Compounds disappear in a certain time window

Hello, during our multi residue pesticides analysis of our usual QC, we started to notice that certain compounds started disappearing. The runs are around 17 min, and the missing compounds are all in the time frame 10-12 min. This happening is inconsistent, as it could be in the morning run they would be there, and later in the day reappear, or they would disappear for several days, see weeks. Are there any suggestions why would this inconsistency be happening, and only in a small time window? We're using an LC-MS system 1290 infinity II LC system +G6495C triple quad.
We've already checked both the sprayer and the transfer capillary, and even tried replacing them, but the problem is still there.

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  • Hey, thanks for your reply, so to answer to you questions:
    - The current column was installed about 5 months ago, the main reason it was replaced at that time was due to the missing compounds.However the problem persisted even after the replacement of both the column and the guard column.
    - The QC we are using are usually extracted blanks, spiked with the pesicide mix, for our routine analysis we are using a tomato blank, with same extraction as the samples we're analyzing
    - Most compounds we have in our method have an aromatic ring, including the ones in the above mentioned time window, are either aromatics, or aromatic ether, or an amino group
    - We still see them in Qual, as the peak is still there, but the response of the those compounds would be 10-1000x or even lower than it's supposed to be (probably disappearing compounds would not be right term then, rather decreasing sensitivity of certain compounds?)
    - On another note, we have 3 compounds which are the most usual and common we see disappearing compared to others. If some other compound is gone, we can only notice it during our daily control cards.


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