Chromatography Forum

hi everyone,

I have been watching a decrease of column pressure over time.

It started at 11.600 psi when new, and has been slowly dropping into around 10.600 psi after almost 1000 injections.

Does this make any sense?
I'm used to see column pressure going higher, not lower.

I have introduced a "several solvent injections sequence" in the end of every sample sequences, in order to improve column cleaning at the end of the day.
It feels like column is lasting longer this way, than with just normal column wash protocols at the end of the race.

Can this might be causing increased column bleeding effect, and it might show continuous lower pressure because of it?

thanks you all for your help!
may the gods of science be with you all

A review of chromatography fundamentals tells us which parameters effect pressure. It pays to address the basics when troubleshooting a problem.

For static isocratic conditions, a reduction in initial column or system back pressure would most likely be caused by changes in the:
(1) ACTUAL Flow Rate (Measure it with a volumetric flask to see if it is lower than the set flow rate. HPLC systems do not measure flow rate. They simply pump at a preset speed). Perhaps you have a leak which has changed the actual flow rate?;
(2) Temperature increase (col compartment or room temp is warmer);
(3) A change in the mobile phase composition due to evaporation, different preparations of the "same" mixture (different people, techniques or methods) OR a defect in the mixing valve resulting in an unexpected composition change. Lower total viscosity equals lower back pressure.
(4) Poor quality column re-equilibration. If you fail to operate the and/or run the method in a consistent way, then variations should be expected.
(5) A change in the physical packing of the support. This is commonly seen in UHPLC applications run at very high pressure (>8,000 psi), which are very rare today. However, if the column is newly equilibrated in a solvent system which differs greatly from the type it was initially packed in, it may change its internal distribution resulting in small changes to the observed back-pressure (the fines could move in position, the overall porosity could change). If true, this should stabilize over time so a log should be kept.

PS a change of 10% in pressure after 1000 injections is probably nothing to worry about!