ABSTRACT
Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS, or FTMS), rst described by Comisarow and Marshall [1], is based on the circular motion of a charged particle in a uniform magnetic eld. This motion is referred to as cyclotron motion and its frequency, ω, is given by the cyclotron equation
ω = zeB m
(8.1),
where z is the charge number of the ion charge, e is the electronic charge, m is the mass, and B is the magnetic eld induction. As can be seen from this equation, the frequency of motion is strongly dependent upon the particle’s mass-to-charge ratio (m/z), making FT-ICR MS analytically useful. In a modern FT-ICR MS instrument, built around a 7 tesla (T) superconducting magnet, for example, ion cyclotron frequencies are typically in a range from a several kilohertz (high m/z) to a few megahertz (low m/z). For example, an ion of m/z 1000 in a 7 T magnetic induction has a cyclotron frequency of approximately 100 kHz. If this ion has a cyclotron orbit radius of 1 cm, then it will travel a distance greater than six kilometers in a single second. Given that the ions are traveling such long distances during the time of detection, it is important that the background pressure in FT-ICR MS instruments be very low.
