Annoying problem keeps popping up before starting runs!

[Consumer Products Guy]

I read a bit about pure ACN not being good for HPLC hardware such as check valves. I suspect that these concern only binary systems where pure ACN would actually contact a check valve.

Though retired now, I had our lab using pure ACN (such as Fisher Optima) in a reservoir in practically all of our Agilent quaternary HPLC units, but with those pure ACN would really be contacting the degasser plumbing and the multi-channel mixing valves, once it got to check valves it would be mixed with other solvent(s) such as aqueous. We never rinsed a column with 100% ACN either, typically rinsing or gradient/clean out would top off with 90% ACN.

[EmpowersBane]

I use a binary Waters UPLC and its high pressure mixing as the mixer is after the pump, hence why pure ACN contacts the check valves. I was advised to always flush the mobile phase lines (in my case A1 and B1) out with 90:10 Water:ACN after a run, that way the check valves (particularly the B or Organic check valve path) are not sitting in pure ACN where it could stick and cause issues.

Why would you not flush a column with 100% ACN? I have always found it a good way to remove any strongly retained molecules after the buffer has been flushed out and a lot of column manufacturers will also recommend doing this as part of the equilibration procedure.