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, N-2, and O-2 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 similar to 1100 K and similar to 4x10(19) m(-3), respectively, and were both located at the pin cathode. In the afterglow, rotational temperatures from OH and N-2(+) yielded drastically different values, with OH temperatures matching those obtained from infrared thermography measurements. The higher N-2(+) temperature is believed to be caused by charge-transfer ionization of N-2 by He-2(+). These findings are discussed in the context of previously reported ADI-MS analyses with the FAPA source.