VIEWS: 2 PAGES: 4 POSTED ON: 9/16/2012
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.
Pages to are hidden for
"FT-ICR MS or simply FTMS"Please download to view full document