Invented a half-century ago by Nobel laureate Wolfgang Paul, the quadrupole mass filter has become an integral part of some of the most sophisticated mass spectrometers. Consisting of four parallel metal rods on which a fixed direct-current voltage and alternating radio-frequency voltage are applied, quadrupole mass filters allow users to selectively filter all ions except for those of a specified mass to charge ratio (m/z), depending on the applied voltage.
So-called triple-quad instruments contain two quadrupoles for mass filtering flanking a third that acts as a collision cell. In MS mode, the quadrupole scans across an m/z range, sequentially transmitting each m/z value to produce an MS spectrum; in MS/MS mode it acts as a selective ion filter. Quadrupole-Tof (Q-Tof) instruments, on the other hand, pair the quadrupole (and collision cell) with a time-of-flight analyzer, allowing high-resolution, high mass accuracy analysis of all ions simultaneously.