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Concept
Orbitrap
Summary
In mass spectrometry, Orbitrap is an ion trap mass analyzer consisting of an outer barrel-like electrode and a coaxial inner spindle-like electrode that traps ions in an orbital motion around the spindle. The image current from the trapped ions is detected and converted to a mass spectrum using the Fourier transform of the frequency signal.
The concept of electrostatically trapping ions in an orbit around a central spindle was developed by Kenneth Hay Kingdon in the early 1920s. The Kingdon trap consists of a thin central wire and an outer cylindrical electrode. A static applied voltage results in a radial logarithmic potential between the electrodes. In 1981, Knight introduced a modified outer electrode that included an axial quadrupole term that confines the ions on the trap axis. Neither the Kingdon nor the Knight configurations were reported to produce mass spectra.
The invention of the Orbitrap analyzer and its proof-of-principle by Makarov at the end of the 1990s started a sequence of technology improvements which resulted in the commercial introduction of this analyzer by Thermo Fisher Scientific as a part of the hybrid LTQ Orbitrap instrument in 2005.
In the Orbitrap, ions are trapped because their electrostatic attraction to the inner electrode is balanced by their inertia. Thus, ions cycle around the inner electrode on elliptical trajectories. In addition, the ions also move back and forth along the axis of the central electrode so that their trajectories in space resemble helices. Due to the properties of the quadro-logarithmic potential, their axial motion is harmonic, i.e. it is completely independent not only of motion around the inner electrode but also of all initial parameters of the ions except their mass-to-charge ratios m/z. Its angular frequency is: ω = , where k is the force constant of the potential, similar to the spring constant.
In order to inject ions from an external ion source, the field between the electrodes is first reduced.
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