Hi all. When I see people put their mobile phase on their systems (~4L), they completely cover up the top of the solvent bottle with parafilm. The use so much, that they you would need a knife to get it off. The question I have is: Is it necessary to seal the solvent bottle that tight? They say the reason for doing it is that they have just degassed their mobile phase and they don't want it to "re-gas". Do they need to worry about a vapour lock (and possibly changing the composition of the mobile phase)?

Thanks for the help

Steve

In any case, it is a bad practice to tightly seal the mobile phase bottles : as they become depleted, they will be put under vacuum and the pump may get priming problems; it is a good practice to cover the bottles while keeping a tiny vent; anyway a good chromatographic system should always come with dedicated bottles and solvent lines fitted with specific caps !
The question of regassing does not apply if you are using on-line degassing devices (helium sparging or better vacuum/membrane system); if not you may have regassing problems, specially if you are pumping 4 liters straightaway !
In this case, you must choose the solution that is the least risky on your particular system : priming problems due to depression or due to regassing.

I think the main reason is the evaporation of the organic phase from the mobile phase which may cause change in the RT and hence sometimes RRT's resulting in wrong or ambiguous identification of impurities in case of related substances method.So if the same mobile phase is used over a long period I personally feel that bottle should be covered with parafilm.But a single layer is sufficient rather than using so much that you need knife to get it off.

I think the main reason is the evaporation of the organic phase from the mobile phase which may cause change in the RT and hence sometimes RRT's resulting in wrong or ambiguous identification of impurities in case of related substances method.So if the same mobile phase is used over a long period I personally feel that bottle should be covered with parafilm.But a single layer is sufficient rather than using so much that you need knife to get it off.

Organic solvents will evaporate through parafilm. Perhaps aluminum foil is better for this purpose.

I've heard a few horror stories about the use of parafilm with some solvents, but I don't recall which ones. As a result I recommend against using parafilm with LC solvents. I doubt if the seal would be tight enough to cause any vapor lock problems. The suggestion of using aluminum foil is a good one. I've used this for years -- it prevents a tight seal (no vapor lock), yet limits air circulation (reduced evaporation??) and keeps out the dust cruddies.