Mass Analyzer · 2 / 3
A mass filter using an oscillating electric field · the workhorse of LC-MS and GC-MS
A quadrupole is a mass filter: four parallel rods create an electric field that combines a DC voltage and a high-frequency RF voltage. At any given setting, only ions within a narrow m/z window stay on a stable path and pass through; all others swing wider and wider until they hit a rod and are lost.
Drag the target m/z — ions that match stay stable and reach the detector; others become unstable and hit the rods.
The small inset (top-right) shows the four rods in cross-section. Opposite rods share the same polarity, and the polarity flips back and forth with the RF, which is what keeps matching ions oscillating on a stable path down the centre.
Unlike TOF, which measures all ions at once, a quadrupole only lets one narrow m/z window through at a time. By scanning the DC/RF settings, it sweeps that window across the mass range to build up a spectrum. It is compact, fast, and robust — which is why it is so widely used.