Can anyone please help me??
I've just come back off holiday and a gradient method that I ran before Christmas showed a flat baseline on the data system(Unipoint - for Gilson LC).
The gradient goes fron 5%ACN to 100% in 7 mins and you can now see the gradient in the baseline.
The solvents are the smae quality and made up exactly the same way and the lamp is brand new.
Can anyone suggest anything to help me??

I guess the gradient baseline is caused by refractive index effect due to the change of viscosity of the mobile phase blend. This can happen, if either the cell window is dirty or, if the detector outlet capillary is clogged.
Try to clean the cell window and to replace the capillary.

Good Luck!

Before trying to clean your flowcell, I'd try switching to solvents and water from another source. The possibility of contamination exists and the fix is too simple to ignore. If the baseline slopes upward from left to right, it's possibly a contaminant in the organic component. If it slopes downward from left to right, the culprit is probably the aqueous/buffer component.
There is a possibility that microbes have built up in your system, including the column. Try taking the column out of the system and running the gradient. If the sloping baseline disappears, it's from material in your column. Also, examine the mobile phase lines for any turbidity that indicates microbial growth
If you are running blank baseline, it's possible that you are looking at such a high sensitivity at an insignificant change in RI looks huge. Make sure that it is significant by injecting a test sample at the concentration at which you usually operate.

Thank-you very much for your suggestions.
I have carried out both sets of suggestions. Unfortunately now the baseline is cycling.
I have changed pump seals and inlet/outlet filters on the pump head as well as changing the column frequently.
Is there any other suggestions??
Am I overlooking something obvious??

Here is something for you to check out. On the Waters web site (www.waters.com)in the service support area (select tech support then instrumentation)there is an interactive troubleshooting guide. I looked at baseling cycling and it listed several causes as well as corrective actions.

A.A.

Steph,


This phenomenon could be caused by many different issues:


First, it's possible to change the sensitivity and make the baseline too small to see. In UniPoint, add a Control Method line to set the detector sensivity. If you don't, it uses whatever was set on the front panel of the instrument. I think this is what "jclark" was suggesting in his last paragraph, above.


Second, an incorrect wavelength would also produce the same symptom. One wavelength might be sensitive to the gradient, and another might not. Again, add a step in your control method to force the wavelength. Otherwise, it just uses the default off of the front of the detector.


Third, it's possible the pumps aren't pumping. It seems you've suspected this, since you already replaced the pump seals and valves. To verify that you're getting the correct flow, from the UniPoint operations window go to "Manual">"Mobile phase" and test each pump independently. Using a volumetric cylinder, measure the flow rate coming out of the detector outlet.


Fourth, there might be nothing wrong! I've made up mobile phases that are degassed to different degrees, or have reagents (such as triethylamine) that are different ages. Yes, the gradient baseline changes, but the chromatography itself is largely unaffected. Are you experiencing poor resolution?


Fifth, there might be something we don't know about in your system. Do you use a flow splitter? Are there any switching valves in front of the detector?


If all else fails, please call Gilson Customer Service at 800-445-7661.