Mass Spectrometry | Selected Ion Monitoring ☆

Selected ion monitoring (SIM) is an acquisition mass spectrometry (MS) scan mode used for monitoring one or a restricted number of ions. Unlike full scan acquisition, the MS analyzer during the SIM acquisition can spend more time for the selected ion, avoiding to acquire other ions that are not of interest for the analyst. This article takes into account the advantages and disadvantage of the SIM MS technique, considering that its use is generally coupled with separation techniques such as gas chromatography or high-performance liquid chromatography. An overview about sample introduction, ionization sources, and mass analyzers (both at low- and high-resolution) used during SIM experiments is here given. However, the increasing request of tandem mass spectrometry instrumentation is supplanting little by little the use of SIM acquisition in place of selected reaction monitoring (SRM), which is the evolution of SIM acquisition with tandem MS. A comparison between high-resolution SIM MS and SRM MS acquisition is also taken into account. At last, new trends of HR MS acquisitions are considered, in which generally the potentiality of data independent acquisition and of retrospective analysis can be utilized at the expense of the use of SIM experiments.