Methonol to ACN

[tarzan]

Dear all..
I have left my c18 column in 100% methanol after the end of my work today.
Tomorrow, I need to start my samples with 93:7 water:Acetonitrile.

So how should i change my eluting solvent ..?
Can I directly go to the 93/7 (Water/ACN) system or should i follow some procedure

with best wishes

[bunnahabhain]

Hi tarzan
I never do such big steps from 100 % organic to > 90 % aqueous. Instead I would first go from 100 % MeOH to 100 % ACN and then add water in 20 % steps, at least 2 column volumes each step. As you are using two pump system this is straightforward. But if you are using isocratic runs with 93/7 % v/v, pre-mixed buffer will give you much better baseline stability, especially when using an RI detector.

Kind regards
Jörg

[LiVD]

Water and methanol are perfectly miscible, so I wouldn't expect any major problems. Do take in to account that certain water-methanol mixtures have a viscosity higher than either water or methanol, so watch out for over-pressure!

[tarzan]

hi.
Thanks for the quick replies ..

Any ways .. how do you guys leave your columns at the end of the day ...

100% organic phase ..ie either methanol/ethanol/ACN

or

in certain composition of organic/water ...?

with best wishes

[LiVD]

I always revert to 100% ACN with a small flow rate (0.005mL/min) on my C18 UPLC column.

[lmh]

It'll be interesting to see what others do.

Since methanol, ACN and water are all completely miscible in all proportions, and assuming this is a modern C18 column (i.e. more-or-less indestructable) you can change it between any of these samples as quickly as you want with no risk, except as LiVD said, there will be a pressure rise on passing between methanol and water because of 50% mix being more viscous than either alone.

I leave my C18 columns in highish organic, either 70-80% MeOH or ACN. The only reason for not going 100% is a personal, and not very logical panic, about gradual precipitation of anything salty. Although column-abuse is not to be encouraged, modern simple reverse phase columns are incredibly robust.