I just got a very wired problem when I prep my QC samples. The stock solution was in DMSO and the sample matrix is a buffer pH=7.4, and I made the QC samples from the stock solution in DMSO.
When I did some pre-validate work, everything was fine except for the QCL (1uM) has 80% recovery, and the QCH had ~93% recovery which was acceptable. However, when I started my validation day1, all the QCs were low. The QCH samples were even 40% lower.
The next day I made another QCH and analyzed it again, it was within +-10%. And I wanted to doublecheck that so I made another QCH, it was 30% low... it was like something is happening between the buffer and the compound. Sometimes it works sometimes not. Has anyone of you met this before? I really help and I've been stuck on this problem for more than a week.

And when I did the investigation, there's no stability issue and I assume there's no solubility problem in DMSO (Every time I dilute the QC stock with pure diluent (50%ACN in water) the concentration came back just fine. )

Thank you very much!