Fundamental properties of a non-destructive atmospheric-pressure plasma jet in argon or helium and its first application as an ambient desorption/ionization source for high-resolution mass spectrometry
Various plasma sources were used as ionization sources for ambient desorption/ionization mass spectrometry (ADI-MS) in the past. In the present study, a plasma jet, termed an atmospheric-pressure damage-free multi-gas plasma jet, which can generate a stable atmospheric low-temperature plasma with various gas species, was investigated and used as an ionization source for ambient MS. First, the OH rotational temperature and electron number density of the plasma were determined spectroscopically. It was found that a relatively low temperature (<350 K) and high-density (1014 cm−3) plasma can be generated in helium and argon. Second, the amount of reactive species in the glow-like discharge was indirectly compared to those typically found in a dielectric barrier discharge jet by means of differences in hydrophilization efficiencies of polyimide films. It was found that the plasma jet was more reactive when the plasma exit was positioned close to the sample (<3 mm). Third, the plasma source was coupled to a high-resolution molecular mass spectrometer (Exactive with Orbitrap mass analyzer) and used for direct solid sample analyses. Commercially available acetaminophen, loratadine, and aspirin tablets were successfully analyzed without any sample pre-treatment. The plasma source was also used for direct solution analysis of model compounds to demonstrate the analytical capacity. Calibration curves were obtained with correlation coefficients of ≧0.9975, and limits of detection were in the picogram to nanogram range for acetaminophen, loratadine, and aspirin.