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Introduction
Scan Types
Thermo Scientific
TSQ Quantum XLS and TSQ Quantum GC User Guide
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Because SIM monitors only a few ions, it can provide lower detection limits and greater speed
than the full-scan modes. 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
analyte. SIM can achieve greater speed because it monitors only a few ions of interest; SIM
does not monitor regions of the spectrum that are empty or have no ions of interest.
SIM can improve the detection limit and decrease analysis time, but it can also reduce
specificity. Because SIM monitors only specific ions, any compound that fragments to
produce those ions will appear to be the target compound. The result could be a false positive.
Selected Reaction Monitoring (SRM)
Selected reaction monitoring (SRM) monitors
a particular reaction or set of reactions, such as
the fragmentation of an ion or the loss of a neutral moiety.
SRM monitors a limited number of parent/product-ion pairs. In product-type experiments, a
parent ion is selected as usual, but generally only one product ion is monitored. SRM
experiments are normally conducted with the product scan mode.
As does SIM, SRM provides for the very rapid analysis of trace components in complex
mixtures. However, because SRM selects two sets of ions, it obtains specificity that is much
greater than what SIM can obtain. Any interfering compound would not only have to form
an ion source product (parent ion) of the same mass-to-charge ratio as the selected parent ion
from the target compound, but that parent ion would also have to fragment to form a product
ion of the same mass-to-charge ratio as the selected product ion from the target compound.
Highly-Selective Reaction Monitoring (H-SRM)
Highly-selective reaction monitoring (H-SRM) is SRM performed at higher resolution (0.4 u
FWHM versus 0.7 u FWHM for SRM).
Intelligent Selected Reaction Monitoring (iSRM)
In intelligent selected reaction monitoring (iSRM) experiments, the TSQ Quantum XLS and
TSQ Quantum GC mass spectrometers perform multiple “primary” selected reaction
monitoring (SRM) scans for each analyte (parent ion) of interest. The mass spectrometer uses
for quantification the sum of the intensities of an analyte’s primary SRM scans.
You specify a trigger threshold for each primary SRM scan. If the intensities of all the listed
primary SRM reactions for an analyte are above their trigger thresholds, and if all these scans
are within a specified retention time window, then the mass spectrometer performs
“secondary” SRM scans to confirm the identity of the analyte. The maximum number of
secondary SRM reactions is 12400.
