Troubleshooting a GC chromatograph can be made easier, if one learns to recognize the symptoms produced by an instrument malfunction, column and detector problems, leaks. Many symptoms appear as unusual peaks as shown below:
Symptom: No peaks (Fig. 1)
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| Fig. 1: No peaks appear after the injection of the sample |
Possible Cause:
Main power off, fuse burned out
Action: Plug in the instrument,
replace fuse
Possible Cause:
Detector off
Action: Turn detector on and adjust
its sensitivity
Possible Cause: No
carrier gas flow
Action: Check the carrier gas lines
and correct the problem i.e. replace empty gas cylinders, possible obstructed
or broken gas lines
Possible Cause:
Injector temperature too low. The sample is not vaporized
Action: Increase injector
temperature.
Possible Cause:
Injector septum is leaking
Action: Replace the septum.
Possible Cause:
The syringe used for the injection of sample is plugged up.
Action: Clean the syringe or replace
it if it is damaged.
Possible Cause:
The FID flame is out.
Action: Check the FID flame. Check
if water vapor condenses on mirror. Light up the flame if needed.
Possible Cause:
Column connections are loose.
Action: Check for leaks. Use a leak
detector. If needed tighten column connections
Possible Cause:
Oven temperature too cold. Possibly, the sample condenses in the column
Action: Increase oven temperature
Symptom: Retention
times or areas are not reproducible (Fig. 2)
Possible Cause:
Septum is leaking
Action: Replace the septum if it is
damaged. If there is a premature septum failure (less than 200 injections)
check also if:
The syringe needle is not straight
The syringe is not installed
correctly
The septum retainer nut is too tight
Possible Cause: Syringe
is dirty or damaged
Action: Replace the syringe if it is
damaged. Clean the syringe with an appropriate solvent if it is dirty.
Possible Cause: Sample
is not stable under the conditions of the analysis
Action: Check the sample stabiblity.
Some samples change with heat or U.V. light. In case the sample is
photosensitive use amber sample vials.
Possible Cause: Sample
volume is too low or too high
Action: Check the sample vials. If
the sample vials are not filled correctly, evaporation or contamination may
affect the analysis. The sample level should be approximately half the volume
of the vial.
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Fig. 2: Retention times or areas are not reproducible
|
Symptom: Poor
sensitivity with normal retention time
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Fig. 3: Poor sensitivity with normal retention times
|
Possible Cause: Insufficient
sample size
Action: Increase sample size. Check
syringe needle for plugging.
Possible Cause: Poor
sample injection technique
Action: Check if the proper
injection technique is used.
Possible Cause: High
attenuation
Action: Reduce attenuation.
Possible Cause: FID
response low
Action: Optimize the flow rates of H2
and air. Use N2 for make-up gas
Possible Cause: Thermal
conductivity response low
Action: Use higher filament current.
Possible Cause: Syringe
or septum leaking when injecting
Action: Replace syringe and/or
septum
Symptom: Poor
sensitivity with increased retention time and broadening of the peak
Possible Cause: Carrier
gas flow rate too low.
Action: Increase carrier gas flow.
Possible Cause: Septum
is leaking.
Action: Replace septum.
Symptom: Irregular baseline
drift (isothermal)
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Fig.4: Irregular baseline drift when operating isothermally
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Possible Cause: The
instrument location is not according to the manufacturer’s specifications .
Action: Instrument should not be
placed directly under heater or air conditioner or any other place where it is
subject to excessive drafts and ambient temperature changes.
Possible Cause: Column
packing bleeding.
Action: Let column to stabilize as
described by the manufacturer.
Possible Cause: Carrier
gas leaking.
Action: Locate leaks and correct.
Possible Cause: Carrier
gas regulators do not work properly.
Action: Check carrier gas regulators
and flow controllers.
Possible Cause: Poor
air or H2 regulation (FID detectors)
Action: Check regulators and flow
controllers.
Possible Cause: Detector
is contaminated
Action: Clean detector according to
manufacturer’s specifications. Raise temperature and bake out detector
overnight.
Possible Cause: Detector
filaments are defective (TCD detector only)
Action: Change the filaments.
Symptom: Contamination
or ghost peaks
Possible Cause: Vial
cap septum is dissolving in solvent
Action: Check the vial septum. If it
is not resistant enough to the solvent you are using try a more resistant type.
Possible Cause: Injection
port septum is giving off volatiles
Action: Make several blank runs with
a small piece of aluminum foil backing the inlet septum. If the contamination
peaks disappear, they were probably due to the septum. Use another type of
septum that is stable to the required injector temperatures.
Possible Cause: Column
is contaminated
Action: Examine the first 10 cm of
the capillary column for contamination holding a light behind it. If possible
cut the contaminated part of the column. Replace or clean the inlet liner.
Possible Cause: The
sample is not stable
Action: Store the sample in a
protected environment and use amber sample vials.
Symptom: Sinusoidal baseline
drift
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Fig.5: Sinusoidal baseline
drift
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Possible Cause: Oven
temperature defective
Action: Replace the oven temperature
sensing probe .
Possible Cause: Oven
temperature control was set to a low value
Set the oven temperature control to
a higher value.
Possible Cause: Carrier
gas flow regulator defective
Replace the carrier gas flow
regulator. Set to a higher pressure in order to stabilize the gas flow.
Possible Cause: Carrier
gas cylinder pressure too low to allow regulator to work properly
Replace the carrier gas cylinders.
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