Proton transfer reaction mass spectrometry has traditionally used drift tubes as ion traps.  However, radio frequency ion funnels offer an attractive alternative, as they improve compound specific sensitivity significantly.  This is due to increasing the effective reaction time and focusing the ions.  The same pressure ranges are required for ion funnels and drift tubes, so the technology is not difficult to implement.  Ion funnels have been shown to favor transmission of ions with high m/z.