retention time shift after fixing a leak.

[SergioM420]

I am running a Agilent 1100 for medical cannabis. We have had very consistent retention times up until these last couple of weeks. I had a leak which caused pressure to drop and retention times to shift. Once I fixed that leak everything was back to normal. A couple days later, seemingly for no reason, the retention times shifted again. This time I was not able to determine a leak of any kind.

After changing the column filter, column and a piece of metal tubing that goes from the heater to the column, I am still seeing this retention time shift. I have taken a couple metal tubing pieces apart and re-connected them. Still nothing.

THCA was coming out just under 6 minutes and is now coming out at around 6.50. The pressure seems to be fine as it is resting at the same bar that it was at when functioning correctly. I am baffled because usually there is a correlation between the pressure and the retention time shift. This time everything looks to be running the same as before. What else could it be?

[uzman]

If it is a quad pump , you may check the gradient valve , if it is isocratic , then consider temperature effects.

[DR]

Batch to batch variations in mobile phase can result in RT changes too.

[EmpowersBane]

Does your mobile phase require pH? I have seen dramatic changes in RT after even one or two units of pH past its optimum value, check that your pH meter is calibrated right. Failing that, a mixing issue seems likely.

[BostonFSE]

run the old column again, if your RT is what you expect than you found the issue.
a new Column will not have have identical RTs as an old column