Proton Transfer Reaction Mass Spectrometer (PTRMS) H3O+-ions are produced by a small electrical discharge and react with trace compounts of the sampling air. The protonated species, mainly acetone, acetonitrile, methanol, acetaldehyde, and other Volatile Organic Compounts (VOC's) are detected by a mass spectrometer. The instrument is supervised by Institute for Meteorologie and Climate Research (IMK), Forschungszentrum Karlsruhe, Germany.

All components of the PTRMS are adapted for the CARIBIC container and are placed into a special rack of unusual size of ca. 26''.