UV baseline movement.

<p>Hi,</p><p></p><p>We have been observing some erratic baseline, I have attached some plots of the TUV and pump.</p><p></p><p>The method we are using is being used on several UPLC, but for this one, baseline moves about resulting in failed %RSD of LOQ. It could be pump related, but we have carried out all the leak/pressure tests and which have passed and the valves/pistons and seals have been replaced.</p><p>Despite the leak test/pressure tests the plots still show what I believe to be cyclic noise. This is the case if run on line A or B.</p><p></p><p>Is there anything else I have missed that could link the two issues. If there is a issue with one of the pumps could it affect the other pump?</p><p></p><p>Thanks</p><p></p><p>Darren</p>


  • Hi Darren - at first glance, it looks like your Primary A pump is the culprit. In your photo of the plot where you're pumping 100%A, the pressure trace drops for your A Accumulator every time your Primary A spikes. This usually indicates a problem with Primary A. But, since you've replaced your seals and pistons already and still see it, your check valve could be an issue. Running the static decay test once more couldn't hurt. It's also possible that the new seals haven't yet fully seated themselves. Pump at a higher pressure through an old column for 15-30 minutes, and try again.

    Also, try running from A2 instead of A1. If your baseline improves, that would indicate your solvent select valve is bad. You could then run out of A2 until it is repaired.

  • Thanks, I''ll give this a try and re post either way.

  • Hi,

    I have tried these out an there is still some signs of an issue. I may change the seals again, just in case. A is worse than B.

    One thing I did not is the large drop in pressure when switching from 100% A to 100%B and back the other way. I have not seen this before. Could this indicate an issue with the vent valve? It would certainly explain the issues we have been seeing. How would we go about checking if the vent valve is OK, other than swapping with another that is thought to be working? How often should we be changing it as part of the PM? As it is ~2K need to make sure it is not working.



  • Hi Darren

    To me it looks as if you pumps are running perfectly OK. The pressure riple is below 1 bar = 14 psi indicating no problem with you pumps at all. The noise on the basline (although in my opinion very small), could be caused by a dirty flowcell, or maybe and old lamp ? I would try and clean the flow cell with a 1% formic acid solution.

    best regards


  • Hello,

    Rune has a good point - the delta is pretty good for you. I generally don't get suspicious until the delta won't drop below 30 psi (~2 bar).

    That being said, you still are obviously not happy with your results...
    I've tested the vent valve before by using a pin plug (see photo).


    Remove the fitting that is in the bottom of your A Accumulator pressure transducer (on the tubing that runs to your vent valve). Install the pin plug into the bottom of your A Accumulator, but don't fully tighten it down. You want your pump to be able to release some pressure at first. Start your static decay test - tighten the pin plug down after the cylinders empty and immediately before the pump starts to compress. If your static decay test passes now, but has failed when connected to your vent valve, that indicates the vent valve is faulty. This technique was given to me by my local Waters engineer, and it's served me well multiple times.

    Of course, all of this is moot if your static decay test is passing already with the vent valve connected. I'm exclusively using mass spec as my detector, so I unfortunately don't have much advice for your UV detector.

    Good luck!

  • Ok thanks,makes sense. I'll give this a go.