Pressure drop and fluctuation

[hamish633]

I am running a Shimadzu LC10 system with 80:20 methanol:water isocratic phase. I keep getting a situation where my pressure drops to zero, then picks up to approximately 50% of normal, and bad fluctuations. The engineers have replaced so many parts its practically a new machine, but I keep getting problems. Does anyone else have any experience of this? I'm getting desperate and the engineers are struggling too.

[Gerhard Kratz]

Be happy, it is not the column!
It comes from your pump head, from the pistons and the check valves.
One check valve minimum does not work properly. The sapphire/ceramic balls in them are too clean and stick in the ceramic seat. Take them out and roll them between 2 of your fingers, than putt them back. Than purge your pump (without the column) to get possible air out of the system. Check flow rate of each channel.
If flow is constant fluctuation is gone. Good luck.

[hamish633]

That's what we thought initially, although we are still seeing problems with a brand new set of check valves. We have even tried a set of ceramic valves with no luck

Would this problem still occur with new valves?

[Hollow]

I'm not familiar with the system. Is it a high or low pressure mixing system?

Either way, I'm also thinking of the check valves. In such a case I've got good experiences with sonicating the CVs for about 10min in nitric acid 6 mol/L. (Flush with water prior and after the nitric acid treatment!)

Or there is a problem with the supply of solvents to the pump e.g. clogged frit in the solvent reservoirs or air in the line?

[sga]

A sudden drop is pressure usually means that there is no solvent in the pump. Therefore I would start at the top and make sure that there is constant solvent flow. I would suggest to by pass the degasser and connect filtered IPA directly to the pump. The solvent filter could be clogged. If you can not replace it, then clean it by sonication.


It is possible that the degasser is not working. Therefore there are large enough bubbles which lead to fluctuation.

I would suggest to use IPA to degas the entire system for approx 30 min (Without a column). Start at a low flow rate and increase slowly. If you still have issues, then there is a problem with the hardware.

If the hardware issues persist and are leading to un-acceptable down time. Then its best to go for a replacement.
On the other hand if that is not possible, then you will just have to keep trying with different components till some thing clicks.

Good Luck !

[DoryFish]

Have the engineers looked at the pressure transducer? I've been chasing odd pressure readings for 6 months, and it's only when the pressure transducer finally fell over that I realized the readings were artefacts.

[hamish633]

Thanks all for the suggestions. In the end we have (hopefully) sorted it - it looks to have been a faulty vacuum pump on the degasser.

[Hollow]

thanks for reply.
it's always good to know what it finally was.