The Quadrupole Mass Analyzer

This analyzer consists of four cylindrical rods onto which is applied both AC and DC electric fields. The four rods are arranged such that they form two pairs, one pair in the horizontal (Y) plane and one in the vertical one (X).

As ions enter the quadrupole, they begin to oscillate in both the X and Y planes. In such a system, the lower m/z ions will be destabilized in the quadrupole whenever the AC component of the electric field exceeds the direct (DC) component. In this instance, the lower m/z ions will quickly be thrown out of the quadrupole and will not reach the detector. This mode of operation makes an effective low mass filter.

If the DC component exceeds the alternating component then the reverse is true. High m/z ions become unstable while lower m/z ions are stabilized by the presence of the alternating component. This makes an effective high mass filter

In the quadrupole, the mass analyzer is created by connecting the two pairs of rods in such a way that the X plane acts as a low mass filter (green path) and the Y plane acts as a high mass filter (blue path). By carefully matching the two fields, only ions of a particular mass (magenta) are able to resonate at the correct frequency and pass through the quadrupole at any point in time.

The quadrupole mass analyzer is a very fast, efficient, and relatively cheap  system , but it achieves only moderate resolving power of about 10000 (FWHP). Is very useful for applications not requiring a high resolution.

F. G. Banica,  2009-03-05