Hello GC forum colleagues,
I have a phenomenon that has perplexed me and I am asking for advice.
I analyse 3 x capsaicinoid compounds and 2x tear gas compounds for their presence or absence as lachrymatory agents in personal defense spray formulations.
A five component standard compound mix in methanol is run in duplicate after initial system solvent blanks and final solvent blanks.
The compounds are: capsaicin, dihydrocapsaicin, nonivamide, chloroacetophenone (CN) and o-chlorobenylidine malononitrile (CS).

The observation is: when run on a DB-1 250um, 1.0um x 30m column they all separate however, if the standard solution is run (duplicates everytime) at the beginning and end of the analytical run, following the solvent blanks, the detector (MSD) response for each of the 5 compounds present in the standard solution increases steadily to peak at approx 30% over 2 x 2 duplicates separated by unknowns and blanks. Importantly, there is no carryover of these compounds in in the 2 x methanol solvent blanks that always follow the STD solution injections.
If, I just inject the standard solution multiple times I see the same steady increase in response for each of the compounds.
It logically seems to be evaporation of the standard solution vial post-piercing but using lvis and individual vials for each injection doesn't change the effect.
The MS is not being autotuned between samples or anything and an autosampler is being used for 2uL injections in split mode. I dont understand why it is always increasing and not ever decreasing.
Any advice strongly encouraged.