Orbitrap

Orbitraps belong to the category of ion traps and utilise the axial oscillation of ions in an electrostatic field to measure their frequency. The frequency is measured by detecting the current generated by the oscillating ions. These signals are then subjected to a Fourier transformation to generate a mass spectrum.

An Orbitrap consists of a central spindle-shaped electrode core and a surrounding hollow cylindrical electrode. The ions are injected into an electric field that is generated between these electrodes. The shape of the electric field causes the ions to move both axially and radially as these move orbitally around the central electrode, but also oscillate axially along the longitudinal axis. The frequency of the axial oscillation depends on the mass-to-charge ratio (m/z) of the ions.
The minimum electrical current that the ions induce in the surrounding electrode is amplified and recorded using ADCs. A Fourier transformation (FT) is applied to the transients recorded in this way and converted into frequency domain spectra. As these frequencies are directly proportional to the square of the m/z ratios of the ions, the measured frequency spectrum can be converted into a mass spectrum.

Orbitraps offer a very high mass resolution (up to 1,000,000) and mass accuracy, often in the range of a few ppm.

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