Spatially resolved spectroscopic measurements of a dielectric barrier discharge plasma jet applicable for soft ionization

An atmospheric pressure microplasma ionization source based on a dielectric barrier discharge with a helium plasma cone outside the electrode region has been developed for liquid chromatography/mass spectrometry and as ionization source for ion mobility spectrometry. It turned out that dielectric barrier discharge ionization could be regarded as a soft ionization technique characterized by only minor fragmentation similar to atmospheric pressure chemical ionization (APCI). Mainly protonated molecules were detected. In order to characterize the soft ionization mechanism spatially resolved optical emission spectrometry (OES) measurements were performed on plasma jets burning either in He or in Ar. Besides to spatial intensity distributions of noble gas spectral lines, in both cases a special attention was paid to lines of N2+ and N2. The obtained mapping of the plasma jet shows very different number density distributions of relevant excited species. In the case of helium plasma jet, strong N2+ lines were observed. In contrast to that, the intensities of N2 lines in Ar were below the present detection limit. The positions of N2+ and N2 distribution maxima in helium indicate the regions where the highest efficiency of the water ionization and the protonation process is expected.