Orbitrap

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An orbitrap is mass spectrometer invented by Alexander Makarov. It consists of an outer barrel-like electrode and a coaxial inner spindle-like electrode that form an electrostatic field with quadro-logarithmic potential distribution. Ions are injected tangentially into this field and trapped because their electrostatic attraction to the inner electrode is balanced by centrifugal forces. Thus, ions cycle around the central electrode in rings. In addition, the ions also move back and forth along the axis of the central electrode. Therefore, ions of a specific mass-to-charge ratio move in rings which oscillate along the central spindle. The frequency of these harmonic oscillations is independent of the ion velocity and is inversely proportional to the square root of the mass-to-charge ratio (m/z or m/q). By sensing the ion oscillation similar as in the FTICR-MS, the trap can be used as a mass analyzer. Orbitraps have a high mass accuracy (1-2 ppm), a high resolution (up to 200,000) and a high dynamic range (around 5000).

Like FTICR-MS the orbitrap resolving power is proportional to the number of harmonic oscillations of the ions, as a result the resolving power is inversely proportional to the square root of m/z and proportional to acquisition time. Scigelova and Makarov^[1] show the resolving power to be 7500 per 0.1 second transient at m/z 400 Th. A transient being the duration that the time domain signal is acquired for. The resolving power decreases further as the m/z value increases so that at 4 times the m/z value (1600 Th) the resolving power has halved to 3750 per 0.1 second. Approximately 0.1 seconds per transient is required for data processing, thus a 0.1 second transient has a cycle time of 0.2 seconds. These effects in combination result in a resolving power of 7500 at m/z 400 and 3750 at m/z 1600 when the orbitrap is operating at 5 acquisitions per second.