The Sure/Seal (TM) septa on top of Aldrich bottles are anything but. As soon as you pierce one of those puppies to extract some anhydrous solvent, you can be certain that subsequent aliquots will be of dubious dryness. I cannot count the number of times I’ve pulled a Sure/Seal bottle from a stockroom only to open the red plastic top and find a gaping hole staring back at me. At that point, why even bother leaving the septum on?
During one of my first-year rotations in grad school, the PI of the lab had a specific policy for using Sure/Seal bottles. Before any solvent was dispensed, you had to:
1) Find a 24/40 natural rubber septum
2) Invert the septum and pull it over the mouth of the bottle
3) Secure the prophylactic septum with a copper wire
Extraction of the solvent would then take place by puncturing both the Sure/Seal and the outer septum. Since the outer septum was made of natural rubber, it would re-seal more tightly than the firm Sure/Seal. (Of course, over long periods of storage, the rubber
dries out crystallizes and the outer septum disintegrates—see comments.)
After you were done setting up the reaction, you were supposed to take one of those yellow plastic 14/20 stoppers and bung it in the hole of the outer septum. Finally, the entire apparatus at the top of the bottle would be wrapped with parafilm.
While the whole process was time consuming, I liked to think that it offered at least some added protection from atmospheric moisture. Milkshake prefers to tape the tops of his Sure/Seals after use, and that seems like a simple alternative. I’d love to see someone run a comparison test of these (and other) methods.