Introduction
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____________________________________________________________________ Scan Types
_______________________ Finnigan LTQ Hardware Manual _____________________
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Two-Stage Full Scan
The two-stage full scan type has two stages of mass analysis (n = 2 scan
power). In the first stage of mass analysis, the ions formed in the ion source
are stored in the mass analyzer. Then, 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. Then, they are sequentially scanned out of the mass analyzer to
produce a full product ion mass spectrum.
The two-stage full scan type gives you more information about a sample than
does SRM, but two-stage full scan type does not yield the speed that can be
achieved by SRM. With two-stage full scan, you spend more time monitoring
the product ions than you do in SRM. Thus, two-stage full scan provides
greater information, but lower speed than SRM does.
To use the SRM scan type, you need to know what parent / product reaction
you are looking for before you can perform an experiment. Thus, for SRM
you might use one-stage full scan type to determine the parent mass spectrum
and two-stage full scan type to determine the product mass spectra for parent
ions of interest. Then, you might use SRM to do routine quantitative analysis
of the compound.
Selected Ion Monitoring
Selected ion monitoring (SIM) is a single-stage (n = 1 scan power) technique
in which a particular ion or set of ions is monitored. In the SIM scan type, the
ions formed in the ion source 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 a SIM mass spectrum.
SIM experiments are useful in detecting small quantities of a target compound
in a complex mixture when the mass spectrum of the target compound is
known. Thus, SIM is useful in trace analysis and in the rapid screening of a
large number of samples for a target compound.
Because only a few ions are monitored, SIM can provide lower detection
limits and greater speed than a single-stage full scan analysis can provide.
SIM achieves lower detection limits because more time is spent monitoring
significant ions that are known to occur in the mass spectrum of the target
sample. SIM achieves greater speed because only a few ions of interest are
monitored; regions of the spectrum that are empty or have no ions of interest
are not monitored.
