High pressure on column.

[oscarBAL]

Hi evey one!
I have two column C18 of 20cm and 4.5um both, in my chromatographic system the presuure with any of this column is arroun 1800- 2800 psi for organic solvent mixes, but when I Start to work with MeOH-H2O at any coposition the pressure go up untill 4500 sudenly, I know that MeOH-H2O mixes increase the presure but I never Has saw so big increase! likely I have to get new columns but these are pretty new, If both columns are obstructed they should have big pressure at any composition of mobil phase? because if I work with H2O-MeCN, MeOH-MeCN or other solvents I don't have pressure over 3500 psi in the worst case.

Somebody have any tip that could help me to retrive my colums??

Thanks,
Oscar.

[Uwe Neue]

The pressure with a methanol-water mixture is at least 1.6 times higher than when you use water, and 3 times higher than when you use pure methanol. So going from 2800 to 4500 is possibly quite normal.

If you give me the flow rates, the solvent composition, and the temperature, I can calculate what the pressure is expected to be. Then you can decide if there is a problem or not.

[oscarBAL]

Thanks, Uwe
The pressure in my column using water is 2900 psi and with MeOH is 1800PSI with 1ml/min. but when I use a mix The pressure go up higher than 4000psi, and at these pressure my postcolumnderivatation system don't work, but they guive you the column so they should know that the pressure have to be below 4000psi.

Thanks in advance.
Oscar

[Uwe Neue]

I forgot to ask you about the column diameter. I need that to calculate the pressure for the column itself. The ratios are right what one would predict.

On the other hand, you say that you are using a post-column derivatization system. Please explain, what the system is, because it will contribute to the pressure. The other thing that I do not understand is why your post-column derivatization system stops if the total pressure is too high. At what pressure will your LC pump stop? There may be some kind of a safety feature built in to prevent post-column reagent to be pumped into the column.

[oscarBAL]

Hi Uwe,
My column is a C18 25cm, 4.6mm, 5um, the LC pump stop at 6000psi, and my post derivatation system is a Pickerin 5200 for carbamate analysis, when the pressure on the system reach 4000psi the postderivatation ponping stop. but the LC can fallow working untill 5900psi.
and yes the post derivatation system stop at pressure higher than 4000psi to prevent, that mobil go into the pickering system, to the reagent reservory.
But if they designit and they give you the column is because they should know that the pressure is lower than 4000psi!
But my colums are pretty new, and everything looks right.

Thanks,
Oscar

[Uwe Neue]

Hi Oscar,

The backpressure for such a column is expected to be around 2000 psi in water and less than 3500 for a 50:50 methanol water mix. The backpressure could come from the post-column system.

In order to test, where the backpressure is, run the column in 50:50 methanol/water without the post-column system. If it is less than 3500, then there is nothing wrong with the columns. Now connect the post-column system and see where the pressure ends up now.

If necessary, we can then troubleshoot the post-column system.

Uwe