Greetings.

We have a HPLC Agilent 1100 with pressure problems. Initially, the pressure was unstable. But we managed to stabilize it by washing the system overnight with acetonitrile. Afterwards, the generated pressure for my method is around 230 bar, which is ok for the column that I am using. We are using a gradient from 80 % H2O to 80 % acetonitrile for 30 min; returning to 80 % H2O at 40 min.

However, after I inject the sample (standard in acetonitrile) two things may happen: (i) the pressure drops with rising percentage of solvent up to 80% (which is normal) but stays at that those values (110 bar) after the system returns to 80 % H2O and stays that way even if I wash the column extensively. The peaks are what I expect but with much lower resolution, which may indicate that the mobile phase is not what it should be. Secondly, the other thing that may occur, is that all my peaks appear right after the the solvent peak, resulting in no column selectivity.

The system pressure without the column is what I would expect.

I really do not know what is happening.

Maybe air in system? Defective injector? The mixer is not working properly? The pump is defective? The filter are not ok? Some minor leak that I could not see? 

Please help.

Thank you so much for your time.

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