6.8: Mass Analyzers: Common Types
The quadrupole mass analyzer consists of four cylindrical metal rods arranged in a diamond carrying a DC voltage and a radio-frequency AC voltage. The motion of ions through the quadrupole depends on the field strength, causing only ions of a certain m/z to resonate successfully and strike the detector at a given field strength. Though the transmission rate for these analyzers is high, the exact elemental composition of the sample is not determined because of low resolution; however, they are cost-effective. The typical resolution of a quadrupole analyzer is 1 Da (or 1 u).
In a time-of-flight (TOF) analyzer, the flight time of an ion from the source through the linear field-free drift tube to the detector is measured in microseconds, and this time is converted to an m/z value. The ions enter the drift tube with about the same amount of kinetic energy. Accordingly, ions with different m/z values travel with different velocities and reach the detector at different times. Lighter ions travel with higher velocity, resulting in a shorter time-of-flight and comparatively poor resolution. Resolution can also be impaired by differences in the amount of kinetic energy imparted to the ions, causing ions of the same m/z to have different velocities. Because of this, a reflectron (or ion mirror) is employed to increase the travel distance to the detector and slow down the ions. More energetic ions experience greater slowdown, so the reflectron corrects the kinetic energy spread and provides a better resolution overall. In addition, TOF analyzers offer high sensitivity.
Three-dimensional quadrupole ion-trap analyzers consist of a ring electrode and two endcap electrodes. A radio-frequency voltage traps the ions, which oscillate in the ring. Increasing the radio-frequency voltage stabilizes heavier ions and selectively ejects them to the detector via apertures in the endcaps. While other mass analyzers allow only a small fraction of ions to reach the detector, the three-dimensional quadrupole ion trap enables half of the ions to reach the detector, making these analyzers more sensitive. However, with a large number of ions stored in the ion trap, the space-charge effects result in decreased resolution.
