A mass peak in a spectrum that corresponds to a compound of known mass (mass standard). Used post-acquisition to adjust the mass calibration for the scan so that other mass peaks may be accurately measured. This peak tends to be a ubiquitous impurity ion such as a plasticiser or polysiloxane degradation product. Typically the lock mass on the Orbitrap is the ion generated from polydimethylcyclosiloxane (PCM) during the electrospray process (protonated (Si(CH3)2O)6); m/z 445.120025)

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