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Quadrapole Mass Analyzers

! Features

Most common system due to low scan times (100 ms), compact design, less expensive than other analyzers

! Ion Trajectory

Four parallel rods are the electrodes where are the positive and negative terminals of a dc source. Variable radio- frequency ac potentials (180 °) out-of- phase are applied to each pair of electrodes.

Ions are accelerated between the rods and must keep a stable trajectory in the xz plane (high pass-mass filter) and the yz plane (low-pass mass filter to get to the detector.

Ions that differ in one mass unit can be resolved by adjusting the center of the band by modulating the ac/dc potentials.

Fourier Transform Ion Cyclotron Resonance (FT-ICR) Mass Analyzer

! Utility

Most complex method of mass analysis but most sensitive of the techniques in common use today. Almost unlimited mass resolution, >106 is routinely observable with resolutions in the 104 to 105 range.

! Ion Trajectory

Ions drift into a spatially uniform static magnetic field of strength, B, that causes the motion to become circular in a plane perpendicular to the direction of the magnetic field. Within this ion-trap, the angular frequency (!c) is inversely proportional to

the m/z value.

F = z" x B

!c

=

" r

=

zB 2#m

m

=

B

z

2p!

or

The presence of ions between a pair of detector electrodes (in the trapping cell) will not actually produce any measurable signal. It is necessary to excite the ions of a given m/z as a coherent package to a larger orbital radius, by applying an RF sweep of a few milliseconds across the cell. One frequency will excite one particular (Fourier transformation allows for all frequencies to measure simultaneously).

Measurement of the angular frequency leads to values for m/z and thus to the mass spectrum. Because frequency can be measured more accurately than any other physical property, the technique has a very high mass resolution.

collector plate

t

excitation plate

frequency spectrum

detector plate

FT

mass spectrum

RF

transmitter plate

B

trapping plate

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