Aspects of surface scanning by direct analysis in real time mass spectrometry employing plasma glow visualization.

Visual monitoring of the Direct Analysis in Real Time (DART) gas impact region during sampling was demonstrated via its metastable plasma glow. It is known that adding neon into helium for DART leads to plasma glow, but this effect has not been used in practice and discussed in the literature so far. A single quadrupole mass spectrometer with a DART SVPA ion source was used for recording of DART mass spectra from different surfaces, using galangin and p-coumaric acid as model analytes. In specific cases, the composition of the mass spectra was clarified using an Orbitrap mass spectrometer. Plasma glow visualization made it possible to track the metastable gas distributions during surface scanning. The influence on the composition of the mass spectra was studied for different carrier gases, i.e. pure helium versus a helium-neon mixture, and for the vacuum pumping rate. The spatial resolution was substantially improved via a DART cap with a narrowed internal diameter, but impaired by a decreased sensitivity. Comparably low signal intensities were obtained for analytes on porous layers due to analyte penetration and metastable gas scattering. Visualization through the plasma glow enables the optimal selection of the coordinates for DART-MS analysis and thus it will support scanning and imaging MS on surfaces, including porous planar chromatographic separation materials.