 3 − 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.

Dopant-Assisted Negative Photoionization Ion Mobility Spectrometry for Sensitive Detection of Explosives

“…However, in 63 Ni source, the reactant ions are dominated by the ions with m/z 32, which is identified as O 2 − and is consistent with the previous study by Waltman et al 41 The ions at m/z 60 were also observed in negative corona discharge source. 42 presented in low ppmv level through the use of 18 O labeled oxygen technique. 43 In our system, the measured concentration of CO 2 in purified air was about 300 ppm by electron impact ionization time-of-flight mass spectrometry, 44 so the ions at m/ z 60 in DANP and UVRI sources are assigned to CO 3 − .…”

A novel non-radioactive electron source for ion mobility spectrometry

“…From pulsed corona discharges it is known that the controlling the flow, i.e. intensity over time and duration, of electrons has an influence on the spectra [33,45]. The possibility to control these parameters exists here, too, and especially the influence on the details of the peak shapes is an interesting topic for further research in our group.…”

“…Nevertheless the exact nature of the ions produced by a pulsed corona discharge ionisation source has yet to be established. For the negative mode the reactant formation in corona discharges has been analyzed recently [33]. Besides the problems on the chemical side are additional problems on the technical side, as the corona discharge needs maintenance due to the electrodes degrading relatively quickly in quality.…”

Atmospheric Pressure Chemical Ionization Sources Used in The Detection of Explosives by Ion Mobility Spectrometry

“…This device was described in detail elsewhere [47]. The NO 2 -reactant ion was created with the DPIS operated at about 4kV and NO 3 -at about 7.5kV.…”

“…Methylene chloride was added to the solutions when a chloride as a reactant ion was desired. For the nitrate reactant ion, the DPIS, located in a dynamic chamber described elsewhere, [47] was used.…”