Chromatography Forum

Hi !

It is my first post. Could you help me ?

I am running an isocratic method on my LC/UV that runs with a mobile phase containing water and acetonitrile. Ils is a quaternery pump with degasser, but I use only A 100% (I tried B 100% too).

Normally the baseline is either totally flat. Sometimes, there is a deviation like :
.........................___________
____________/

.... that I couldn’t explain. The pressure is constant. The LC/UV was not used for one year. I have changed the filters of the pump.

This is a problem for the repetability of the injections.

Have you got an idea ?

DCHIMIE

More details would help:

What type of UV detector (variable wavelength or photodiode array)?

What detection wavelength?

What sensitivity (range)?

Is the baseline ramping up and then stabilizing, or is it an abrupt increase?

Does the baseline come down? If so, it it gradual or does it mirror the increase?

Here are a few possibilities that are consistent with what you have described so far:
- you are seeing the effects of your degasser cycling on and off.
- your lamp output is fluctuating (more likely with a photodiode array detector)
- temperature fluctuations (e.g., lab air conditioner coming on/off)
- voltage fluctuations.

A flow fluctuation can cause the same kind of shift, but then you would expect to see a pressure change.

Disconnect the column and do the troubleshooting

First, I will degas the solvents, although there is a online degasser. Try sonicate the solvent for 15 minutes (for volatile solvents, use less minutes) under vacuum;

Second, let the UV detector stabilize for a period of time since the UVD had not been used for a long time, and watch the baseline.

Also note any changes of environment, temperature .........

Good luck

Hi !

Thank you for your remarks.

The UV detector is a variable wavelength at 254 nm. I inject caffeine at 0.25 g/L six times (method for the validation of UV injector).

The increase (or decrease) is abrupt. The baseline come down or up.

I have degased the solvant.

I will test what you suggest.