Principal of Operation
A quadrupole field is formed by four electrically conducting, parallel rods(shown in below figure). Opposite pairs of electrodes are electrically connected. One diagonally opposite pair of rods is held at +Udc volts and the other pair at -Udc volts.
An rf voltage supplies a signal(+V Cos wt) to first pair of rods and (-V Cos wt) to second pair of rods, which appear as oscillating hyperbolic potentials between the four rods. Ions from the ion source are injected into the quadrupole through a circular aperture. As the ions moves down the quadrupole, they undergo transverse motion in x- and y- planes.
The dc electric fields tend to focus positive ions in the positive plane and defocus them in the negative plane. The ions exhibit oscillations with increasing amplitudes until they finally collide with the electodes and become neutral particles. The lighter the ion in mass, the smaller the number of cycles before it is collected by the electrode.
By controlling the ratio Vdc/Vrf, the field can be controlled which allows to pass ions of only one m/z ratio down the entire length of the quadrupole. By simultaneous ramping the dc and rf fields, ions of various m/z ratios are allowed to pass through the mass filter to the detector and an entire spectrum can be produced.
The operation of a quadrupole mass analyzer is usually treated in terms of a stability diagram that relates the applied DC potential (U) and the applied RF potential (V(t)) and the RF frequency (omega) to a stable vs. unstable ion trajectory through the quadrupole rods. :
The ions are passed within a 60ocone around the axis, therefore does not require focusing slits. Increasing the rod length (5-20 cm) increases the resolution. If the rf frequency is increased, the lenght of the analyzer can be reduced. Rod diameters are also a factor: increasing the rod diameter increases the sensitivity by a large factor, whereas decreasing the diameter increases the mass range.
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