Chromatography Forum

During a normal isocratic run, the pressure is constant at 55 bar (B1:73%ACN/A1: 27%water). In a recent run, the pressure was 55 bar for about 100 injections (10 minutes each). Then during one 10 minute run, the pressure gradually decreased from 55 bar to 52 bar then remained constant at 52 bar for the final 100 injections. A significant retention time shift corresponded with this pressure change. The pump passes pressure, leak, flow rate, and gradient mixing tests. Any ideas what could have caused this pressure drop?

Were peak retention times the same?. Where all samples prepared the same?. Was mobile phase added during the run?.

Assuming your pressure transducer isn't telling lies, low pressure can only come from lower backpressure or lower flow. If peak RT is the same, and all samples were similar, I'd go with the dishonest transducer.

Backpressure is controlled by the fluid flow resistance, and dissolution of insoluble/immiscible material, leaks, instrument and/or column temperature changes, or lower mobile phase viscosity can change backpressure. Take your pick.

Bruce Hamilton

The samples were separate vials of the same standard. The mobile remained the same throughout the entire run. The retention time was approximately 4.7 minutes for all injections before the pressure drop and approximately 4.5 minutes for all injections after the pressure drop (this causes the run to fail system suitability with a >2% retention time shift). The system was tested for leaks and none were found. The system passed a pressure test up to 400 bar. The pressure and retention times were very consistant before and after the pressure drop. The pump seals, check valves, and frits were all replaced and/or cleaned. Because of the R.T. shift that cooresponded with the pressure drop, I doubt it is the pressure transducer. Temperature was maintained at a constant 40C for the entire run. It's a mystery to me, but I am not comfortable running on the instrument without and explaination. We lost a lot of samples dut to this problem.

OK - I wasn't certain that the subsequent 100 also had different RT. Lower pressure and shorter retention time suggests there was either more acetonitrile, or the column shrunk ( eg changed guard, something dissolved ).

I'd go with more acetonitrile, and my first assumption would be that the water solvent reservoir inlet sinter/frit may be blocking up. I'd clean all the sinters/frits in the pump. Your pump valves could also be sticking. Has your water quality/system changed recently?.

I'd make up the mobile phase as a mixture and use that. Does the column still show lower pressure with an accurately-made mobile phase mixture?. If it reverts back to the high pressure, your mixing valve may faulty, if it stays low, check pump flows at pressure. Good luck.

Please keep having fun,

Bruce Hamilton

My interpretation would be that tronic used a pre-mixed mobile phase and that his pump simply does not compensate pressure changes (it keeps going at the same stroke frequency even if the resistance of the column drops).

How about degasser? different amout of air was involved?
Distribution valve? its property changed a bit.
how about repeating the original 200 injection withour changing anything? to see how often this can happen.
Repeating the above only with a new column? to see if column related.
Repeating the above only wiht a new LC? to see if LC related.
It is interesting to me. Thanks for sharing.

Thanks for the ideas. I have decided to utilize the weekend by monitoring the pressure over 3 days. Next week I will look at the affects of pre mixing the solvent. I will let you know what happens.