Chromatography Forum

I am getting very high backpressure on my analytical system, to the point where Unipoint stops my method. I am using a Gilson 322 (H2) pump and Luna C18 250 x 4.6 5u column, I have replaced several parts including column, outlet frit, bellows and piston seal (on organic H2 pump which were in a very poor state) but have not managed to solve the problem.
I notice that there is occasionally a large amount of bubbles in the outlet tubing (to waste after the column)
I would appreciate any suggestions to try as I am running out of ideas!
Thanks

Faulty sensor? Take off the column and pump at max flow rate while Checking flow and pressure reading. May provide clue.

I would try what Bill has suggested, but after that, perhaps the setpoint for your max pressure (the point at which your instrument will shut off) is too low for the method you are running.

Nevertheless, a little more background please would be useful, ie mobile phase(s), pressure you are seeing, when does the pressure start increasing (during equilibration, after an injection, during the gradient - if there is one), etc. Information such as this can help with the troubleshooting process since it seems you have replaced almost every mechanical thing with the pump and its processes.

Bill: If I remove the column I get a small pressure reading in the range 14-32psi, the flow reading in Unipoint gives me the flow-rate I have input whether in a method or manual control

David: My mobile phase is MeCN/H2O, my gradient is from 10-95% MeCN as I attempting to run a scout gradient for method development. The preesure increases from 14psi to 4000psi at a flowrate of 1 mL/min in a matter of minutes (at which point Unipoint cuts my method off as the high pressure limit is breached) and can occur during manual control as well at 50:50 MeCN/H2O at 1mL/min, I have not run it at 100% MeCN.

Thanks for your responses
Kate

if you can run 10 ml /min with no column and get low pressure then the problem is the column. probably a plugged frit. Do you use an inline filter ahead of column. These cheap Upchurch devices saved me many a column.

If I understand you correctly, you have replaced the column, and you still get the same results. Therefore I would not chase the column.

Is it possible that you have something stuck in the detector inlet tubing? You would not see this, if you disconnect the column. If this is correct, you can check, if the pressure does go crazy, if you run your gradient through the column and the column is not connected to the detector. If this works out to be OK, then you either have a plugged detector inlet line, or proplems with the connection of this line to the column.

Amplifying a bit on Uwe's post:

The "standard" diagnostic routine for high pressure is to start disconnecting fittings from outlet end of the system and moving upstream until you disconnect one and the pressure drops. Just downstream from there is your blockage.

A common source to these problems are a PEEK fitting that has been too tighly connected.

PEEK fittings that are tightly connected for several times tend to require more violence and result finally in a closed tube. Remember to cut an end piece each time a PEEK tubing is reconnected.

Einar, are you talking about stainless ferrules on PEEK tubing? With PEEK ferrules on PEEK tubing there have been no problems here (mostly 0.1 mm ID PEEKs, though). Might be a matter of heavy-handedness?