No abstract

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.

During the development of a plasma imaging system, luminous streaks and instabilities occurred in a pulsed rf glow discharge operated in Ar at reduced pressure. Sputtering sources with a flat cathode having an area of 10-50 cm 2 were designed to reproduce the surface spatial structure of the cathode in the plasma emission, providing optical emission spectrometry with spatial resolution for the chemical analysis of the cathode material. Unfortunately, depending on the discharge parameters and design of the discharge cell, spot patterns appeared and disappeared, and at times were stable or unstable. The behavior and possible origin of these patterns are described in this article.

As helium availability declines in many parts of the world and helium prices rise, it is desirable to limit helium use by plasma-based sources for ambient desorption-ionization mass spectrometry (ADI-MS). Here, helium flow to the flowing atmospheric-pressure afterglow (FAPA) source is interrupted to reduce total helium consumption. The FAPA glow discharge can be sustained for up to several minutes after the helium flow is halted, without damaging the source. As a result, a stable discharge can be maintained indefinitely by introducing relatively short periods of helium flow just before the discharge extinguishes. When helium flow is restored, the signal rises and reaches a steady state after 20 s; signal integration after this time yields mass spectra that are very similar, sensitivities that are 71% of those under continuous-flow conditions, and comparable precision.