Direct Analysis in Real Time (DART)
The DART negative-ion reagent mass spectra are virtually identical for nitrogen, neon and helium. However, negative-ion sensitivity increases for DART gases in the following order:
nitrogen < neon < helium
This is due to the increased efficiency in forming electrons by Penning ionisation and surface Penning ionisation as the internal energy of the metastable species increases.
You can read more about it in this .pdf file
http://www.jeolusa.com/SERVICESUPPORT/Applicat ionsResources/AnalyticalInstruments/DocumentsD ownloads/tabid/337/DMXModule/693/Command/Core_ Download/Default.aspx?*******=302



