Patent ID: 7759655

Claim:
A mass spectrometer comprising: an ion volume to receive atoms or molecules of a sample; an electron source to inject electrons into the ion volume to ionize the atoms or molecules in the ion volume; a quadrupole mass filter positioned to receive ions from the ion volume, the quadrupole mass filter including a voltage source configured to scan the amplitude of voltage applied to rods of the mass filter over a preset range to progressively change the mass-to-charge ratio of the transmitted ions; a reflection mechanism configured to selectively reflect electrons through the ion volume; and a programmable controller programmed with a procedure to cause a potential to be applied to a reflector of the reflection mechanism as a periodic series of pulses, the duration and frequency of the pulses defining a duty cycle; wherein the programmable controller is programmed with a procedure to vary the duty cycle of the applied potential during scanning of the amplitude of voltage applied to rods of the quadrupole mass filter such that the ionization efficiency is adjusted concurrently with the mass-to-charge ratio of the transmitted ions.