lock mass

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)