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Habitual monitoring of the GC baseline signal will lead to
longer uptime and better quality data. Important attributes
of the baseline are background level, drift, and noise. Each
of these can provide useful information and aid in
diagnosing instrument problems. The focus of this article
is use of the baseline to diagnose and correct leaks in
GC/ECD analysis.
Baseline Drift
ECD baselines showing drift, or a wandering effect, are
often an indication of leaks or pressure control issues. More
specifically, an uncommon rise or fall in baseline signal
followed by recovery during the instrument run
should cause high suspicion that leaks or pressure
control issues are the cause.
Leak Detection
A leak can occur at numerous places within the instrument
but is most common in the sample’s flow path from
introduction to signal collection. A leak detector should
be used to quickly assess the issue. All accessible
connections from the inlet to detector should be checked.
If no issues are found using the leak detector, it is
possible that the leak is too small to locate. In this case, There should
be mild disruption of test chromatograms.
Quick Tip: Insulation covers within the GC oven can hide leaks.
Electronic leak detectors typically operate by thermal conductivity and are recommended for use
during instrument maintenance. A quick Internet search
returns many different hand-held or portable electronic leak detectors that are offered by
instrument manufacturers and consumables suppliers.
Quick Tip: Using liquid or soap (Snoop®), to find leaks within the
instrument often causes serious contamination problems.
Systematic Checking
It is best to begin systematic checking of all frequently accessed
components if the leak detector is not successul in identify the issue. Any
recent configuration changes or maintenance
performed should be scrutinized first. The GC inlet (refer to the picture below) is a
possible source of leaks due to periodic maintenance and
re-assembly. Ensure that septa are changed as per the
manufacturer's guidelines and that o-rings are not
overused. Inlet liners must be free of cracks or chipped edges
and properly suited to the instrument and application. Confirm
that the nuts securing the top and bottom of the inlet are
correctly secured.
Quick Tip: Do not over-tighten GC inlet nuts. A standard
guideline for tightening is finger tight plus and additional quarter turn.
GC Oven
Continued instrument use can lead to loosening of the
fittings within the GC oven due to expansion and contraction
during oven cycles. These can include column connections
to the inlet and detector, guard columns or retention gaps,
and Y-connectors. The nut securing the ECD detector stem
is susceptible to loosening as well (refer to the picture below).
Each of these connections
should be inspected for obvious issues, disconnected, and
reassembled. It is important to always replace ferrules
during this process, and discard the used ferrule.
Re-assembly
Remember to recheck each component and fitting during
re-assembly using the leak detector. If baseline issues
persist after following this method, contact the instrument
support personnel. The issue may be related to a
malfunctioning controller or a leak within the internal
plumbing of the GC instrument.
Jamie Hoban may be reached by email at j.hoban@apgqa.com or by phone at 800.272.4442.
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