Mass Analyzers: Common Types

The quadrupole mass analyzer consists of four parallel rods, across which are applied a DC voltage and a radio-frequency AC voltage.

Here, the field strength causes only ions with particular m/z values to resonate with the field and strike the detector. These analyzers offer a low resolution.

Another common analyzer is the time-of-flight analyzer, wherein ion flight time is measured from the source through the linear field-free tube to the detector.

All ions have about the same kinetic energy, so lighter ions have higher velocities, resulting in shorter flight times.

To improve the resolution, reflectrons, or ion reflectors, are used. They slow down the more energetic ions, correcting kinetic energy differences between ions of the same mass.

Another alternative is the three-dimensional quadrupole ion trap, consisting of a ring electrode and a pair of endcap electrodes. A radio-frequency voltage traps the ions, which oscillate in the ring.

By increasing the radio-frequency voltage, progressively heavier ions stabilize and selectively eject to the detector.