Are your ghost peaks coming from the GC column, or something else?

 

This is a common GC troubleshooting question.  Since determining the source of the ghost (contamination) peaks can be difficult, I decided to write this post to (hopefully) help our customers.

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GC column carryover

When I operated an instrument on a daily basis, it was only a matter of time before something caused ghost peaks in my chromatogram.  So how did I narrow down the source of the ghost peaks?  First, I would cool the injection port.  Then, I would program the GC so that I could simply press the Start Button to acquire data for an analytical analysis without injecting any sample, standard, solvent blank, etc.  In other words, with a cold injection port, and without injecting anything, I would acquire some data while heating the GC oven.  In most cases, there were no ghost peaks in my chromatogram, or if there was, they were much, much smaller.  This told me that the ghost peaks were not a result of my GC column.  However, if the ghost peaks remained, I would trim 1-loop (approximately 0.5-meters) from the inlet side of my capillary column and perform a 10 minute bake-out by setting the GC oven to 20°C less than the column’s maximum isothermal temperature.

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Injection port septa

If the GC column is not causing ghost peaks, where should you look next?  One of the most common sources is injection port septa.  I wrote the following post to help identify and fix this issue.

Troubleshooting Injection Port Septa

To test, replace the injection port liner after thoroughly cleaning the injection port.  Then replace your injection port septa and make sure your septum purge is working properly (by measuring the septum purge flow with a flow meter).  Keep the injection port cool and without injecting anything, obtain some data like described in the GC column carryover paragraph above.  This same test can also identify potential off-gassing contamination from the injection port O-ring (if applicable to your instrument).

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Syringe

If the baseline does not contain ghost peaks, heat the injection port and acquire an analytical analysis without injecting anything.  If the baseline still does not contain ghost peaks, inject a solvent blank into a heated injection port.  If you observe peaks, your syringe, blank solvent, and/or rinse vial solvent may be the problem, and you should review the posts below.  Or, your injection port may not be as clean as you think it is.

Troubleshooting GC Syringe Issues

Troubleshooting GC Syringe Issues – Part 2

syringe 2

 

Vial Septa

Another potential source of ghost peaks is from your sample/standard vial septa.  So how do you test for this?  I used to remove the septa from the vial cap and make an injection (manually or using an autosampler).  Do you still see peaks?  If not, these septa are likely the cause.  If you do, then switch to a septa made from different material and repeat the experiment.  Remember to only inject once from a vial, as piercing the septa multiple times may lead to ghost peaks.

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Carrier Gas

If nothing listed above helps you determine the source of the ghost peaks, take a closer look at your instrument and/or carrier gas.  Testing carrier gas is relatively easy; cool your injection port and GC oven temperature to 35°C while letting the carrier gas flow for several hours (do not cool the detector).  With the GC oven and injection port still at 35°C, start acquiring data with the instrument.  Then, increase the injection port and oven temperatures.  If you notice ghost peaks, then suspect the carrier gas.  (Example of a carrier gas filter trap shown below.)

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Split Vent line and trap

If you normally operate the GC in splitless mode, you need to check one more thing.  The ghost peaks may actually be coming from the split vent line and/or split vent trap.  Remember that the split vent line is seldom heated, so material (compounds) introduced into it may be condensing out, and vapors may be working their way back into the injection port.  To test, set the injection mode to split (2:1 split should be fine).  Inject a few solvent blanks.  Without letting the instrument switch to splitless mode, acquire data for a sample, standard, or solvent blank.  If the ghost peaks disappear while in the split mode, then the split vent line and/or trap are likely the problem, and both should be replaced.  (Example of a split vent trap shown below.)

split vent 2

I hope something here has helped you identify and fix the source of your ghost peaks.  For additional information on troubleshooting ghost peaks, I suggest you read Jaap de Zeeuw’s blog post Poster on sources for “Ghost-Peaks” in Gas Chromatography which contains links to five articles he has written about ghost peaks.

Thank you.

2 Responses to “Are your ghost peaks coming from the GC column, or something else?”

  1. Dear Alan –

    Nice piece! Always challenging troubleshooting ghost peaks… I am wondering why the septas mounted on S/SL injection ports always are heated? An injection port design where heat transfer from the injection port to the septa seat in top is eliminated, would cause less bleeding from the septa?

    With kind regards
    Lars Kurstein, Copenhagen

  2. Alan Sensue says:

    Hi Lars:

    As always, thank you for reading my post.

    I think your question would be better suited for an instrument manufacturer, but I look at it this way, the injection port is heated, and some of this heat travels to the septa, rather than the septa being directly heated. In one way, this indirect heating actually has a benefit; a warmer septa is more pliable (softer) than a cold septa, which is why septum used for cold-on-column injection ports are typically predrilled. If they were solid, it would be much more difficult to push a fine syringe needle through them, which could lead to septum coring and bent needles. In addition, this is also why we do not recommend our red BTO septa for lower injection port temperatures; they are very difficult to pierce and are easily cored.

    I do agree that cold septa bleed (off-gas) much less than hot septa, but if the septum purge is working properly, this should not be an issue.

    I hope this helps answer your question.

    Alan

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