A method for obtaining high accuracy mass spectra using an ion trap mass analyzer includes adjusting operating parameters of the analyser to enable a reverse mass scan in a mass selective resonance ejection mode and setting the trapping field to trap ions in a mass-to-charge ratio which has a lower limit close to the mass-to-charge ratio of an ion of interest. A method of determining chemical shift includes adjusting operating parameters of the analyzer to enable forward and reverse mass scans and calibrating the spectra obtained from the forward and reverse mass scans.