FT-ICR MS or simply FTMS by R7OqsR

VIEWS: 2 PAGES: 4

									                 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
radius r.
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
                                 to radius > ion trap radius to
                                 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