Dear All,

After washing/cleaning (at 1 ml/min, in same setup: SP8800, precision isocratic pump) four orphan NovaPak C18's (they were not end capped) columns with methanol, I found that two caused a slight increase in the pressure readout and two caused the pressure readout to drop (after about five minutes) from 1234 psi to about 786 psi.
My questions:
Does this change in pressure indicate anything good/bad about the columns? (Besides it's inside dry and wet state)
If so, is the decrease or the increase in pressure an indication of a good (usable) column?

I thank you in advance, best regards Joe

It is not impossible that the columns were stored from different solvents, with some of them containing a salt or a buffer. To get the column completely clean, wash them with water and then go back to methanol. You may now get consistent backpressure.
It is generally not a good idea to reuse old columns with an unknown history. If you are doing critical method development work in a regulated environment, I would definitely not recommend to do that. If the work is not that critical, and does not need to be reproduced later, it is less of a problem. But don't be surprised if 4 columns with unknown history give different results.