The APi-TOF is a high-resolution time-of-flight mass spectrometer. It is built and developed by Tofwerk AG (Thun, Switzerland), and Aerodyne Research, Inc. (Billerica, MA, USA). Aerosol is sampled at atmospheric pressure through a critical orifice, and then passes through quadrupoles and ion lenses which focus and guide the sampled ions into the mass spectrometer. In the commonly used configuration, the specified mass accuracy is < 20 ppm, and the specified resolving power 3000 Th/Th. The range of practically detectable mass-to-charge ratios is subject to "tuning", and usually reaches from ~30 to ~1500 Th. There is no dedicated ionization of the sampled aerosol, so only naturally charged ions are detected.