Chromatography Forum

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.

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

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.