Understanding the flowing atmospheric-pressure afterglow (FAPA) ambient ionization source through optical means

The advent of ambient desorption/ionization mass spectrometry (ADI-MS) has led to the development of a large number of atmospheric-pressure ionization sources. The largest group of such sources is based on electrical discharges; yet, the desorption and ionization processes that they employ remain largely uncharacterized. Here, the atmospheric-pressure glow discharge (APGD) and afterglow of a helium flowing atmospheric-pressure afterglow (FAPA) ionization source were examined by optical emission spectroscopy. Spatial emission profiles of species created in the APGD and afterglow were recorded under a variety of operating conditions, including discharge current, electrode polarity, and plasma-gas flow rate. From these studies, it was found that an appreciable amount of atmospheric H₂O vapor, N₂, and O₂ diffuses through the hole in the plate electrode into the discharge to become a major source of reagent ions in ADI-MS analyses. Spatially resolved plasma parameters, such as OH rotational temperature (T_rot) and electron number density (n_e), were also measured in the APGD. Maximum values for T_rot and n_e were found to be ~1100 K and ~4 × 10¹⁹ m^–3, respectively, and were both located at the pin cathode. In the afterglow, rotational temperatures from OH and N₂⁺ yielded drastically different values, with OH temperatures matching those obtained from infrared thermography measurements. The higher N₂⁺ temperature is believed to be caused by charge-transfer ionization of N₂ by He₂⁺. These findings are discussed in the context of previously reported ADI-MS analyses with the FAPA source.