Negative Base line in PDA

[rc_12321]

Hi all , a vey good evening

We are facing a strange problem with PDA Water's alliance system .
We developed a method on Zorbax-C18 column with following parameters.

M.P-A-93:7:0.1 (Water:ACN:TFA)
M.P-A-13:87:0.1 (Water:ACN:TFA)
flow-0.4 mL/min Wavelength-265 nm Column Temp-25 C

Gradient(Time/%B)-- 0/10, 5/10, 20/50, 45/85, 50/90, 60/90, 62/10,70/10

We found a negative drift at exactly rt of active.This is found in PDA systems ,not in DAD systems.We r digging our heads to find out the cause for this.Please help us.

[unmgvar]

can you show a picture?

[Michal.Dousa]

The above described problem is know phenomenon in gradient elution (especially using trifluoracetic acid in mobile phase and low wavelength at UV detection). In my experience, you have to lump it .

[tom jupille]

The above described problem is know phenomenon in gradient elution (especially using trifluoracetic acid in mobile phase and low wavelength at UV detection).

Garbage peaks and drift at low wavelength *are* common problems with TFA, but in this case the problem is a negative dip, and 265 nm is not "low wavelength".

Per unmgvar's suggestion, seeing a chromatogram would help (instructions are posted here: viewtopic.php?t=2617