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

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.