Negative-mode mass spectrometric study on dc corona, ac corona and dielectric barrier discharge ionization in ambient air containing H2O2, 2,4,6-trinitrotoluene (TNT), and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX)

Abstract

In this work, a miniature dielectric barrier discharge (m-DBD) ionization source was developed, and a comparative study of dc corona, ac corona and the m-DBD ionization in ambient air was made. As a very simple miniature dielectric barrier discharge ion source, a tip-sealed fused silica capillary (i.d.: 0.2 mm, o. d.: 0.35 mm) inserted with an acupuncture needle (body diameter: 0.12 mm) was used. While dc and ac corona discharge are glow-like, the m-DBD was more or less arc-like and it gave more abundant ions originating from fragmentation of analytes. When H2O2 vapor was introduced in the m-DBD plasma, the major ions O3−• and CO3−• generated in air were quickly annihilated by the reactions with H2O2. For 2,4,6-trinitrotoluene (TNT) introduced in the m-DBD plasma, [TNT − H + O]− (m/z 242), [TNT − CH3 + O]− (m/z 228), and [TNT − NO2 + O2]− (m/z 213) were detected which were almost absent in dc and ac corona discharge ionization. Their precursors may be the fragment neutral radical species generated from TNT exposed in the m-DBD plasma.Density functional theory (DFT) calculations predicted that O3−• and CO3−• react with H2O2 to form O2−•⋅⋅⋅H2O⋅⋅⋅⋅⋅O2, and O2−•⋅⋅⋅H2CO3, respectively. The stabilities and structures of the adduct ions of O2−•, HO2−, O3−•, CO3−•, HCO3−, NO3−, and HCO4− with H2O and H2O2 were also examined by highly accurate calculations.