quadrupole mass analyzer
/ KWOD-roo-pole /
Imagine four parallel metal rods arranged like the legs of a square table, with ions flying down the gap between them. Now wobble the voltage on the rods in a clever rhythm so that only ions of one particular mass can thread the wobbling field steadily, while all others swing wider and wider until they crash into a rod. The quadrupole is this rhythmic gatekeeper.
A quadrupole mass analyzer is a set of four parallel rods to which combined steady (DC) and oscillating (radio-frequency) voltages are applied. For a given setting, only ions of one narrow m/z have a stable path all the way through; the rest are filtered out. By smoothly changing the voltages, the analyzer scans through m/z values one after another, building up a spectrum.
It matters because quadrupoles are compact, fast, robust, and inexpensive, which is why they are the most common analyzers in routine labs and the standard partner for chromatography. The caveat is that their resolution and mass accuracy are modest compared with time-of-flight or trap instruments, so they are workhorses for everyday targeted analysis rather than for resolving the finest mass differences.
In a GC-MS, the quadrupole is parked on a single m/z to watch only one fragment of a target pesticide as it elutes; ignoring all other ions cuts the background and makes the trace measurement far cleaner.
A quadrupole acts as a tunable filter that passes one m/z at a time.
A quadrupole is often described as a 'mass filter' rather than a true mass measurer, because at any instant it transmits only a selected m/z and rejects the rest. Three quadrupoles in series form a triple quadrupole, a mainstay of tandem mass spectrometry.