Heavy PAH standard stability

[Peppels]

Good evening,
After some attemps, I've finaly achieved some good calibrations. The issue I'm now trying to solve is related to the last compounds of my PAH Standard which usually get underestimated after just a few days.
I'm referring to the family of the four Dibenzo(a,l)pyrene, Dibenzo(a,h)pyrene, Dibenzo(a,e)pyrene and Dibenzo(a,i)pyrene.
Once I've calibrated the instrument, the calibration works perfectly up to ten days, but the last four compounds, usually degrade from 25ppb to 13/14ppb right after few days, although the verification standard is freshly prepared.
Has anyone noticed something similar?
Thank you.

[MichaelVW]

Does the issue persist after you clip column and put in new injection port consumables?

Do the peak shapes and/or raw abundances change at all?

[Peppels]

Does the issue persist after you clip column and put in new injection port consumables?

Do the peak shapes and/or raw abundances change at all?

Yeah, same situation. Also after replacing consumables and cutting the column, this four compounds are characterized by a short time calibration stability. Looks like they are lost from the standard as they were lighter compounds, or like the instrument loses sensitivity related to the dibenzo- compounds.