Let’s kick this off with something simple, rubber septa. There are a few different designs that have come out over the years (pictured), and I want to talk about the kind that looks a little like a smurf house, upside down.
Now, the stated purpose of these septa is to seal size 19 ground glass joints, especially during anhydrous reactions. Drying glassware requires heating to 130ºC or so, and using a glass stopper at that temperature would instantly freeze the joints together. Once you’ve got the septa on you can apply a steady stream of nitrogen while the flask cools to room temperature.
Unfortunately for the original designers of the smurf septa, it leaks like a sieve. The bottom half isn’t an effective stopper on its own, and folding down the top flaps onto a 130ºC flask almost always leads to an exercise in burn management. Worse, even with the flap down the seal tends to be a little loose, and so if you’re using an inert gas balloon you’ll probably need to wrap the whole joint with parafilm. The newer, frilled septa form a better seal (even with the flaps up), and so over the last few years the smurf septa has fallen out of use.
If this outdated septa is so bad, why am I so fond of it? There is one job that it it absolutely perfect at: drying small quantities of precious compounds.
Unless I’m dealing with over a gram of compound, every one of my final products is ending up in a 4mL vial at some point. The vials are cheap (about $0.40 each), air tight, and durable (I can tell from experience that they’ll survive a fall onto linoleum, even if you throw them straight down).
The problem of course, comes when you’re trying to get your compound INTO the vial. Transferring solids from the original reaction flask is likely to leave at least 15% behind, and good luck getting even half of an electrostatic compound past that tiny neck. Transferring dissolved compounds is even worse, as the vials don’t have ground glass joints and so can’t be fitted onto a standard rotovap adapter. Normally, your best option is to leave the vial in the back of your hood and hope nothing happens to it while the solvent evaporates.
Now, here’s where our tragically mis-designed septa really shines. While the bottom portion is a horrible seal for ground glass joints, it fits perfectly onto the screw threads of our vials. Better, the upper flap will form an air-tight seal with your standard size 24 male ground glass joint, allowing easy pairing to rotovap adapters. Stick a nice, wide bore (16G) needle through the top of the syringe, fill the vial no more than 2/3 full, and you have the perfect means of both transferring, drying, and storing that 15mg compound you just spent a week making. To dry the sample under high vacuum simply leave the vial’s cap on loose and storing the vial in a small vacuum chamber or dessicator.
One final word. Because they’re an older design, you may not have a supply of these septa sitting around. As best as I can determine they are item CG-3022-96 from Chemglass’s catalogue, but I make no promises. I managed to find a pack of 100 in one of our teaching labs.