A process that ensures the mass axis of the spectrum is accurate. Typically calibration is performed prior to any other analysis and consists of the accumulation of a series of spectra that are used to generate a mathematically modelled fit of the measured mass of the ions in the spectrum to their actual masses. The Orbitrap is calibrated using a mixture of caffeine, a peptide denoted MRFA and ultramark. This calibration is valid across the mass range 100-2000 Da. Further calibration can be achieved during a run (lock mass) or via a post run recalibration of the acquired spectra.