The Effects of Added Hydrogen on a Helium Atmospheric-Pressure Plasma Jet Ambient Desorption/Ionization Source

Abstract

We present mass spectrometric data demonstrating the effect that hydrogen has on a helium-based dielectric-barrier discharge (DBD) atmospheric-pressure plasma jet used as an ambient desorption/ionization (ADI) source. The addition of 0.9 % hydrogen to the helium support gas in a 35-W plasma jet increased signals for a range of test analytes, with enhancement factors of up to 68, without proportional increases in background levels. The changes in signal levels result from a combination of changes in the desorption kinetics from the surface and increased ion production in the gas phase. The enhancement in ADI-MS performance despite the quenching of key plasma species reported in earlier studies suggests that ionization with a H2/He plasma jet is the result of an alternate mechanism involving the direct generation of ionized hydrogen.

The Effects of Added Hydrogen on Noble Gas Discharges Used as Ambient Desorption/Ionization Sources for Mass Spectrometry

“…The sharp peak corresponding to plasma ignition disappears and the signal peaks later in time. The dramatic changes in temporal profiles with changes in gas composition, also seen with the addition of H 2 to He [9], suggest that the analyte molecules are not simply desorbed by heating up the glass substrate. Thermal images of glass slides exposed to the discharge add evidence that there is a non-thermal component of the desorption process.…”

“…The chemicals used in this study were chosen to match previous studies [9,10,20,21] done with pure helium and hydrogen-doped helium. They included caffeine (Sigma Aldrich, Steinheim, Germany), diphenylamine (Brigham Young University chemical surplus, source unknown), anthracene (Spectrum Chemical Mfg.…”

“…It should be noted that the optimal amount of hydrogen may vary as operating parameters such as power, frequency, source configuration, and source geometry relative to the mass spectrometer change [9].…”

“…It has been brought to our attention by two research laboratories through private communication that the results reported previously showing signal enhancement with hydrogen-doped helium [9] have not been successfully reproduced in their labs. They report using different types of mass spectrometers with presumably different voltage settings that did not exactly correspond to those used in our lab.…”

“…The simplest hydrogen source is molecular hydrogen. Previous work in our laboratory [9,10] has shown the benefits of using hydrogen-doped helium as the plasma gas for a highfrequency dielectric barrier discharge (DBD) source. These benefits included enhanced signals and faster desorption rise times.…”