Noise and inconsistent ISTD injections

Hello

Our lab has a method where we us methanol as the solvent and have begun to have issues with random injections having a lot of noise or unknown peaks show up on our back column and sometimes in our front column. For our front column, we have been having an issue of our ISTD peak having varying area counts on random injections. For example, we could have 3 injections in a row with an area count of 350 for our ISTD (which is normal) but it will then suddenly jump to 800-1000 area count for the next injection. We will see this happen throughout a run and it only affects the ISTD. All other compounds do not see the same jump or change.  I have include a coupe of images for the back and front from 2021.

When we preformed our validation in 2016 and we did not experience any of these issues (see below image - Front Column 2016, but now these issues are happening more often for us and making it difficult to obtain good runs. There were no changes to the GC during this time and all of the parameters have been consistent, so we are having a hard time trying to pinpoint the issue. Any assistance will be great.  Also we run another method on this GC and do not experience any of these issues.

Thank you

Tony

  • Just trying to see if anyone has an idea of why we are having this issue.

  • Again I am following up to see if there are any ideas why we are having this issue.  Is there another forum or place to ask this question?

  • Hi

    Better to call your local agilent support to see onsite...

  • Hi chemlab1989,

    not an answer, but a request for more information. You mentioned a front and a back column.

    • How is your system configured?
    • Which phases are the two columns?
    • What type of injector do you use? What are its working parameters?
    • What's the injection volume? Injection solvent - sample vs. ISTD?
    • How do you add the internal standard?
    • Anything additional

    As the peak shapes seem ok and other peaks are not impacted, it might be something with either the (addition of the) internal standard or something in the injection system.

    Regards,

    Norbert

  • Hi Dr. Know

    Here is some of the information you have requested.  Please let me know if you need any further info.

    • How is your system configured?   FID detector using helium as carrier gas, nitrogen as makeup gas.
    • Which phases are the two columns? Polyethylene glycol (PEG) stationary phase for both
    • What type of injector do you use? What are its working parameters? Heated Split/Splitless Injector.  Temperature is set to 210C and split ration 1:1
    • What's the injection volume? Injection solvent - sample vs. ISTD? 1uL injection volume.  Injection solvent is methanol.  The ISTD is already contained within the extraction solution.
    • How do you add the internal standard?  It is added to our calibration standards during their creation.
    • Anything additional - as stated above we use this GC instrument to run another method that uses water as the extraction solution and we do not experience this issue.
  • Hi again,

    210 °C is a little low for 1 µL methanol at 1:1 split ratio (would be worse for water though). I would increase the injector temp to 250 °C.

    • Are both columns in the same injector and/or detector? If yes, do they have different dimensions? Which?
    • What liner do you use?
    • Do you only have the ISTD in the calibration solutions?
    • How is it mixed with the sample?

    The strange thing here is you see a bigger peak area after 2 injections. Is it staying constant after injection #3? Then your problem would be the first 2 injections with the low area count. 

    In which region do you reside? Could you talk to one of our local application engineers directly?

    Regards,

    Norbert

  • Hi Dr. Know

    Please see the responses:

    • Are both columns in the same injector and/or detector?   Yes
    • If yes, do they have different dimensions? no same dimensions Which?  30m x 0.53mm x 1um
    • What liner do you use?  Double gooseneck
    • Do you only have the ISTD in the calibration solutions?  No it is in the sample extract also
    • How is it mixed with the sample?  We create an extraction solution with the ISTD and use that

    The first 2 chromatogram examples I provided were different injections, 1 from the Front and 1 from the Back.  The 3rd example is from our validation which did not experience this issue.  The random peaks and ISTD fluctuation happens randomly and is not consistent.  That is why I am having a hard determining  what the issue is.  I am in Illinois, so yes I could look into talking with an engineer.

    Thank you
    Tony

Was this helpful?