Direct analysis in real time (DART) is an example of adesorption-based atmospheric pressure ion source [108]. An electrical discharge in a heated helium (or nitrogen) gas stream createsenergetic neutral metastable helium species that flow from an orifice directed as a narrow beam at the layer. The interaction of the metastable helium species with the atmosphere produces predominantly charged water clusters (for positive ion spectra) and oxygen ions (for negative ion spectra), as well as other species.The collision of the hot gas plasma with samples on the layer surface is responsible for ionization by proton or charge transfer and release to the gas phase by thermal desorption and possibly spluttering.