The nitrate Chemical Ionization–Atmospheric Pressure interface–Time Of Flight mass spectrometer (nitrate CI-APi-TOF) utilizes a long time of flight chamber (LTOF). The instrument includes the highest performance TOF analyser that is currently offered by the company Tofwerk. The LTOF offers a wide mass range up to 2000 Th and a very high mass resolving power (up to ca. 12000 Th/Th). The ion source uses a corona discharge needle to produce NO3-(HNO3)m (usually m<=2) primary ions from nitric acid. The sample flow rate is around 8 standard liters per minute. It enters the ion drift tube where it is surrounded concentrically by the sheath gas. The primary ions are directed towards the center of the sample flow by means of an electrostatic field so that they can interact with sampled compounds. These compounds can be ionized through proton-transfer reaction or via clustering with the primary ions.
The LTOF uses an ion precipitator integrated in its sampling line, in order to remove all ions from the sample flow during ion-induced nucleation experiments. A flow of dry nitrogen is added in front of the pinhole instrument so that when the ions travel through the nitrogen, water molecules are effectively removed from the core ions. The LTOF is calibrated for gaseous sulfuric acid measurement (Kürten et al. 2012), and a know mass-dependent transmission efficiency allows to derive concentrations for others masses (Heinritzi et al. 2016). In fact, the LTOF from Frankfurt University is the reference instrument providing the sulfuric acid and the HOM concentration during the CLOUD experiments at CERN.