Noise and inconsistent ISTD injections


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


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