SEC Columns, RI noise

[Noser222]

Hi,
We are having a problem with noise in our RI detector, though it's not really baseline noise as much as it is a long term oscillation in the baseline. We have 3 SEC columns in series and run the flow at 0.5 mL/min with 0.1M ammonium acetate into a Wyatt DAWN EOS system and then into our Waters 2410 RI detector. How likely is this baseline oscillation a sign of the columns going bad? We have tried moving the columns onto different pumps and detectors and even put the columns in a heater to be sure that temperature wasn't the problem. Any ideas would be appreciated.
Thanks,
Brent

[Uwe Neue]

Please explain the long term of your baseline fluctuations (the time for a cylce, how regular ist etc.)! With an RI, I would bet on things like a column heater, a heater/vent blowing on the instrument in winter or an airconditioner in the summer and related things.

[Benjamin]

Dear Nosse2,

Most cases of RI drift or noise are always due to pump fluctuations, Mobile phase changes, and temperature fluctuations. In your case it may also be that your columns have been contaminated with some long retained compounds, and these are slowly bleeding into the RI detector.

Make sure your columns are clean, your mobile phase is perfectly mixed and stable, and your system has good temperature control.

Good Luck,

Benjamin

[HW Mueller]

The light scattering detector does not show an oscillation?

[Noser222]

The Light-Scattering detector has also shown a baseline fluctuation in a simliar pattern. The time period of the noise is about 10 min, and it seems to be a pretty regular pattern. We did put the columns in a heater set to 30 degrees C and are still seeing it.
I should also note that when we ran water as the mobile phase the long-term noise was still visible but less intense. When we returned to ammonium acetate it was present at the same level again. We had not experienced this problem until recently; ie. we have run ammonium acetate as the mobile phase for about a year and a half with no long term noise in the baseline of the RI or light-scattering.
So in summary, We have tried 3 different HPLC pumps and 2 different RI detectors and controlled the temperature of the columns.

[HW Mueller]

Did you measure flow fairly accuratly over several of your "periods"? Is it constant?