Microplasma-based flowing atmospheric-pressure afterglow (FAPA) source for ambient desorption-ionization mass spectrometry

A new direct-current microplasma-based flowing atmospheric pressure afterglow (FAPA) source was developed for use in ambient desorption-ionization mass spectrometry. The annular-shaped microplasma is formed in helium between two concentric stainless-steel capillaries that are separated by an alumina tube. Current-voltage characterization of the source shows that this version of the FAPA operates in the normal glow-discharge regime. A glass surface placed in the path of the helium afterglow reaches temperatures of up to approximately 400 °C; the temperature varies with distance from the source and helium flow rate through the source. Solid, liquid, and vapor samples were examined by means of a time-of-flight mass spectrometer. Results suggest that ionization occurs mainly through protonation, with only a small amount of fragmentation and adduct formation. The mass range of the source was shown to extend up to at least m/z 2722 for singly charged species. Limits of detection for several small organic molecules were in the sub-picomole range. Examination of competitive ionization revealed that signal suppression occurs only at high (mM) concentrations of competing substances.