The Quadrupole Mass Analyser

The quadrupole analyser is an ideal detector for chromatography as it is capable of fast scanning and uses low voltages which make it tolerant of relatively high operating pressures, such as those encountered in LC-MS. 

The quadrupole is classified as a low-resolution device, i.e. it is capable of measuring the m /z ratio of an ion to the nearest integer value, and thus is unable to provide the elemental composition of an ion. 

Because these analysers do not employ magnets, peak switching for selected ion monitoring can be done more quickly without hysteresis effects, which makes this system ideal for depth profiling, where it is necessary constantly to switch among masses. These instruments do have the disadvantage of loss of transmission and mass 

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Beside the most commonly used ion separation systems, such as the quadrupole mass analyser, the ion-trap mass analyser and the double-focusing and tri-sector mass analyser, a new system has encountered increasing interest. This system, the time-of-flight (TOP) mass analyser offers fast scanning opportunities coupled with the possibility of detecting ions with high m/z ratios. This provides a useful tool for rapid quality control of sensorial profiles as well as the detection and identification of toxins and proteins or protein-bound substances [42]. 

The quadrupole mass analyser is a mass filter . Combined DC and RF potentials on the quadrupole rods can be set to pass only a selected mass-to-charge ratio [36]. All other ions do not have a stable trajectory through the quadrupole mass analyser and will collide with the quadrupole rods, never reaching the detector. The operation of a quadrupole mass analyser is usually treated in terms of a stability diagram that relates the applied DC potential and the applied RF potential, and the RF frequency to a stable vs unstable ion trajectory through the quadmpole rods. A schematic diagram of a quadmpole mass filter is shown in Fig. 10.5. 

In essence, the quadrupole mass analyser is a mass filter a combination of DC and AC potentials on the quadrupole rods can be set so that only a selected mass-to-charge ratio can pass the device. All other ions exhibit unstable trajectories through the quadru-... 

Ions are extracted from the plasma through a water-cooled sampling aperture and a molecular beam is formed in the first vacuum stage and passes into the high-vacuum stage of the quadrupole mass analyser. 

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