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How does a quadrupole mass spectrometer select ions of a particular mass?

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Figure 1: An electron bombardment ion source in cross section. An electron beam is drawn from the filament and accelerated across the region in which the ions are formed and toward the electron trap. An electric field produced by the repeller forces the ion beam from the source through the exit slit. — mass spectrometry Figure 1: *An electron bombardment ion source in cross section.* An electron beam is drawn from the filament and accelerated across the region in which the ions are formed and toward the electron trap. An electric field produced by the repeller forces the ion beam from the source through the exit slit.

A quadrupole mass spectrometer selects ions of a specific mass by utilizing a combination of static and radio-frequency (RF) voltages applied to four parallel cylindrical electrodes.

When positive ions travel along the axis of these electrodes, the applied potentials create oscillating ion trajectories. By carefully adjusting the electrical parameters—specifically the static voltage and the RF voltage—the device acts as a mass filter:

Selective Transmission: Only ions of a specific mass-to-charge ratio maintain a stable trajectory that allows them to pass through the quadrupole.
Defocusing: Ions with other masses experience unstable trajectories, causing them to be defocused and lost from the ion beam before reaching the detector.1

This mechanism allows for rapid data acquisition and high transmission efficiency. Because the device is compact and its resolution can be adjusted by changing the electrical parameters, it is frequently used as the analyzing element in gas chromatograph–mass spectrometers.1