negative baseline

[Shiveirou Raomai]

I have been trying to run hplc-UV for quite a long time. but everytime baseline become negative.what could be the reason?can anyone please suggest some solutions?
wavelength 203nm,flow rate 1ml/min, solvent-water and acetonitrile at different ratio. initially there was no problem but few weeks back negative baseline appeared even while monitoring and running blank. I have changed the column twice but to no avail.

[bisnettrj2]

Post your analysis parameters, your instrument type, your UV detection wavelength, your mobile phase, flow rate, etc.

[Gerhard Kratz]

Especiallyat low wavelength you feel that your system is not in an equillibrium. Maybe it was not flushed well and you have still a solvent in the system what does not belong to your mobile phase, or you have a temperature problem, or your gradient pump system is not working propper. But without details it is impossible to give you better support.