Yes, check valve was the most likely culprit here. Good luck.

Hi Dagmara.

I think the solvent flask is on top of your pump it's not possible the solvent return in another flask (if was this your problem) because the fluid transport in this case is only gravitational. The problem could be mixture valvule, the salt in mobile phase could damage menbranes inside (in this case it's necessary replace it).Your report and experience I thought you had cleaned all canals with water for removal salts, and after use isopropanol remove bubbles. It's possible the check valvule was blocked, if you could disassemble this valvule and put in a sonic bath for 10 minutes, this procedure will unblock the valvule.

Hope my comment help you.

Hi Marcos, thanks for comments. The solvent is not moving to another flask but moving to another pump. Pumps are connected by capillaries only with the mixing chamber, so I think this is the way the solvents get from pump A to B instead of flowing further to the injector.

If there is no other solution then I will have to disassemble the mixer elements. Which element is more likely to be clogged: mixing chamber, damper or the mixer? I don’t think I can wash the dumper, though. Does anyone have some experience with those parts?

• Dagmara wrote: " the screw of the capillary connected to the purge valve (pump A) and the screw of the outlet valve were covered with salt crystals" - This implies that the fittings may be loose (leaking). If so, please tighten them, disconnect the column and run the line to a beaker, then flush the system down with water (no buffers).

You may have just a leak at a fitting, a salt clogged Solvent Selection Valve (The plastic block with 'A' and 'B' between the two pump heads), stuck check valve or salt encrusted piston seals.

The problem is most likely that salts have been deposited on the pistons and piston seals inside the pump (you may also just have a leak at one of the fittings and are only noticing it because of the dried salt residue). The pump(s) must always be rinsed of all salts after use (every day). Never leave the HPLC system in salt buffer, wash it with a solution of mostly water with some MeOH (e.g. 80% water / 20% MeOH) after use. Deposited salts on the pistol seals will cause a leak in the system. The dried buffers cystals create openings for the liquid to pass around the seals resulting in even more buffer and salt deposits. The pump(s) should be run (column disconnected and bypassed) run with a solution of pure HPLC grade water to try and rinse the material off. Run through the prime purge valve first  (10 mls), then close the valve and run through the system. Additionally, the pump heads should be dissambled, parts rinsed with water and cleaned. Seals and piston should be checked for wear and if worn, replaced.

I have added a link to an article, "HPLC PUMP SEAL WASH & FLUSHING THE HPLC " regarding the flushing of the HPLC system which may be of interest to you below.

http://hplctips.blogspot.com/2012/08/hplc-pump-seal-wash-flushing-hplc.html

 I think I found the problem. I removed the outlet valve and opened it. Two sapphire balls and many plastic rings fell out. And one plastic part was definitely broken. There were also 2 tiny metal pins that looked like they should be  fixed together (but I'm not sure). So, I will order a new valve and hope the system will work. Anyway, thanks for all valuable sugesstions. I learned something new.

Yes, check valve was the most likely culprit here. Good luck.

Check valve, see the back pressure, and also have a look onto pump seals, salts are nasty stuff and can block capillaries, open the valve/purge valve, and inject solvent such as water or methanol via syringe slowly while running the pump.