does a HPLC pump seal properly?

Discussions about HPLC, CE, TLC, SFC, and other "liquid phase" separation techniques.

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Hi to all,

There i go, I am seting an equipment and I am using a shimadzu LC 20AD, but not for HPLC, I am using it to enter my reactive into the reactor. I mix it with the carrier gas through a "T" and after that they goes together. I am having a problem, I am seeing my reactive when the pump is not pumping, so I think it is working like a "vacuum tube" (I hope these are the correct words) and the gas is sucking in the liquid. I know that I could put a valve before the "T" to stop it but I have the doubt if the pump would not seal properly and I have to do some maitenance and change some broken joint.

Does anyone have some idea?
Thanks in advance for your response.
Is your T above the pump's reservoir?

You could just have a sticky check valve. It could be that what you're pumping is the problem, or it could just be the result of passive mixing at the T. Either way, you may be able to use gravity to minimize the problem at least a little bit.

The more you do not tell us, the less we can help you...
Thanks,
DR
Image
Do you have enough backpressure after the pump?
- the reservoir above the pump will feed the solvent autonomously
- the gas flow in the T will additionally create a vaccum and suck on the solvent line
So, sounds plausible to be normal for me

I'm not familiar with the shimadzu pump, but if there are just passive check valves and the gpv doesn't close the line, then I guess there's just the backpressure from the tubing/column that normaly prevents the solvent from flowing.

maybe adding enough length of 0.1 mm ID tubing (or a dedicated backpressure regulator) between pump and your 'T' may improve the situation.
of course, a valve would work best.
I'd go for the dedicated back-pressure regulator, because if you genuinely want absolutely zero-flow when the pump is set to zero, the tubing won't do it (tubing generates no back-pressure at no flow). Like many HPLC systems, the Shimadzu pump expects a substantial back-pressure trying to force the flow in the wrong direction. It's in no way designed to handle the situation where the liquid is actually trying its best to go forwards, not backwards!
lmh wrote:
I'd go for the dedicated back-pressure regulator, because if you genuinely want absolutely zero-flow when the pump is set to zero, the tubing won't do it (tubing generates no back-pressure at no flow). Like many HPLC systems, the Shimadzu pump expects a substantial back-pressure trying to force the flow in the wrong direction. It's in no way designed to handle the situation where the liquid is actually trying its best to go forwards, not backwards!


Exactly, HPLC pump check valves only work to allow flow forward and prevent reverse flow, otherwise they would not flow under pressure. We have used a 100psi back pressure fitting before which is simply a check valve held closed by a spring, so that no flow is allowed until you system pressure exceeds 100psi, that would probably work in this situation since even under hard vacuum there should be less than 100psi pressure differential on the system.
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