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FT-ICR Mass Spectrometer
FT-ICR
stands for Fourier Transform Ion Cyclotron Resonance. This type of
mass spectrometer measures the ion's cyclotron frequency and
calculate the corresponding mass. It is a non destructive
technique : ions are trapped for long period of time and can be
manipulated using electric and magnetic fields. It provides some
unique capabilities: very high resolution, high sensitivity,
possibility of extended MS/MS experiments.
The
physic behind is the motion of a charged particle in a uniform
static magnetic field. A force is exerted onto a particle when it
is moving perpendicular to the magnetic field direction. The
particle, (in our case an ion) is deflected into circular orbits,
providing it doesn't encounter any obstacle (vacuum). The
cyclotron frequency is the frequency at which an ion of a given
mass rotates inside a given magnetic field. This frequency depends
only on the field magnitude and the ion's mass-to-charge ratio. By
solving the equation of motion, we obtain the following formula:
ωc=q.B/m with the units [rad/s]=[
Coulomb].[Tesla]/[ kg ].
The
FTICR MS generates series of carefully timed events in order to
measure the cyclotron frequency of the ions.
a)
Introduction of the neutral sample: A small quantity of gas is
pulsed inside the vacuum chamber.
b)
Ionization of the neutral sample: Using electron impact, a
short beam of electron is accelerated across the cell, effectively
ionising the neutral gas sample. The resulting ions are trapped
inside the cell by a combination of magnetic and electric fields.
c)
RF excitation of the ions: The ions are put into circular
motion by using RF pulses. Each ion is rotating accordingly to its
cyclotron frequency.
d)
Detection of the induced signal: When the ion's orbit is close
enough to the detection electrodes, they induce an image current
that's amplified and recorded, for all the ions simultaneously.
e)
Signal mathematical analysis : A Fourier Transform is
performed on the raw signal in order to get a frequency spectra. A
mass spectra is then easily obtain by applying the previous
formula.
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