A quadrupole mass spectrometer is a device which can separate ions by their mass and pass them to a detector, where they are counted. Gas molecules are ionized prior to entering the quadrupole zone, so that they have a charge and can be isolated in the tuned quadrupole zone. Heavier or lighter ions are deflected so that they cannot pass through to the detector. Therefore a selective scanning of a mass range (specifically mass to charge ratio) is possible, producing peaks where the detector has counted the desired mass range of ions or charged fragments.
