Constant pressure option on Agilent 6890 GC w. Empower

Hi We are having an issue with the constant presure option on an Agilent 6890 GC controlled by Empower2.The issue is that we run a method GC44 with the following column: DB-WAX 60 m* 0.25 mm (film 0.25 um)
The method is run at constant pressure 43 psi. Carrier gas: He.
It’s a gradient method starting at 70 degrees up to 240 degrees after 63.5 min.
·         But by a mistake the column information in the method was not set correct (in instrument method 25 m* 0.32)  (Chromatogram 1)
·         Later the mistake was found and info was changed to correct 60 m* 0.25 mm (Chromatogram 2)I’m presume that the incorrect information in instrument method (at constant pressure 43 psi) results in calculation of flow from wrong info in instrument method, giving that the flow is greater – because the instrument is trying to obtain 43 psi in a shorter and wider column. Or maybe I’m wrong?·         But why does the retention times not change?
·         And why does the areas of the peaks increase around 8 times ?I need an understanding of this issue to decide what to do with this deviation from the method. I hope soneone can give an explanation.Chromatograms are attached.Best regards


  • Hi

    It is a few years back that I did work the last time with a GC. However looking at the chromatogramm my first idea is:

    Whatever you changed has not impacted the flow on the column but it does affect heavily the split ratio.

    Looks like you did simply decrease the split ratio by your change but you still having the same flow on the column what I would expect.

    What means that you bring 8 times more substance onto the column.

    Regards  Reinhard

  • Hi Reinhard

    This sounds like a plausibly explanation, thanks for your help.

    best regards


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