Method of high mass resolution scanning of an ion trap mass spectrometer

Abstract

A method of using a quadrupole ion trap mass spectrometer for high resolution mass spectroscopy is disclosed. High resolution of a mass spectrum of a desired species is achieved by first using a slow scanning rate and by first ridding the trap of unwanted ions. Accurate mass calibration is achieved by using a reference compound of known mass and using a second supplemental AC dipole voltage to eject the reference ions at nearly the same time as the sample ions of interest are ejected from the trap. This eliminates the need to scan the trap between the masses of the sample and reference ions. Space charge in the trap is held constant, thereby eliminating a major source of mass axis instability, by using the results of one scan to control the ionization time during the next scan. Preferably, during ionization a broadband supplemental dipole voltage is applied to the ion trap to rid it of unwanted ions. During a portion of the ionization time the broadband signal is constructed to retain only sample ions in the ion trap, and during the remainder of the ionization time the broadband signal is constructed to retain both sample and reference ions in the ion trap. By adjusting the relative lengths of the two portions of the ionization time the total space charge in the ion trap can be held constant notwithstanding variations in sample ion concentration.