Introduction
Scan Types
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______________________ Finnigan LTQ Hardware Manual _______________________
SIM can improve the detection limit and decrease analysis time, but it can
also reduce specificity. In SIM, only specific ions are monitored. Therefore,
any compound that produces those ions appears to be the target compound.
Thus, a false positive result can be obtained.
Selected Reaction Monitoring
Selected reaction monitoring (SRM) is a two-stage (n = 2 scan power)
technique in which parent ion and product ion pairs are monitored.
In the first stage of mass analysis, the ions formed in the ion source are stored
in the mass analyzer. Ions of one mass-to-charge ratio (the parent ions) are
selected and all other ions are ejected from the mass analyzer. Then, the
parent ions are excited so that they collide with background gas that is present
in the mass analyzer. The collisions of the parent ions cause them to fragment
to produce one or more product ions.
In the second stage of mass analysis, the product ions are stored in the mass
analyzer. Ions of one or more mass-to-charge ratios are selected and all other
ions are ejected from the mass analyzer. Then, the selected ions are
sequentially scanned out of the mass analyzer to produce an SRM product ion
mass spectrum.
Like SIM, SRM allows for the very rapid analysis of trace components in
complex mixtures. However, because you are monitoring pairs of ions (one
product ion for each parent ion), the specificity obtained in SRM can be much
greater than that obtained in SIM. Thus, you are very unlikely to get a false
positive result with SRM. To get a false positive result, the interfering
compound must do the following: First, it must form a parent ion of the same
mass-to-charge ratio as the selected parent ion from the target compound.
Second, it must also fragment to form a product ion of the same
mass-to-charge ratio as the selected product ion from the target compound.
Consecutive Reaction Monitoring
Consecutive reaction monitoring (CRM) is the multi-stage (n = 3 to n = 10
scan power) analog of SIM (n = 1) and SRM (n = 2), in which a multi-step
reaction path is monitored. In the first stage of mass analysis, the ions formed
in the ion source are stored in the mass analyzer. Ions of one mass-to-charge
ratio (the parent ions) are selected and all other ions are ejected from the mass
analyzer. The parent ions are excited so that they collide with background gas
that is present in the mass analyzer. The collisions of the parent ions cause
them to fragment to produce one or more product ions.
In the second stage of mass analysis, the product ions are stored in the mass
analyzer. Product ions of one mass-to-charge ratio are then selected and all
other ions are ejected from the mass analyzer. The selected product ions now
become the new parent ions for the next stage of mass analysis. The new
