We use Agilent 1100 series HPLC's in our lab. When a column is attached and the purge valve is open (ie purging the system), I had always been taught to turn off the pump, close the purge valve, and then turn the pump back on; versus shutting the purge valve off with the pump still running and the column attached. The rationale behind this was that the sudden pressure "increase" could "harm/ruin" the column packing. Has anyone ever heard of this?

I don't use Agilent, but I think the concept of sudden pressure increase is general.

You should not increase pressure suddenly because the packed gels (silica gels) in column will be pressed and the surface of gels in column will go down from the normal level. The result is that there is a space in the column at the surface, which is not good. Because you have to know that in the process of making HPLC column the gels are just being packed into column, there is no glue between them or between them and column - This is what I know from other. (if anything is not correct, anyone please tell me)

The sudden pressure increase may bring packed gels in the column more being packed or rearranged in other way. - This is what I guess from my idea.

I agree that a sudden pressure increase can damage a column but this depends very much on the column you use and how this pressure increase is produced. Certainly it is very safe to shut down the pumps and then close the purge valve, but normally it`s no problem to just lower the flow and then close the valve. For systems that are very sensitive towards air bubbles (like most PEEK-systems) this is sometimes the best way to get the pumps primed.

Thanks for the information regarding the column packing and pressure.