Dylan Stiles has a post up on distilling HMPA, which will be familiar to anyone who's worked with the solvent. The problem is, HMPA doesn't come dry, and it has be be dry to be any good. You can take the wimpy way out and dump a load of dessicant into a fresh bottle, but the only way to be sure is to distill the stuff.
Well, that's one of the problems with it. The other one, as Stiles mentions, is that it's carcinogenic. His advice - not to soak your genitalia in it - is sound. And that prompts me to update an old post from a few years ago here, to get it into the "How Not to Do It" archives. Longtime readers may recall it, but it's worth bringing back:
One fine afternoon in graduate school, I was peacefully advancing the cause of science when one of the guys from down the hall came into my lab. "What's HMPA smell like?" he asked. "Holy (excreta)!" I answered, "You think I know? Probably like it tastes, I guess." He told me that one of our recent postdocs was distilling it, and he was afraid that the smell in the lab was, well, HMPA, odd as that might sound.
I went down the hall to investigate, and came upon the single stupidest distillation rig I've ever encountered. There it was, a two-liter round-bottom flask with a heating mantle on it, boiling and bumping away on the high vacuum line. (OK, fair enough, if you're going to distill the stuff, you might as well get it over with.) On top of this lunking load of toxic solvent was the smallest still head in the group, a tiny little 14/20 short-path job that looked, in that context, to be about the size of the cherry on top of a triple banana split. This thing wasn't even slowing the hot HMPA vapors down much. My friend had a right to be suspicious, because yes, that smell probably was the springtime-fresh aroma of HMPA itself.
Unfortunately, I can't say much about the bouquet of this caricinogenic substance. You'd have to track down our Spanish post-doc and ask him; he was basically showering in the stuff. I stayed out in the hall while I ranted at him, and as he informed me that this was how they did it in Barcelona. "Well, go to Barcelona and do it!" I shouted, looking around to see if there were drops of solvent starting to run down the walls yet.
I left him peacefully distilling away, confident in his technique. Sometimes I wonder what's become of the guy. . .
(Note: no slur is intended on Spanish post-docs - I've worked with lunatics from all over the world, and as far as I can see, none of us are safe. Besides, if there were any country where people didn't make idiotic mistakes, they'd have taken over the world long ago, you'd think. . .)
Update: I should add that I haven't used HMPA myself in at least fifteen years. It's a no-win solvent in drug research. For one thing, it can be quite difficult to remove from your samples, and you have to make sure that it's all gone before you assay anything. If your reaction will only work with HMPA in it, it's as good as dead, because no scale-up group will use it. And if it'll work with something else besides HMPA, well, you should have just used that to start with and saved yourself the trouble.