Chromatography Forum

We have recently been seeing negative peaks and strange baseline activity on one of our HPLCs. The lamp is brand new, and these methods work fine on our other HPLCs. The HPLC is an Waters alliance 2695 with a Waters 996 PDA detector.




Any ideas to what could be going on?

Thanks,
Mark

If it's an electronic problem (lamp or diodes) it should still show up under "no flow" conditions, so the first thing I would do is to shut off the pump and monitor the baseline for a couple of hours.

If the baseline looks ok, go back and recall (or record) the pump pressure trace to see if the pressure drops when you see the baseline anomalies. If it does, you have a flow problem (sticking check valves or partial loss of prime).

If the pressure trace looks OK, watch the detector outet line during and just after one of the baseline anomalies (yes, it will be boring!) looking for airbubbles (which would suggest a degassing problem or a leak in the pump intake line(s)).

Thanks for the reply Tom. I did as you suggested and monitored baseline at 0 flow and still getting those peaks. So it sounds like it is an electronics problem. The below chromatogram is only showing a 10 minute window but it is dropping negative peaks seemingly at random. Even though the lamp is practically brand new I am going to try swapping lamps from another PDA to see if anything changes.

Did you find the problem for these negative peaks ? Did it help to change lamp ? Did you see the problem in every injections or just random once in a while ?

Hello,
Did you ever find what the problem was?
I have a similar problem with my Waters 996 PDA detector.
Thanks!

We have recently been seeing negative peaks and strange baseline activity on one of our HPLCs. The lamp is brand new, and these methods work fine on our other HPLCs. The HPLC is an Waters alliance 2695 with a Waters 996 PDA detector.




Any ideas to what could be going on?

Thanks,
Mark

while monitoring the baseline in empower, pull out the flow cell out of the light path, (2 phillip screws, perfectly safe to remove, pull the whole thing straight out towards you) observe what happens to the baseline. If the baseline goes perfectly flat for 5-10 min, you have a dirty flow cell. If it stays the same, you likely have a bad lamp (even though new) or its not inserted correctly in the detector.
If bad flow cell, waters sells a flow cell lens replacement kit, if you cant afford to replace the entire flow cell (~$1k)