Characterization of Direct-Current Atmospheric-Pressure Discharges Useful for Ambient Desorption/Ionization Mass Spectrometry

Two relatively new ambient ionization sources, direct analysis in real time (DART) and the flowing atmospheric-pressure afterglow (FAPA), use direct current, atmospheric-pressure discharges to produce reagent ions for the direct ionization of a sample. Although at a first glance these two sources appear similar, a fundamental study reveals otherwise. Specifically, DART was found to operate with a corona-to-glow transition (C-G) discharge whereas the FAPA was found to operate with a glow-to-arc transition (G-A) discharge. The characteristics of both discharges were evaluated on the basis of four factors: reagent-ion production, response to a model analyte (ferrocene), infrared (IR) thermography of the gas used for desorption and ionization, and spatial emission characteristics. The G-A discharge produced a greater abundance and a wider variety of reagent ions than the C-G discharge. In addition, the discharges yielded different adducts and signal strengths for ferrocene. It was also found that the gas exiting the discharge chamber reached a maximum of 235 °C and 55 °C for the G-A and C-G discharges, respectively. Finally, spatially resolved emission maps of both discharges showed clear differences for N₂⁺ and O(I). These findings demonstrate that the discharges used by FAPA and DART are fundamentally different and should have different optimal applications for ambient desorption/ionization mass spectrometry (ADI-MS).