Sampling from flask under vacuum

[ClearLight]

I was curious as to how one proceeds with sampling reaction mixes for tlc when the reaction vessel is under vacuum?

  Is a rubber septum w/ a screw on adaptor (like thermometer) used for extraction of contents? Stainless steel needle used?   Any wisdom greatly appreciated on how I can monitor my reaction w/out breaking the vacuum.

[Aurelius]

get another evacuated flask with a septum and use a canula (sp?) to transfer a small aliquot from one flask to another.

[ClearLight]

What, i've got a flask in a mantle, w/ a fractionating or reflux column on it, and I get another flask under vacuum and somehow transfer an aliquot over? How do i do that without disturbing the apparatus?  This is not very clear or detailed as to a method here..

[LaBTop]

The Glassware Gallery, Cannulas:

http://www.ilpi.com/inorganic/glassware/cannula.html

  :

""A cannula  is a long thin metal tube that is used to transfer air-sensitive liquids or solutions. Cannulas come in different bore sizes and lengths. A typical cannula is two to three feet long and is made up of 16-22 gauge (.047"-.016") tubing. The ends are pointed so they can be inserted through septa. In many respects a cannula is like a very long double-ended syringe needle.""

You'r right, it is not as simple as it sounds, to take a TLC sample from a vacuum system.
Even an injection needle with a long tip through a rubber stopper in one neck of a multi-necked reaction flask can't be used UNDER vacuum.
However when it would be a very special one with a little valve IN the needle connector it would work.
Then you could suck up a few drops of your reaction mix, close the valve, and take off the injector part with your drops of reaction-fluid in it. LT/

PS: you can silversolder such a tiny valve to a long injection needle, or a long cannula with the top part of a cut injection needle.
However just connecting a glass valve with on one end a long hard silicone tubing (goes through a rubber stopper into your flask) to a cut injection needle (use only top part of needle, valve BELOW it) with a thin piece of vacuum tubing (preferably hard silicone) will do the job also, just connect the sucking part to the cut needle.

[Osmium]

You will have serious problems making that cannula thing work when everything is under vacuum, since it requires a pressure difference.

> Even an injection needle with a long tip through a rubber
> stopper in one neck of a multi-necked reaction flask
> can't be used UNDER vacuum.

Yes of course it can!

I'd use a regular rubber septum, syringe and a needle. Why complicate things? The septum will seal itself when the needle is pulled out, no need for valves and all that.

[LaBTop]

The cannula drawing is just to show what a cannula looks like, don't get the impression that that setup answers your problem.
I always prefer to have a fixed setup, thus my thingamadingy. When you open its valve with the syringe attached, suck up and close the valve, no infringements on your high vacuum have been made.
In fact the same as Osmium does, but no intrusion by alien materials,   .
I hate to loose vacuum when multiple needle sticks have made a crack in the septum, and a tiny vacuumleak occurs, fucking up all future data readings like bp at expected temp, oxidizing precious reactions etc.
I know, Os, antfuckin behavior. LT/

[yellium]

If you really need to take a sample while your vessel is 'under vacuum' there's a simple solution: stick the needle three of four syringes through a rubber stopper, but don't let them make contact with the solution you want to sample. If you want to make a TLC, push the syringes so the needle dips in the solution, take a sample and remove the hole thing through the septum.

[yellium]

Another solution is to use a slightly modified schlenk addition funnel (so that it has a larger valve (or a valve with a larger opening) than usual.) Stick a septum on top, close the valve, push a syringe with a very long needle through the septum, evacuate the  addition funnel, open valve, push the needle through the hole in the valve, take your sample, take out the needle, close the valve and be done with it.

[RoundBottom]

while this wouldn't work for air sensitive material like the cannula, it would be perfect for swapping out a flask with forerun, without having to break the vacuum and restart.

take a standard vacuum adapter.  below the vacuum outlet, put a stopcock.  when it's time to swap out the forerun, close the stopcock, remove the flask, insert new flask, open stopcock.  there will be a small increase in pressure with an ungreased stopcock, and another increase in pressure when the stopcock is opened again (the receiving flask is full of air, right!).

i haven't seen one of these in any catalog, and would love to get one.  let me know if you've seen it in the wild.

[RoundBottom]

wow, that's an industrial looking aparatus. i gotcha on the part numbers, thx.

while i have no doubt that this will perform the job spectacularily, i was hoping for something much simpler.  a basic vacuum adapter with a stopcock insterted after the vacuum connecter seems like an easy thing, but you'd think there would be one available already, so there's probably some limitation i can't see.

[raffike]

I'd suggest to take that 1 liter one,it's costs only 7 bucks more than 500 ml one

[ClearLight]

Standard item, do a search for "fraction cutter" and you'll see the pix i posted of mine.  That works great for distillations, but I was more interested in monitoring reactions to completions... I'll pick up some 14/20 septums and 24/40's and use those for my samples... will probably need a big - ass needle and a glass syringe for the extraction...

[ClearLight]

Syringe needle adapter 24/40 w a septum.. 12 replacement septums available, and needles come in 6-9 or 12" lengths.. glass syringe 1-12cc's are available.  So that's the trick for this now..

THX!

[Osmium]

I prefer those silicon rubber stopcocks/septa, which you can fold over the outside of the glass joint. They can be reused dozens of times.
You don't need glass syringes either. The regular disposable plastic ones are fine even for shit like BuLi.

Those long needles are pimp, once you have them you will use them all the time e.g. when you need a small amount of solvent out of a nearly empty tall bottle with a 20 or 50ml syringe since you can measure the required amount easily.
The needle should be all steel, no plastic parts.

[Lilienthal]

Maybe I missed something... But how should it be possible to remove something from a flask under vacuum by applying vacuum in a syringe? Or are you talking about inert-gas filled flasks?

[Osmium]

Well, what kind of vacuum are we talking about here? Some lame ass slight underpressure or oil pump?