base line problems

[mahjoob]

I have problem with my HPLC, I have a ghost peak when the the percentage of acetonitrile increased from 6% TO 40 % the peak eluted about 2.5 minutes if I made sudden change from 6% to 40% ACN, and base line did not come to normal level after this peak. if i made the increment of ACN linear there will be no be peak but the base line become higher than before and irregular.
the gradient is solution B 6%, solution A 94% from start to minute 5.5 then A 60% and B 40 from minutes 5.55 to minute 12, the A 94% B 6% from minute 12 to minute 20
solution B is ACN HPLC grade solution A is either water or 10 mM potassium dihydrogen phosphate with PH of 4.5, no difference in baseline between using water or phosphate buffer
flow rate 1ml/min wave length 261nm

any one could help please

[tom jupille]

One possibility is that you are seeing a refractive index shift with composition (yes, UV detectors are somewhat sensitive to RI). Try running the separation on another instrument (preferably of the same model). If they both look the same, then it is a question of detector optical design, and your only real option is to live with it. If one looks significantly worse than the other, then the flow cell may be out of alignment.