Exposition

Quadrupole with Pre-Filter

Exhibit no. 666

The quadrupole mass analyzer separates ions based on the stability of their trajectories in the oscillating electric fields.

It consists of four parallel rods with a hyperbolic or circular cross-section. Typical rod diameters are between 5 and 12 mm with rod lengths between 100 and 200 mm. The opposing quadrupole rods are connected together electrically, and a radio frequency (RF) voltage and a DC voltage is applied between one pair of rods and the other. Quadrupole mass analyzers are usually operated at a fixed frequency typically between 1 and 10 MHz.  Within the region between the rods, the quadrupole electric field affects the motion of the ions in the XY plane, while the ion motion along the Z-axis is not affected. The quadrupole mass analyzer acts as a mass filter. Only ions of a certain mass-to-charge ratio will reach the detector for a given ratio of RF and DC voltages: other ions have unstable trajectories and will collide with the rods. This permits the selection of an ion with a particular m/z or scanning for a range of m/z-values by continuously varying the applied voltage. The quadrupole mass analyzer was developed in parallel with the quadrupole ion trap by the Nobel prize-winning mass spectrometry pioneer, Wolfgang Paul.

The exhibit is a quadrupole mass analyzer with 15.5 mm diameter cylindrical rods equipped with a prefilter at the entrance. The prefilter was used to eliminate negative effects of the fringing field and thereby increased ion transmission. It also prevented direct contamination of the quadrupole. RF voltage with the same frequency and amplitude as on the main rods was applied to the prefilter. The exhibit comes from a simple quadrupole GC/MS detector Automass III manufactured by ThermoQuest in 2000. The quadrupole was designed for the mass range of 10 – 1000 Da.

Wikipedia: Quadrupole mass analyzer
Wikipedia: Wolfgang Paul

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