Chromatography Forum

Hello, I now have problem with my UHPL binary system. No flow in mobile phase B but can run isocratic on A fine. I am setting up a new method and was interrupted to to another task and left my system with mobile phase in the lines but stopped the flow. My take was that I had 50% water 50% ACN with 10 mM Ammonium Acetate and that would not be likely to precipitate due to the high water content. The next day everything started fine but in the middle of my run 2 hours after starting the flow my pump B was starting to lose pressure and now it cant even create a prime flow.

My routine is to always flush with 50/50 MeOH/H20 in the end but when there is a shorter interruption where you intend to resume the run this seems unpractical. A possibility is to set a very low flow rather than no flow to keep the plungers moving

I don't know if I made the wrong judgement but in the future I would likely set the flow to keep plungers moving such as 10ul/min flow. It is very hard when you get interrupted to know if you will resume in two hours or the next day.

Is it likely that the 10 mM buffer in my 50/50 H20/ACN could be the cause for the loss of pump funktion 2 hours into the run, a stuck valve should cause a problem at the start? The LC method is based on the manufacturers protocol and works well in general when running samples and it is possible to dissolve 10 mM ammonium acetate in 95% ACN 5% H2O so I thought 50/50 would be safe?

I am curious whether I made the wrong judgement or if I was just unlucky? In the future I will keep plungers moving a little just to be safe.

"Never leave buffer-containing mobile phase stagnant in an LC system" is good advice. For now, if this were my problem, I would remove and sonicate the check valves (or swap in a spare set if I had one) and flush that side of the system as a minimum. As long as it was apart anyway, i would probably take the opportunity to do a full PM.

Thanks for advise. Did the sonication meoh/ipa/h2o/meoh but did not resolve the problem. Will replace check valves with new and never leave buffered mobile phase without flow in the future. Would a very liw flow be enough, like 10ul/min.

Just hope new valves will be enough. I have given the system proper flush with meoh/h20.

... forgive me if I'm teaching a grandmother to suck eggs (where did that strange expression come from? I mean if you know this already). But you can usually hear whether the check valves are stuck, once you've taken them out. If they're the normal passive ruby-ball-in-a-little-cartridge type, then they do rattle distinctly if you shake them right next to your ear in a reasonably quiet place. If they don't rattle, they're stuck.

Well these are ruby but on watets acquity system with active plunger i2v controller. Did try cleaning sonication without success. Fortunately we get excelent and quick support and with the new valve in place everything works fine.

Thanks for the trick with the ratteling ruby. Will be usefull when trying to unstick valves with cleaning, unless all ms pump noise have damaged my hearing to much. Will test it on my spare valve.

On our older systems (Agilent 1100) I would have a standby method that ran at about 0.1ml/min to keep things flowing through the weekend or whenever the instrument finished the analysis sequence. Newer systems can probably run even lower flows and keep things from sticking.

Be careful if you flush out the buffer. Flush with 100% Water before using the 50/50 Water Methanol as some buffers can crash out with that much methanol.

Sonicating check valves isnt great advice because once check valves start to stick and give atypical pressure traces, its much better to replace the check valves. Sonication is only a very temporary fix. Sticking check valves, especially on UPLC systems, rarely get better.

Thanks for the advise, I am guilty of trying to save old parts with the hope of restoration but I will follow this advise and not waste my time for temporary fixes, it is an uhplc system. A lost sample is far worse than having to replace checkvalve. Waters were kind enough to send us a new valve and a spare. Replacing was a very easy quick operation. Thanks for the advice!