Fig. 1.

Schematic representation of a typical PTR-MS. The instrument consists of an ion source (1) in which H₃O⁺ primary ions are produced, a drift tube (2), where the trace gases from samples are ionized by the proton-transfer reaction with H₃O⁺ ions, a collisional dissociation chamber (3), where cluster molecules dissociate, and the detection unit, where ions are mass filtered with a quadrupole mass filter (4) and quantified by a secondary electron multiplier (5)