TCD Signal Degradation

[BilNerd]

Hey Everybody,

I've recently inherited an old HP 5890 GC that was poorly serviced for awhile. In the course of getting it running again there have been a few problems.
First the column clogged up requiring replacement. After that was sorted we started to have problems with the TCD signal degrading over time. There haven't been any changes in settings during this period, but the signal intensities have been decreasing rapidly. Even trying to increase the amount of sample has not increased the peak sizes. I've spoken with tech support and other operators and no one has a firm idea of what might be wrong. Today the peaks have finally just disappeared. Any help on this is greatly appreciated.

[Bruce Hamilton]

You have to try and work through this systematically. You haven't descried the behaviour of the background signal, it could be increasing to smother the peaks, or it could be decreasing due to poisoning of the TCD. Go back and check signal at start of runs, and see if any trend is obvious.

Gases to the TCD and GC.. Check their quality, and ensure that you have good, fresh drying and purification traps on the gases. Ensure all gas flowa are as expected by measuring them at the detector outlet. Check that the detector temperature is correct ( electronic readouts can lie - just see if it's hot when set to 100C and hotter when at 150C) , and is suitable for the sample.

Take the column out ( capillary PLOT or packed ), and replace with an empty, cleaned short length of restrictive tubing. Adjust flow/pressure to typical. Inject an inert gas sample different to your carrier, eg Ar if you use N2 or He. Do you see an immediate large peak?

If so, replace the column, reset conditions, but don't connect to detector. Condition at maximum temperature for at least several hours preferably overnight, cool and connect to the detector. Set up instrument, inject a suitable standard mixture to test the column as per manufacturer's performance test. That becomes your baseline case.

If it's OK, the column is OK, if the peaks are absent, you have to look at another column. Once you know that your instrument, column, and TCD are OK, you can then look at your sample, does it have water or some other reactive impurity, such as sulphur compounds?.

If you didn't see an immediate large peak in the initial test, then your detector is poisoned or electronics dead, or your sample injection system is defective. the Agilent manuals are available online if you don't have them.

We really need alot more detail about your analysis, but one of the effective ways to kill a TCD is to allow it to stay hot with oxygen ( air ) present. Make sure your switching on/off protocols protect the filaments.

Please keep having fun,

Bruce Hamilton

[JI2002]

What's the reading of the TCD signal? If it's zero, the filament could be beoken.

[AICMM]

Bilnerd,

What carrier are you using? What kind of samples are you injecting? You could be poisoning your TCD.

Best regards

One other comment. If you do the troubleshooting that Bruce is suggesting, make sure your detector is off whenever you break the column seal since a big influx of air could oxidize your filament.

Best regards.