Exactly, you want as much as possible to limit any exchange with the atmosphere, so you shouldn't use a desiccator, which would evaporate some water, and induce isotopic fractionation. In any case, you want to use a very tight cap, and put parafilm on top of it to really isolate the bottle. To limit exchange even within the bottle, two possibilities: freezing it as you suggested, or getting the bottle as full as possible, we do that by adding small glass balls inside the bottle, which limit any air in the bottle, and thus the exchanges.

Thank you very much for your advices Dr Casado!

I will aliquote into fractions, close tightly with parafilm and freeze. I also heard from a colleague that after thawing, in case of not using all, keep it in the refrigerator instead of freezing it again, because thawing leads to some extent of hydration.