MASS SPECTROMETRY RESOURCE

    The two opposite rods in the quadrupole have a potential of +(U+Vcos(wt)) (labelled '+' on the Fig. 1) and the other two -(U+Vcos(wt)) where 'U' is the fixed potential and Vcos(wt) is the applied RF of amplitude 'V' and frequency 'w'. The applied potentials on the opposed pairs of rods varies sinusoidally as cos(wt) cycles with time 't'. This results in ions being able to traverse the field free region along the central axis of the rods but with oscillations amongst the poles themselves. These oscillations result in complex ion trajectories dependent on the m/z of the ions. Specific combinations of the potentials 'U' and 'V' and frequency 'w' will result in specific ions having a stable trajectory through the quadrupole to the detector. All other m/z values will have unstable trajectories and will hit the quadrupoles and not be detected (see Fig. 1). The mass range and resolution of the instrument is determined by the length and diameter of the rods.

    Quadrupole mass spectrometers generally have two configurations in the modern laboratory. They are very commonly used in conjunction with either gas-chromatography or liquid-chromatography as a simple high throughput screening system. Quadrupoles can also be placed in tandem to enable them to perform fragmentation studies - the most common set-up is the triple quadrupole (QQQ) [3] mass spectrometer which enables basic ion fragmentation studies (tandem mass spectrometry MS/MS) to be performed.