Chromatography Forum

I noticed a higher than normal back pressure for a couple of batches on my UHPLC, but the chromatography looked good. The somewhat higher pressure was present since the previous pre-column install, I assumed that the pre-column was getting occluded with fine particulate from the last few batches, so I swapped it for a new one from the same box (Box of 3). (Note: these are true pre-columns with their own, new fittings, not a replaceable filter in a holder.)

I reassembled the unit and started it. The pressure soared up instantly and the pump stopped at the high pressure limit.

Not sure what's wrong, so I removed the pre-column and column as a unit and tried to pump solvent through the bare fitting to check the pressure, but the tubing appears to be completely blocked at the end fitting from the UHPLC tubing. No flow at all! Visually, I can see nothing wrong with the threads or the ferrule, but I haven't used a magnifier or microscope to look at the end.

Is it possible that I got a bad lot of pre-columns and they somehow damaged the tubing fitting to the column?

Any repair possible or order a new section of SS tubing?

Typically, diagnosing this is a matter of sequentially loosening fittings until the "clog" is found via pressure drop or even more obvious sign.
Once you find the pinch point, repair or replace whatever it is if it is not amenable to repair via trimming, filing, ferrule replacement etc. (this is where we can't help you more because you did not tell us much about the plumbing involved).

Depending on the dimensions of SS and availability of tools/fittings/tubing, you may well be able to fabricate a suitable replacement far faster then replacing it via ordering.

Yes, we have traced the blockage to the final piece of SS tubing that connects to the column. It has a label of "0.1mm" diameter and a length of ~35 cm. One end connects into a high pressure switching valve in the autosampler and the other feeds into the column end.

I believe the precolumn fitting inner surface has damaged the open end of the tubing extending through the ferrule, completely sealing off the end. Is there any way to reopen the tubing passage without trimming?

I have cut and repaired many SS HPLC lines before, but this one I am unsure, because it is very thin UHPLC tubing and the SS line seems to go into a thicker, rigid sleeve section about 5 cm from the column end (presumably to reinforce the tube running into the fitting and ferrule assembly). I may be able to trim off the old ferrule and install a new one, but I wanted to check if this may be a lost cause with UHPLC tubing.

Do not try and "repair" the damaged/plugged tubing. This is a special type of pre-made tubing with brazed ends. Throw the damaged tubing and fittings in the trash. Order a new SS line with the correct types of HPLC end fittings (nuts) and ferrules to make the required new connections. Correctly swage the new fittings onto the tubing, then flush the line before use.

BTW: Here is a link to a free article on HPLC tubing and fittings: http://www.chiralizer.com/uploads/8/9/7 ... aphers.pdf