by
lmh » Tue Mar 30, 2021 2:51 pm
The pressure instability issues that the original poster described are probably not a degasser/air-bubble issue, and probably do relate to failed seals. Shimadzu use a diaphragm on the back of the pump plunger to act as a seal wash pump, drawing wash solvent into the space behind the plunger seal as the plunger moves backwards. This is quite a neat design, but it moves quite a bit of solvent, so they use a recycling wash solvent. The original poster noted that the wash solvent reservoir is gradually filling up (rather than disappearing through evaporation) which is absolutely a diagnostic feature of a leaky piston seal on a Shimadzu system. Once you have leaky seals, you also get irregular pressure, particularly as I think this pump continues the Nexera's design of parallel rather than series pump heads. If the seal in one has partially failed, the pressure will drop dramatically every time that side is doing the pumping. (Incidentally, this can be an issue with priming, as it's quite possible to have a Nexera pump that's bubble-free and working fine on one side, but not the other, so when you purge, it looks OK, but you're only purging half a pump...)
I'm sorry, original poster, I can't find the specifications I once had on how much volume those seals are supposed to pump before they fail. But if you're using your system really heavily, even at analytical flows 0.6mL/min plus, I wouldn't be at all surprised if the seals fail in under a year. You may not be doing anything wrong. The trouble is, they are quite expensive. The other problem is that replacing them is a gamble: if you replace the pistons at the same time, it's even more expensive, but if the pistons are in any way scratched, they will trash a new set of seals very, very rapidly... so it's a matter of having a look at the pistons and hoping you can judge whether they're damaged.
Don't be too disheartened; you may need to replace seals, but if these new pumps are similar to the Nexeras, they're actually pretty decent pumps, and not hard to service. We have to change seals in ours a bit more than we'd like, but they give us good, reproducible gradients and cover a wide range of flow-rate.