Understanding the Flowing Atmospheric-Pressure Afterglow (FAPA) Ambient Ionization Source through Optical Means

Abstract

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 H(2)O 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 ~1100 K and ~4×10(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.

Halo-Shaped Flowing Atmospheric Pressure Afterglow: A Heavenly Design for Simplified Sample Introduction and Improved Ionization in Ambient Mass Spectrometry

“…Similarly, diffusion into the discharge and residual nitrogen in the lines are the likely sources of ionized nitrogen dimer and nitric oxide observed in the spectrum. 13 These species are important for charge-transfer ionization, so their presence in the mass spectral background is desirable. 14 The background mass spectrum also reveals that the h-FAPA source has an ion distribution similar to that of other plasma based ADI-MS sources, and therefore retains their flexibility.…”

Molecular typing of meningiomas by desorption electrospray ionization mass spectrometry imaging for surgical decision-making

“…This led to subsequent development of multiple atmospheric pressure ionization sources including methods in which the energetic beam consists of metastable gas-phase atoms and reagent ions (i.e. DAPCI [21–23], FAPA [24], LTP [25, 26]), energetic droplets (i.e. EASI [27]) combinations of laser radiation and ESI (i.e.…”

Helium conservation by discontinuous introduction in the flowing atmospheric-pressure afterglow source for ambient desorption-ionization mass spectrometry

“…8 The owing atmospheric-pressure aerglow (FAPA) is another helium-plasma-based ADI-MS source that offers attractive performance for a wide range of analyte and sample types, 3,9 and is the subject of the present study. 10,11 This heated ow can desorb material from the surface of a solid or solution sample and energetic species from the plasma can ionize the resulting desorbed molecules. 9 When helium ows through the discharge cell, a stable glow discharge is sustained and both charged and high-energy species ow out of the cell though the capillary in a stream heated by the plasma.…”