Chromatography Forum

Hey,

I have problems with my UV detector giving some kind of cyclic UV noise. My first thought was that something must be wrong with the pumps, but the cyclic pattern continues if I stop the flow so it must be the uv detector. I've tried to plug the system into a computer UPS to "clean" the electricity.

It is a waters 406 detector and I am monitoring 232nm. The fluctuation is quiet small, but I often use small quantities, ie~5 mAU peaks, so I need to sort the problem.

Here is a sample chromatogram:

For how long did you see this pattern after shutting off the pump?
Do you have the same inconsistency of pattern as when the pump is running? Maybe this is a nother case of an interference from a machine that runs intermittandly?

You have air bubble in your detector cell. Flash detector with 100% ACN for 2-5 minutes and it will go away.

Several minutes...

Same noise with the pump running and when it is not running.

I have thought about interference, we have another UV detector so I think I will hook that one up to see if it is the detector or interference...

Dodgy, isn't it?

try adding a small amount of backpressure on the flow cell (like 200 psi or so.)

Conny, maybe you didn’t actually stop the flow?
Because, as Vlad pointed out, it’s a clear picture of an air bubble in the flow-cell.
Also, it’s a good idea to apply a moderate backpressure (f. ex. 200 psi) on the flow-cell, in order to avoid that kind of problems in the future.
Best Regards

I appreciate your suggestions and will try it, but I don't think it can be an air bubble or something similar as it cycles without any flow...

Would you please show data generated without any flow? The data you’ve shown is with flow on.
Best Regards

Conny, if I understand your replies correctly then the periodicity stops a few minutes after you stop the flow. It is not extremely likely that a detector fault would do this, it would more likely continue indefinatly. Your chromatograms also didn´t show the undulation continuously. Either your flow did not stop immediatly after shutting off the pump (so the air bubble act continued), or you have an intermittant interference. Can´t think of anything else presently. I agree that it would be helpful/interesting to see how the baseline looks for some time after shutting off the pump (and please check whether the flow stops when you shut off the pump).

Right, you need to do similar several minutes after flow has stopped. And magnification can make any tiny bumps look larger, your scale is quite sensitive.

To paraphrase Yogi Berra "from a distance even the elephant's skin looks smooth".

Check your UV lamp. I've noticed similar problems when my diode array's lamp is starting to go out.

bisnettrj2, did your lamp do the "similar thing" intermittendly also?

Yes, the problem was intermittent, and was solved by replacing the lamp. I'm guessing that when the lamp is beginning to fail, the voltage is unstable, and you begin to see intermittent oscillations in the baseline. Just a theory, though. If you have a spare lamp around, replace it and see if the problem goes away. It's an easy fix, in any case, if it is the lamp causing the problem.

I've also seen detector fluctuations with zero flow that were intermittent, replacing the lamp fixed it. But more common to be pump issue, but this can happen.

A worn out lamp’s oscillations are much more regular. So, in this particular case the data strongly indicate an air bubble in the flow cell.

Best Regards