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.

<figcaption>The Waters Q-Tof Premier shown here is configured with an electrospray source. Sample is delivered by an online liquid chromatography system (1). Following ionization at the NanoLC source at left (2), the particle beam travels through the ion guide (3) and into the quadrupole (4). In MS/MS mode, selected ions are fragmented by collisions with argon gas in the T-wave collision cell (5). The reflectron (6) helps to minimize the kinetic energy spread of the ions and focus the ion beam towards the detector. (The instrument is shown operating in the W optics mode, whereby the ions are passed through the reflectron twice, doubling the instrument's flight path and increasing its resolution.) Click to view enlarged diagram</figcaption>
The Waters Q-Tof Premier shown...

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