The Atmospheric Pressure interface Time of Flight Mass Spectrometer is used to measure the chemical composition of naturally charged ions (positive or negative) present in the atmosphere at very low concentration (few hundreds per cc). In the APi part, the sample air passes through a critical orifice with a flow rate of 0.8 l min-1, after which the ions are guided by two quadrupoles and an ion lens to the time-of-flight mass analyzer. In this interface the pressure is progressively reduced from atmospheric level to 10-6 hPa inside the mass spectrometer. The APi-ToF has a high sensitivity and resolution, allowing the study of ion-molecule interactions, such as the initial steps of nucleation.

The Api-ToF maintained by the smog chamber group is a commercially available instrument produced by TOFWERK.