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I need to run NMR on samples of anhydrous ammonia. The boiling point of anhydrous ammonia is -33.34 deg C. In order to be able to have enough time to acquire spectra, I would like to seal the NMR tubes. Once the sample tubes are sealed, the ammonia will build up pressure until it reaches equilibrium and exists as a liquid at room temperature (at approximately 10 atm of pressure). I know NMR tubes exist which can easily withstand these sorts of pressures, but my main problem is in finding a way to seal the tube. I have tried sealing the tubes by melting the glass, but the heating renders the glass weakened and unable to withstand the pressure. How might I be able to seal these samples effectively? Thanks for any help.

asked Feb 23 '12 at 08:34

grant's gravatar image

grant
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To get a good seal you will need a pre-constricted tube and will need to seal it with a hot sharp flame from a glass-blowing torch or similar. The sample tube will need to be under a good dynamic vacuum at the time (connected to a vacuum line), and the bottom of the sample will have to be frozen in liquid nitrogen, of course. With practice, you can get very good seals this way. Move the torch around the constricted part of the tube and let it collapse in on itself due to the vacuum. I believe that this is the way that commercial sealed samples (e.g. test samples) are made. For liquid NH3, you will probably want to use thick walled tubes.

WARNING: While the tube is warming up, treat it as a bomb - just in case you have created a pin-hole leak and have condensed liquid oxygen from the air into the tube. And yes, we have had it happen here. Also, if you are using a solvent with a vapour pressure of 10 bar, you must handle the sample at all times with the appropriate safety equipment. Eye protection at the very minimum.

Your NMR facility manager may also have some opinions on loading pressurized samples into the instrument. I would probably insist on using an old probe that I don't care all that much about.

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answered Feb 28 '12 at 08:14

Kirk%20Marat's gravatar image

Kirk Marat
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updated Feb 28 '12 at 11:39

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You should probably use this high pressure NMR tube, it is rated for pressure up to 13.6 atm http://wilmad-labglass.com/Products/522-PV-7/ It also comes with a swagelok thread so you don't have to flame seal it.

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answered Feb 28 '12 at 14:09

Damodaran%20Krishnan%20Achary's gravatar image

Damodaran Krishnan Achary
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You have to use liquid nitrogen (or other cooling agent) for cooling ammonia (solidifying) before sealing.

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answered Feb 25 '12 at 05:45

Zdenek%20Moravec's gravatar image

Zdenek Moravec
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