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Concept
Fourier-transform ion cyclotron resonance
Summary
Fourier-transform ion cyclotron resonance mass spectrometry is a type of mass analyzer (or mass spectrometer) for determining the mass-to-charge ratio (m/z) of ions based on the cyclotron frequency of the ions in a fixed magnetic field. The ions are trapped in a Penning trap (a magnetic field with electric trapping plates), where they are excited (at their resonant cyclotron frequencies) to a larger cyclotron radius by an oscillating electric field orthogonal to the magnetic field. After the excitation field is removed, the ions are rotating at their cyclotron frequency in phase (as a "packet" of ions). These ions induce a charge (detected as an image current) on a pair of electrodes as the packets of ions pass close to them. The resulting signal is called a free induction decay (FID), transient or interferogram that consists of a superposition of sine waves. The useful signal is extracted from this data by performing a Fourier transform to give a mass spectrum.
FT-ICR was invented by Melvin B. Comisarow and Alan G. Marshall at the University of British Columbia. The first paper appeared in Chemical Physics Letters in 1974. The inspiration was earlier developments in conventional ICR and Fourier-transform nuclear magnetic resonance (FT-NMR) spectrometry. Marshall has continued to develop the technique at The Ohio State University and Florida State University.
The physics of FTICR is similar to that of a cyclotron at least in the first approximation.
In the simplest idealized form, the relationship between the cyclotron frequency and the mass-to-charge ratio is given by
where f = cyclotron frequency, q = ion charge, B = magnetic field strength and m = ion mass.
This is more often represented in angular frequency:
where is the angular cyclotron frequency, which is related to frequency by the definition .
Because of the quadrupolar electrical field used to trap the ions in the axial direction, this relationship is only approximate.
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