Chromatography Forum

I am running a confidential gradient HPLC method on a conventional C-18 column. Mobile phases are water and acetonitrile, both with 4% phosphoric acid. Column pressure is about 2,000 psi. When I use the 4% phospohoric acid in acetonitrile, there is significant seepage of the mobile phase from the top of the nut on the tubing on the outlet check valve. It seems that this mobile phase is very "slippery". Does anyone else have any experience with phosphoric acid/acetonitrile mobile phases or has anyone else experienced this? Any explanations?

What type of HPLC system are you using? Do you see leak when you are not using phosphoric acid? Other mobile phases?

If its really outlet check valves leaking you might want to replace to check valves.

What type of HPLC system are you using? Do you see leak when you are not using phosphoric acid? Other mobile phases?

If its really outlet check valves leaking you might want to replace to check valves.

Dionex Ultimate 3000 HPLC. No leak or only minor seepage when using pure acetonitrile or methanol.

The leak is not from the check valve itself but is coming out the top of the nut that connects to the outlet check valve. I have already tightened the nut but the leak is still there.

I have also ordered new check valves and new tubing for the pump head to install if necessary.

I am curious if anyone else has had leaks with phosphoric acid in acetonitrile as a mobile phase.

Is the pressure of the system normal? How does the pressure compare with and without phosphoric acid?

Be aware that at certain concentrations buffers can precipitate when organic solvent concentrations get too high. However, I don't have a handy chart for solubility of phosphoric acid in acetonitrile.

Also be aware when using buffers like phosphoric acid that systems should be rinsed regularly to remove buffers from various system components. Salts can build up on various parts causing scratches, clogging, or corrosion.

This is a known problem of the Ultimate when mobile phase with TFA is used. Apparently the material of the steel capillary / ferrule corrodes under certain conditions. This phenomenon is more pronounced for the part mainly used with organic. Your phosphoric acid content is unusual high; most probably there is the same problem.

Some years ago (~5?), Dionex have changed this capillaries at the pump. New systems are now shipped with a special type of the Viper capillaries to solve this problem.

I recommend you to ask the Thermo/Dionex service for the new type of capillaries and the dedicated torque wrench. Also replace all capillaries between pump heads and injector with Vipers, it is a matter of time that leakages will occur also on other places were there conventional capillaries are installed.

4% Phosphoric acid? Please don't drink it
Are you sure it isn't supose to be 0.4%?
I can't see what would be the rationale for using so high concentration.

Best Regards

Danko: You are correct. I rechecked my calculations and it is 0.4% phosphoric acid. Dang those pesky decimal places.

Klaus I.: This is what I suspected. Thank you for the information about the tubing. I will replace the tubing and try again.

Thank you all for your helpful questions and suggestions. I will try to remember to post again when I have the problem solved.

UPDATE: I installed a piece of PEEK tubing in the pump where the leak was taking place. This temporarily solved the leak problem until I can obtain replacement tubing. My retention times are now stable. Thanks again for the replies to my problem.