Improved accuracy of mass measurement with a quadrupole ion‐trap mass spectrometer

Mass Measurement accuracy of better than 0.015% on protonated gramicidin S (nominal chemical average molecular weight 1141.5 Da) and its first three isotopes has been achieved using reasonant ion ejection in the quadrupole ion‐trap mass spectrometer operated at unit resolution. Analyte and Calibrant ions are subjected to identical injection and trapping conditions when desorbed by Cs + ions from a split prone‐tip and co‐injected into the trap. When the mass analysis scan rate is slowed, enhanced resolution is achieved for all ions but the mass range accessed in a single scan of the digital‐to‐analog converter, which controls the mass analysis scan, is limited making calibration difficult. A peak‐matching procedure is described for use at these higher resolution conditions in which the offset voltage which determines the origin of the RF amplitude ramp is adjusted in such a fashion that the analyte and calibrant ions occur at the same point in the mass analysis scan. This procedure gave an improved mass measurement accuracy of better than 0.007% for the protonated gramicidin S. ions.