never get a stable baseline

[jcg200]

I ran the system with the degaser OFF. Please see links for the baselines with and w/o flow.

https://docs.google.com/file/d/0Bw8z-jp ... sp=sharing
https://docs.google.com/file/d/0Bw8z-jp ... sp=sharing

The 20-min cycles remain after I turned the degaser off. But the cycles disappear when the flow is zero. Could it be the pump, the check valve that causes the cyclic noise?

[tom jupille]

You're making progress! Next question, are you pre-mixing your mobile phase or mixing on-line? If you're mixing on-line, it could be a proportioning problem. To diagnose, try premixing your mobile phase and put all inlet lines in the same reservoir.

[aceto_81]

Do you also record pressure?
Maybe there is a relationship between pressure and noise?

Ace

[jcg200]

There is no solvent mixing issue. The mobile phases used in the last two chromatograms are water.

Could it be a faulty pump? How to record pressure in a Waters 2695 system?

[aceto_81]

In your instrument method.
The tab 2690/5 > Channel > enable channel.

Ace

[Peter Apps]

How are you controlling your column temperature ? The 25 C that you give is very close to default room temperature, and this can make it difficult for a column oven not to overshoot its set point. What happens if you increase column temperature to say 30C. If you do not have a column oven you may be seeing the effects of fluctuating room temperature (although I would be surprised if they were so regular.

Peter

[remesquaddie]

When troubleshooting a system, do remove the column, that makes it easier to find out what hardware if any is not functioning properly.
With column OFF, place instaed a long piece of very narrrow ID tubing to create some back pressure. Then run your baseline. If you do NOT get a good baseline, then your problem probably is , A: an inlet check valve failing, B: solvents need degassing, or [ and I have seen this], a broken plunger.
THIS is 905 of most failing baseline stability ssiues.

Dear Forum:

Please help me with the baseline problem. I set up Waters 2695 Alliance with a 2998 PDA detector using the following parameter:

Column: Whatman Partsil 10-SCX
Column T: 25 celcius degree
Wavelength: 210 nm
Mobile phase: 25% 25 mM KH2PO4 adjusted to ph=4.5; 75% acentonitrile

After I flushed the system (with column and detctor flow cell) with water and 100% methanol, I still couldn't get a stable baseline.
Here is what the baseline looks like:
https://docs.google.com/file/d/0Bw8z-jp ... sp=sharing

Please help trouble shooting the baseline.

Thanks so much!

[ChromatChic]

What was the resolution to this issue? I seem to be having an identical situation. Wavy baseline (~17minute cycles) even after removing column, flushing system, no flow or high flow etc. I was also performing ion chromatography when it started. After backflushing the flow cell, I had a steady baseline for four injections and now I cannot get it back. Pressure is stable. Could be the back flushing happened to rinse different part or the flow cell is now too contaminated? I've run this assay many times before without issue. Please let me know if you found a cause.
Thanks!