Orbitrap

An 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 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 1990-s started a sequence of technology improvements which resulted in a commercial introduction of this analyzer by Thermo Fisher Scientific as a part of the hybrid LTQ Orbitrap instrument in 2005.

In an 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 thus their trajectories in space resemble helices. Due to properties of quadro-logarithmic potential, 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 ions except their mass-to-charge ratios m/z. Its angular frequency is: ω=√(k/(m/z)) 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. As ion packets are injected tangentially into the field, the electric field is increased by ramping the voltage on the inner electrode. Ions get squeezed towards the inner electrode until they reach the desired orbit inside the trap. At that moment ramping is stopped, the field becomes static, and detection can start. Each packet contains a multitude of ions of different velocities spread over a certain volume, therefore ions will move with different rotational frequencies but with the same axial frequency. It means that ions of a specific mass-to-charge ratio spread into rings which oscillate along the inner spindle.

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