Optical characterization of miniature flexible micro-tube plasma (FμTP) ionization source: A dielectric guided discharge

Abstract

Miniature DBD has been obtaining more and more attention. A recently developed miniature flexible micro-tube plasma (FμTP) has proved to be an excellent soft ionization source for mass spectrometry and ion mobility spectrometry. In this paper, the optical characterization of the novel FμTP ionization source was carried out, aiming to cast light on the plasma propagation mechanism in one discharge cycle. To provide a clean environment for investigating the plasma propagation, the plasma was totally sustained in a capillary tube. It was found that in one discharge cycle, three discharge developments, namely two dielectric guided discharges in both positive and negative half cycle, and a negative glow discharge in the negative half cycle. Compared with the conventional dielectric barrier discharge, there is no formation of a dissociative plasma, which improves the soft ionization capacity of the present source. In addition to the whole light emission from 200 nm to 1100 nm, the lines related to the reactive species including He 706 nm, N2+ 391 nm, and O 777 nm were investigated spatially and temporally. Due to the effect of the attached charges, the plasma of the negative half cycle propagates faster than that of the positive half cycle. It was also found that the plasma in the positive half cycle propagates more discretely than that of the negative one, while the negative one has a larger emission intensity due to the overlap of the signal in both temporal and spatial scales. Further on, the electric field distribution was proved to be in Gaussian shape along the diameter of the capillary, while the negative one has a flat shape. By changing the concentration of N2 in He, the electron collision and penning ionization for generating N2+ were experimentally proved and identified. This study identifies the typical discharge process of the FμTP in one discharge cycle, indicating the plasma is indeed a dielectric guided discharge.