How a quadrupole mass filter works

This is a simple explanation of a complicated device. A quadrupole is a set of four rods with a space down the middle. The ions enter this space. The rods are electrically connected to eachother in opposite pairs. A constant (DC) voltage and an alternating (AC) voltage are applied to the two pairs of electrodes.

The alternating electric field makes the ions go off into spirals as they pass down the quadrupole. The constant voltage drags them in one constant direction, towards one pair of electrodes.

A small ion will be dragged a large distance by the alternating field, and will find itself in stronger and stronger regions of field. It will quickly collide with an electrode and disappear.

A very large ion will not be affected much by the alternating field, but will gradually drift in the constant part of the field (the DC part). The alternating field is not strong enough to drag it back as it wanders, so it also collides with an electrode, and is lost.

An ion that is the right size drifts slightly in the constant part of the field, but is always dragged back by the alternating part. The alternating part, however, is not quite strong enough to make it spiral out of control into an electrode. Thus an ion just the right size is stable in this quadrupole field and reaches the end, where it can be measured.

The stability of an ion in a quadrupole (its chance of making it through the quadrupole without wandering so far from the "safe" region in the middle that it hits an electrode and is lost) therefore depends on the sizes of the alternating and constant fields. It is possible to draw stability diagrams describing whether an ion is stable or not at any given pair of voltages, AC and DC.

The next page describes how a mass spec exploits the shape of this stability diagram.