Falling down baseline problem !!!!

[ChromaMaster]

recently i used a mobile phase (70% KH2PO4 Buffer + 30% Acetonitrile , pH 6.5 ) and Column C18(4.6*250),5micron , UV detection @ 210nm . the system was keeping fall in baseline and took 4 hours to get stable baseline , what is the problem ???????????

[danko]

The column has been polluted. Or brand new.
Maybe you should've pumped/mixed some more ACN in the flow (60 - 70 %) for a while.
Beware of high phosphate concentration (> 0.05 M) if you follow my advice.
Best Regards

[ChromaMaster]

The column has been polluted. Or brand new.
Maybe you should've pumped/mixed some more ACN in the flow (60 - 70 %) for a while.
Beware of high phosphate concentration (> 0.05 M) if you follow my advice.
Best Regards

i used a new Waters symmetry column, should i do any treatment with a new column before use ???

[danko]

My suggestion is:

1. pump some 20 - 30 mL 20/80% water/ACN
2. pump some 20 - 30 mL of you mobile phase
3. Again 20 - 30 mL 20/80% water/ACN
4. Equilibrate with your mobile phase (20 - 30 mL)
5. Ready to run your samples.

Best Regards