We have characterised a new Atmospheric-Pressure-interface Time-of-Flight Mass Spectrometer, equipped with an octapole ion trap for accumulating the sampled ions before orthogonally accelerating them into the mass analyser. The characterisation has been carried out using ion standards produced by electrospray ionisation, that were subsequently mobility-selected by a differential mobility analyser operated at atmospheric pressure. Our results show that the detection sensitivity (or limit of detection) of the mass spectrometer is in the parts per quintillion (i.e., 10−3 parts per quadrillion, ppq; which is ∼ 30 ions cm−3) range with temporal resolutions of 1 s. When increasing the temporal resolution up to 1 min, the detection sensitivity can be reduced to the 10 parts per sextillion (i.e., 10−5 ppq; which is ∼ 0.3 ions cm−3) range, enabling the system to measure gaseous ions of extremely low concentrations. In contrast to other mass spectrometers that employ spectra accumulation to improve the detection sensitivity for atmospheric observations, ion accumulation amplifies the signal without increasing the noise level; something that among others is highly important for probing short-lived ionic clusters during new particle formation events in the atmospheric environment. We also show that the mass spectrometer has a transmission of up to 1 %, and a mass resolution of 23 000 for ionic masses of ca. 600 Da, while it can be used in ways to induce collision dissociation of the sampled ions by tuning the operating conditions of the Atmospheric-Pressure-interface stage.