The CompassCDS Smart Time Update feature

Automating Retention Time and Timed Event
Updating using the CompassCDS Smart Time
Update Function
Graham Shelver Ph.D., Bruker CAM, Fremont CA.
Abstract
Chromatographic column ageing as well as other instrumental and chemical effects can cause peak
retention times to vary over time from the original values set in a method. Various approaches have
been used to compensate for these variations including changing instrument control parameters such
as carrier gas flow rate and using reference peaks to adjust the target retention times. However, none
of these approaches can be used to adjust both the peak retention time targets and timed peak
processing events on the chromatogram where these deviations occur. The CompassCDS Smart Time
Update feature can be used to automatically correct both retention times and timed events on
chromatograms where such deviations occur.
Introduction ............................................................................................................................................. 2
Automatically changing instrument flow rate to correct of changes in chromatographic retention
time. ......................................................................................................................................................... 2
Use of reference peaks to correct for chromatographic variations ................................................... 3
The CompassCDS Smart Time Update feature .................................................................................... 4
The CompassCDS Smart Time Update feature and timed peak processing events ........................ 4
The CompassCDS Update Ratio function ............................................................................................ 6
Manually Applying Smart Time Update using the Update Ratio function ......................................... 7
Conclusions ............................................................................................................................................ 7
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Introduction
Changes in chromatographic retention times can be caused by both random factors like
changes in injection technique or unexpected components in particular samples as well
as systemic factors including gradual column ageing and small instrumental parameter
changes such as leaks in either the HPLC or GC system.
There are several approaches that can be used to compensate for such changes. One
is to simply replace the column if it is determined that this is the root cause of the
problem. If a column has been in service for a long time and is demonstrating
deteriorating in the form of gradually decreasing peak retention times, this would be a
good solution. However, in order to achieve this, the system must be shut down, the
column replaced and the system re-equilibrated. This cannot be done during a series
of automated runs. It also usually requires a certain level of expertise and could be
disruptive to a routine laboratory’s workflow. Also, it may be that the column is not the
source of the problem.
Automatically changing instrument flow rate to
correct of changes in chromatographic
retention time.
An alternative to replacing the column is to incrementally increase the flow rate of
carrier gas for GC or mobile phase for HPLC. When applied to GC, this is termed
Retention Time Locking or RTL. This can be done during a series of automated runs
without operator intervention. However, this approach can have several limitations.

The correction can only be made on the next run. When the problem is
detected on a particular analysis, the instrumental parameters will be adjusted
for the next run. If this was a problem specific to one injection or one sample,
the correction will be made for the wrong injection.

Increasing the flow rate can cause changes in the chromatographic
characteristics because the Van Deempter curve is not constantly proportional
to flow rate. Usually as the flow rate of the carrier in either HPLC or GC is
increased, the resolving power of the column is decreased. This could lead to
changes in resolution.

This approach actually changes the analytical method and, if this is a validated
method, the new method may have to have an abbreviated validation
procedure, especially if the initial validation highlighted a limited flow rate range
for the method.

This approach is difficult to apply to HPLC. For gradient HPLC increasing or
decreasing the flow rate changes the nature of the gradient.

This approach can only be used on a few, vendor specific chromatographic
instruments under the direct control of the data system.
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Use of reference peaks to correct for
chromatographic variations
It is far better to address changes in peak retention characteristics of a
chromatogram through chromatographic peak processing. Using software, each
chromatogram can be treated individually to correct for retention time variations.
Usually these issues can be corrected for simply by the use of one or more
reference peaks. Reference peaks are easily identifiable peaks, usually larger
than most of the peaks in the chromatogram and, whenever possible, well
resolved from other peaks. They are usually set to have a larger detection time
window for than the other peaks. When a reference peak is detected at a time
earlier or later than expected, the target retention times for the identification of
other peaks around the reference peak are adjusted proportionally. For
example, if a reference peak is detected at a retention time 5% earlier than
expected, the target retention times of the other peaks in the chromatogram are
adjusted to be 5% earlier than the time reflected in the peak table. Normally,
this adjustment is done for that run only and does not effect the retention times
listed in the peak identification table which remain unchanged.
Figure 1 CompassCDS Retention Time Reference Peak Input.
In Figure 1, the peak labeled TCAA is designated as a reference peak. Its retention
time window has been expanded to +/- 0.6 minutes to make it easier to identify if the
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chromatography has changed. The peak identified as TCAA will be the peak with the
largest height in this retention time window.
The CompassCDS Smart Time Update feature
While reference peaks can be used to correct for random chromatographic variation,
systemic variations can be a more serious problem.
Column retention time
characteristics can change over time. This is sometimes termed column ageing. As
columns age, the retention times of the reference peaks can migrate beyond the pre-set
time windows. These changes usually occur gradually but during the analysis of an
extended series of samples, can be quite significant. CompassCDS can utilize such
gradual changes to constantly adjust target retention times and thereby ensure
ongoing, accurate peak identification.
Figure 2 CompassCDS Smart Time Update Ratio parameter
When the Smart Time Update feature is activated in CompassCDS, as shown in Figure
3, the Update Ratio percentage is used to change the retention times in the peak table
for both the reference and non reference peaks. If a reference peak is detected 1%
earlier than expected then all of the retention times in the analytical method peak table
are decreased by 1%. In this way even a significantly larger gradual change in
retention time (for example 10%) can be corrected by the Smart Time Update function.
Proper peak identification can be maintained even after significant changes in the
chromatography .
The CompassCDS Smart Time Update feature
and timed peak processing events
Adjusting only peak retention time targets is not enough as timed events, which are
used to maintain proper peak processing, must also be adjusted to correspond to the
adjusted peak retention times. Without this adjustment, improper peak processing will
give incorrect results. The CompassCDS Smart Time Update feature will also
automatically adjust timed events so that proper peak processing is maintained
throughout each run.
As the retention time for a peak decreases, for example 5% as shown in Table 1, the
CompassCDS Smart Time Update feature can adjust the times for the timed events in
the integration table 5%. Importantly, the modified times are used for peak processing
on the chromatogram where the change in the reference peak retention time occurred
and not just on the subsequent run.
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Event
Retention time of
reference peak
Horizontal forward
start
Horizontal forward stop
Horizontal forward next
peak start
Horizontal forward next
peak stop
Original
times
8.7
Time offset for
reference peaks
5% earlier
New times
6.8
5% earlier
6.46
8.2
8.4
5% earlier
5% earlier
7.79
7.98
9.2
5% earlier
8.74
8.27
Table 1 CompassCDS Smart Time Update adjusts expected peak retention times and
timed events
The effect of timed event adjustment is shown in Figure 3 below. In this example, the
retention times of all of the peaks including the reference peak Pentachlorobenzene
have become 5% shorter. In the lower chromatogram, the CompassCDS column
ageing factor was turned off. The peak processing times events (shown in the figure)
were maintained at the original times. Here the carefully chosen timed events do not
function properly because they are being executed at the original and now incorrect
times.
The timed events in the upper chromatogram have been automatically adjusted in the
timed events table by the CompassCDS Smart Time Update feature to be 5% shorter.
These now execute at the correct time relative to the peaks in the actual chromatogram
giving the correct peak processing and the correct results.
Figure 3 CompassCDS Smart Time Update adjusts timed events
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The CompassCDS Update Ratio function
Although column ageing and instrument variation problems can result in changes in
chromatography, now and again, individual samples can also cause significant retention
time shifts. These changes may often only occur with one anomalous sample and then
the chromatography returns to normal. If retention times and timed events were to be
changed by a large amount, based on such an outlier chromatogram, subsequent
chromatograms could be incorrectly processed. The CompassCDS Smart Time Update
feature can be used to adjust for this possibility. The Update Ratio (entry value shown
in Figure 2) determines how rapidly CompassCDS will react to sudden changes in the
retention times of the reference peaks. This ratio determines how much of an
adjustment of retention time targets and timed events is done due to the effect of any
one chromatogram.
The effect of a ratio of 25% is shown in Figure 4. Here the change in retention time of
the reference peak in chromatogram 6 only causes a small change in the timed events
and peak identification table. This entry allows CompassCDS to successfully adjust to
gradual changes in retention times but not to over react to “one of a kind” events.
Figure 4 Effect of CompassCDS Update Ratio when one of a kind sample effects are
detected.
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Manually applying Smart Time Update with the
Update Ratio function
Sometimes, automating changes to retention times in the peak identification or timed
events table might be considered unwise. It may be better for chemist to first assess
the chromatograms before any changes are made. In such cases, the CompassCDS
column Smart Time Update makes implementing these changes easier. If the chemist
determines that the changing chromatogram warrants changes to the peak and timed
events table, a new chromatogram can be run with Smart Time Update turned on and
the Update Ratio set to 100%. All of the tables will be updated accurately without the
possibility of missing a peak or event.
Conclusions
The CompassCDS Smart Time Update feature meets all of the criteria required for
successfully compensating for variation in retention time originating from any source.

Fully automatic

Can be used on any LC/GC system, even those not being controlled by
CompassCDS software

Does not require re-validation of an analytical method as the instrument control
parameters are not modified.

Uses the commonly accepted reference peak method to determine the amount
of change in peak retention time from injection to injection.

Corrects timed peak processing events so that peak are not only identified
correctly but they are also processed correctly

Can be set to minimize the “correction” due to unique sample based variations

Can be used in either manual or automatic mode.
CompassCDS Smart Time Update minimizes the effect of
column ageing and keeps accurate peak identification on track.
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