58 Agilent 7890 Series Troubleshooting
3 Chromatographic Symptoms
If noise appears suddenly on a previously clean baseline, do the
following:
• Consider recent changes made to the system.
• Bakeout the inlet. See the following:
• To Bakeout Contaminants from the Split/Splitless Inlet
• To Bakeout Contaminants from the MMI Inlet
• To Bakeout Contaminants from the Purged Packed Inlet
• To Bakeout Contaminants from the COC Inlet
• To Bakeout Contaminants from the PTV Inlet
• To Bakeout Contaminants from the VI Inlet
Bakeout can reduce septum bleed and other contaminants.
New septa may contribute noise from bleed of low molecular
weight material. If noise decreases when inlet temperature is
lowered, this is a likely cause. Use only high quality septa and
store them where they cannot become contaminated.
• Verify the purity of carrier and detector gases. If a tank was
replaced recently and the old one is still available and still
has some gas in it, try the older tank to see if noise
decreases.
If the new gas is so badly contaminated that it saturates
traps, changing to the old one may show little improvement
until the traps are replaced or regenerated. This problem is
most common with nitrogen carrier gas. Deal with a reliable
gas supplier.
• For the TCD, check for ambient air pressure fluctuations at
the GC. Air currents from a fan or air conditioner blowing
across the GC may interfere with gas exiting the detector.
This is a possible, though not very likely cause of noise since
detectors are well protected. Switching off the air current
source or shielding the detector exit identifies this problem.
Install the TCD outlet restrictor (G1532-60070).
• Loose connections in the detector or its signal path generate
noise.
• Verify proper reassembly after recent maintenance.
• Inspect the detector for contamination.
If noise increases gradually to an unacceptable level, check the
following possible causes:
• Bakeout the detector (FID, TCD, uECD).
• Inspect the detector for contamination. Replace parts as
needed. (See the 7890 Series Maintenance manual.)
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