The selected ion monitoring (SIM) experiment is a single stage (scan power of n = 1) technique that monitors a particular ion or set of ions.
As with all scan types, the ion source forms ions that enter the analyzer assembly. The quadrupole mass analyzer isolates the user defined m/z range. The ions pass from the quadrupole mass analyzer and move to the ICRM quadrupole assembly where they are stored, then transmitted to the dual-pressure linear ion trap where they enter and are trapped in the high-pressure cell, pass to the low-pressure cell, and are mass analyzed. The resulting mass-selected ions are finally detected by the ion detection system.
You can use SIM experiments to detect small quantities of a target compound in a complex mixture when you know the m/z of the target compound. Therefore, SIM is useful in trace analysis and in the rapid screening of a large number of samples for a target compound.
Because SIM monitors a narrow range of m/z, it can provide lower detection limits and greater speed than the full scan mode. SIM achieves lower detection limits because the finite instrument trapping space is utilized by ions of interest and not as much by other ions, which results in a higher dynamic measurement range.
SIM can improve the detection limit and decrease analysis time, but it can also reduce specificity compared to SRM. Because SIM monitors only specific precursor ions and not their product ions, any compound that is isobaric will appear to be the target compound, which can result in a false positive.