# FT-ICR MS or simply FTMS by R7OqsR

VIEWS: 2 PAGES: 4

• pg 1
```									                 FT-ICR MS or simply FTMS
Invented in 1974.
Begin with same discussion used for magnetic sector mass
analyzer.

How does an ion with a velocity vector (v) behave in a magnetic
field (B)?
The definition of a
magnetic field (B):
F=vxB
The ion undergoes a
force perpendicular to
velocity and magnetic
field vectors, and is
deflected through a
circular path with
Ions with different m/z will be deflected with different radii of
curvature.

In FTMS the ion is simply allowed to continue its circular
motion in an “ion trap”. The physics of how an ion behaves in a
magnetic field is the same.
Starting as before with the fact that
centripetal force = magnetic force

A little algebra and the concept of angular frequency we get:
wc = zB/m
The unperturbed ion cyclotron frequency
All ions of a given m/z have the same ICR frequency
independent of velocity. (An increase in velocity results in an
increase in the radius of rotation, thus keeping the ICR
frequency constant.)

Rearrangement of the 1st equation gives the orbital radius:
r = mv/zB = z-1b-1(2mkT)0.5
For a 3 Tesla magnet (not large) at room temperature:
100 amu ion r = 0.08 mm
10,000 amu ion r = 0.8 mm
50,000 amu ion r = 1 cm
Small orbits – easy to build ion traps of this size.

Ion cyclotron resonance not by itself useful. Trapped ions are
excited by an oscillating (RF) field.

The RF field used to excite the
ions to a larger radius (for later
detection), increase ion KE for
dissociation, or accelerate ions
remove.
As long as the RF field is on, and is of the same frequency as
the ion cyclotron resonance frequency (dependent on m/z), the
ion will continue to absorb energy, increase velocity, thus
increasing its orbital radius. All ions, regardless of m/z, can be
excited to the same radius by varying the time of excitation with
frequency.
The RF field also makes all the ions coherent (spatial coherence
to form ion packets), thus making detection possible.

The “image” current that is detected when the RF excitation
pulse is turned off has a frequency, the ion cyclotron frequency,
dependent only on m/z, not velocity.

FTMS is almost completely analogous to FTNMR with respect
to data generation and different types of excitation pulse
sequences.

FTMS Resolution: Dependent on the ability to detect different
frequencies, which can be done with high precision and accuracy.
Data below shows the high resolution and the ability to obtain
mass spectra of large molecules with ESI FT-ICR MS.

```
To top