A flowing atmospheric pressure afterglow as an ion source coupled to a differential mobility analyzer for volatile organic compound detection

Abstract

Atmospheric pressure glow discharges have been widely used in the last decade as ion sources in ambient mass spectrometry analyses. Here, an in-house flowing atmospheric pressure afterglow (FAPA) has been developed as an alternative ion source for differential mobility analysis (DMA). The discharge source parameters (inter-electrode distance, current and helium flow rate) determining the atmospheric plasma characteristics have been optimized in terms of DMA spectral simplicity with the highest achievable sensitivity while keeping an adequate plasma stability and so the FAPA working conditions finally selected were: 35 mA, 1 L min(-1) of He and an inter-electrode distance of 8 mm. Room temperature in the DMA proved to be adequate for the coupling and chemical analysis with the FAPA source. Positive and negative ions for different volatile organic compounds were tested and analysed by FAPA-DMA using a Faraday cup as a detector and proper operation in both modes was possible (without changes in FAPA operational parameters). The FAPA ionization source showed simpler ion mobility spectra with narrower peaks and a better, or similar, sensitivity than conventional UV-photoionization for DMA analysis in positive mode. Particularly, the negative mode proved to be a promising field of further research for the FAPA ion source coupled to ion mobility, clearly competitive with other more conventional plasmas such as corona discharge.

Flowing Atmospheric Pressure Afterglow for Ambient Ionization: Reaction Pathways Revealed by Modeling

“…In contrast, the so-called flowing atmospheric pressure afterglow (FAPA) has two separate regions: the plasma ignition zone (between two electrodes) and the afterglow, which is generated outside the chamber. The analytes are introduced into the latter region and therefore do not affect the interelectrode plasma conditions . The FAPA source was introduced by Andrade et al in 2008 and has been applied both for the ionization of compounds in the gas phase , and for desorption–ionization for the direct analysis of solid compounds, such as metal complexes and drugs, as well as liquid compounds and recently liquid crystals employed in display devices. − ,, Importantly, the FAPA source can be tuned in hardness, enabling it to produce mainly molecular ion, fragment ion, or atomic ion mass spectra. ,,− …”

“…Among the many plasma-based ambient MS sources, the FAPA source has certain advantages. It seems less affected by ionization matrix effects, can produce higher gas temperatures, and directly generate “negative reagent ions”. ,, Its simple construction and DC power, as well as its high sensitivity and abundance of reagent ions, make the FAPA source a suitable candidate for exploring unique plasma processes and chemistries. ,,− Furthermore, the FAPA source represents an easy-to-handle alternative ionization source to the traditional corona discharge used in ion mobility analyzers. , …”

“…To further enhance the application potential of the FAPA source in analytical chemistry, its behavior must be better understood. This can be achieved by experiments, as illustrated, for example, in refs , , , . Some FAPA source parameters were found to greatly influence reagent ion production, such as the discharge current, ,, gas flow rate, ,, and the anode and MS front-plate potential .…”

“…This can be achieved by experiments, as illustrated, for example, in refs , , , . Some FAPA source parameters were found to greatly influence reagent ion production, such as the discharge current, ,, gas flow rate, ,, and the anode and MS front-plate potential . Also the effects of geometrical parameters, such as the spatial location within the afterglow and the distance between the electrodes, have been studied.…”

“…Some FAPA source parameters were found to greatly influence reagent ion production, such as the discharge current, ,, gas flow rate, ,, and the anode and MS front-plate potential . Also the effects of geometrical parameters, such as the spatial location within the afterglow and the distance between the electrodes, have been studied. In addition, both the positive- and negative-ion modes were studied for a variety of analyte types …”