Thank you very much for your answer.
I need an expert like you to be present in my lab all the time
I had service and system validation 4 weeks ago (authorized person did it) and everything was OK. I'll check once more everything maybe I'll find something. I always filter all samples and standards prior to analysis. I already increase flow of the mobile phase from 1 ml/min to 1,2 ml/min and decrease retention time from 11,5 minutes to 9,4 minutes. What do you recommend, how much I can increase flow ?
According to your opinion, do you think that I should wash the column with methanol instead of acetonitrile ? Is it possible that washing with acetonitrile cause changes in retention time ?
Thanks once more.