Fourier-transform ion cyclotron resonance
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[2] and Alan G. Marshall at the University of British Columbia.
[3] The inspiration was earlier developments in conventional ICR and Fourier-transform nuclear magnetic resonance (FT-NMR) spectrometry.
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.
Because of the quadrupolar electrical field used to trap the ions in the axial direction, this relationship is only approximate.
The cyclotron motion is still the frequency being used, but the relationship above is not exact due to this phenomenon.
Additionally the masses are not resolved in space or time as with other techniques but only by the ion cyclotron resonance (rotational) frequency that each ion produces as it rotates in a magnetic field.
This provides an increase in the observed signal-to-noise ratio owing to the principles of Fellgett's advantage.
[4] A review of different cell geometries with their specific electric configurations is available in the literature.
Several closed ICR cells with different geometries were fabricated and their performance has been characterized.
Nested ICR cells with double pair of grids were also fabricated to trap both positive and negative ions simultaneously.
The most common open cell geometry is a cylinder, which is axially segmented to produce electrodes in the shape of a ring.
The central ring electrode is commonly used for applying radial excitation electric field and detection.
DC electric voltage is applied on the terminal ring electrodes to trap ions along the magnetic field lines.
[6] Open cylindrical cells with ring electrodes of different diameters have also been designed.
Several ion axial acceleration schemes were recently written for ion–ion collision studies.
[8] Stored-waveform inverse Fourier transform (SWIFT) is a method for the creation of excitation waveforms for FTMS.
The SWIFT procedure can be used to select ions for tandem mass spectrometry experiments.
FTICR-MS is able to achieve higher levels of mass accuracy than other forms of mass spectrometer, in part, because a superconducting magnet is much more stable than radio-frequency (RF) voltage.
[10] Another place that FTICR-MS is useful is in dealing with complex mixtures, such as biomass or waste liquefaction products,[11][12] since the resolution (narrow peak width) allows the signals of two ions with similar mass-to-charge ratios (m/z) to be detected as distinct ions.
[13][14][15] This high resolution is also useful in studying large macromolecules such as proteins with multiple charges, which can be produced by electrospray ionization.
Because the isotopic peaks are close to each other on the m/z axis, due to the multiple charges, the high resolving power of the FTICR is extremely useful.
[17] Although CID and IRMPD use vibrational excitation to further dissociate peptides by breaking the backbone amide linkages, which are typically low in energy and weak, CID and IRMPD may also cause dissociation of post-translational modifications.
This is quite useful in analyzing phosphorylation states, O- or N-linked glycosylation, and sulfating.
