Proton transfer reaction mass spectrometry at high drift tube pressure

An ion drift tube operated at a total pressure of 10Torr (∼13mbar) coupled to a mass spectrometer is described. Proton transfer from water molecules was used to detect organic compounds and calibrations of the instrument yield normalized sensitivities that are in accord with what is expected for operation at this pressure. For example, the sensitivity for acetone was found to be 240Hzppbv^-1 for 1MHz of reagent ion signal while that for isoprene was 120Hzppbv^-1. Issues particular to using this technique at pressures of 10Torr, considerably higher than the conventional pressure, are addressed. Detection limits for a 1s integration (S/N=2) depend on the cleanliness history of the system but can reach as low as 20pptv.