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This then forces a compromise between adding many
compounds per segment, reducing individual SRM
dwell times and sensitivity, and adding segment breaks
between closely eluting peaks, which causes the risk of
false negatives due to shifts in peak retention times outside
of acquisition windows because, for example, a large bit
of matrix coelutes with a peak.
The TSQ 8000 system takes an approach called Timed-
SRM that eliminates this compromise. Timed-SRM
removes the limitations of segmented SRM by centering
acquisition windows on the retention time of each peak
and allowing for acquisition window overlap, so that
acquisition windows for all nearby eluting compounds are
not forced to start and stop at the same time (Figure 5).
The user simply needs to enter the retention time of each
compound, and the instrument method takes care of the
rest, eliminating the need for creating segments.
High Compound Capacity Methods
One of the primary challenges of modern pesticide
analysis is the sheer number of pesticides that need
monitoring in order to meet international standards. This
is particularly true in food analysis where products are
transported across country borders, requiring exporters
to meet the regulatory demands of many countries. Triple
quadrupole instruments help meet this demand due to
the high selectivity of MRM analysis, which allows for
spectral separation of coeluting peaks due to unique
reactions in the collision cell. This enables monitoring of
more compounds in a single chromatographic run without
prohibitive interference. However, due to the targeted
nature of the MRM process, individual scan events must
be created for each pesticide to be monitored, placing a
strain on the amount of time devoted to the monitoring
of each compound, and thus the sensitivity of the analysis
of each compound.
With a traditional style analysis, this issue can be partially
resolved by slicing up the acquisition list into discreet time
segments, so that all transitions are not being monitored
at the same time. However, this can quickly lead to
problems when analyzing more than 50 pesticides in one
run. This is because, due to the density of the peaks in the
heart of the method, it is difficult to find a time for a
segment break when no target peaks are eluting.
Acquisition windows
centered around
retention time
Acquisition windows
allowed to overlap
Figure 5. The TSQ 8000 system Timed-SRM Acquisition list, showing SRM acquisition windows centered on retention times and overlapping nearby transitions.