orthogonal acceleration mass spectrometry orthogonal acceleration time of flight mass spectrometry oa-TOF mass spectrometry Mass spectrometry where the sample is ionised and the ions are accelerated to the same (known) very high kinetic energies by an ion source that works by sampling an ion beam travelling in a direction orthogonal to the axis of the flight path in the mass-to-charge analyser. The time taken for each ion to reach a detector at a known distance is measured. This time is dependent on the mass-to-charge ratio of the ion. CHMO:0000798 oa-TOF MS