Low pressure-alternating current glow discharge ion source for mass spectrometry

Abstract

For social security and safety, the sensitive explosives detection is crucial. Mass spectrometry (MS), a unique technique that may immediately reveal information about molecular structure, is considered as the “gold standard” in analytical science. Both science and technology now frequently employ MS-based techniques. This study developed an air-based low-pressure alternating current glow discharge (LP-acGD) ion source for MS to investigate its ionization efficiency in both positive and negative ion modes. The LP-acGD ion source gave O₃^−•, CO₃^−•, HCO₃⁻, HCO₄⁻, NO₂⁻ and NO₃⁻ ions that appeared at m/z 48, 60, 61, 77, 46 and 62, respectively in negative ion mode, some of these contributed to the formation of adduct ions. In positive ion mode, only protonated acetone, [Acetone + H]⁺ m/z 59, was detected at the beginning of the total ion current when 15 and 5 V were applied at orifices 1 and 2, respectively. However, the background ion signals were stabilized by protonated acetone and water cluster ions [(H₂O)_n + H]⁺ (n = 3, 4) as applied voltages were 40 and 0 V at orifices 1 and 2, respectively. Hydrogen peroxide, 3,4-dinitrotoluene, nitric acid, hexafluorobenzene, triacetone triperoxide and basil leaf were taken as model samples to investigate the efficiency of the ion source. In negative ion mode, H₂O₂ and HNO₃ gave [H₂O₂+O₂]^- m/z 66 and [HNO₃+NO₃]^- m/z 125, whereas 3,4-DNT and HFB exhibited mostly molecular ions. In positive ion mode, TATP yielded fragment ions of [C₃H₆O₂+H]⁺ m/z 75 and [C₃H₆O₃+H]⁺ m/z 91. The present method has been validated for detection and quantification of H₂O₂ and HNO₃. Ion formation mechanisms are also discussed.