I am conducting long NMR experiments at 30°C on samples that contain some volatile compounds and chloroform-d as lock solvent. In order to avoid the loss of the analytes and the solvent, I would like to flame-seal the NMR tube. I managed to do so by using a torch lighter. However I found in some bibliographical sources that, the tube must be placed under vacuum.
My question is : Is it mandatory to place the NMR tube under vacuum when sealing it by flame ?
The vacuum is necessary to remove any oxygen present so that it does not oxidise your sample. You can purge your sample with Nitrogen gas and then seal it so that a neutral environment is created.
As said by Shalini Pandey you could purge it with some inert gas, to ensure that your sample last longer without oxidizing, but since your compounds are volatile, i would recommend for you to seal the opening of the tube while lowering the sample temperature, once I tried to do this without lowering the temperature of the sample and I removed the TMS from the sample with the heat needed to seal it.
Flame sealing is possible but a lot of hassle, I think you would be better off investing in some J Young tubes that way you can seal them with a valve and use them with inert gases etc.
The purging with inert gas prevents the oxidation of the sample in the tube. As Lewis pointed out that sealing can be safely done if the tube is already fused with a ground joint through a constricted opening. We have done similar exercise long ago for air and moisture sensitive organometallic compounds and the spectra were recorded in Germany as we did not have the necessary facilities at that time.
To seal NMR tubes under vacuum, you need to freeze the solution with liquid nitrogen, otherwise it will evaporate. The operation is rather tricky because of the connection between the tube and the pump. As soon as you melt the glass, the hot glass is "sucked in" ; you may have to practice on old/broken tubes. Another problem I encountered with volatile solvents (CH2Cl2) and reactive gases (BF3) is the decomposition of the vapors on hot glass surfaces during sealing. You could see some fumes and colors on the internal glass surface that make an efficient sealing difficult or ineffective. This is a good reason to cool the liquid, but under an inert gas, to avoid humidity condensation and have a "blanket" of inert gas above the liquid. Argon is good (heavier that air) to purge the tubes, but more expensive that nitrogen.
Thank you all for your answers and the valuable information you brought in. It really is helpful.
You can also try Tetrachloroethane-d2 if you want to heat without sealing.
Dear Salem Baroudi many thanks for asking this interesting technical question. Flame-sealing of NMR tubes was quite common in the 1980's and 1990's when chemists wanted to measure highly air-sensitive organometallic compounds. A potentially helpful YouTube tutorial video is available, although it does not particularly deal with NMR tubes:
Flame sealing a sample in a glass tube under vacuum
https://www.youtube.com/watch?v=icZ62A1_dCo
Nowadays we use only screw-cap NMR tubes which can be filled with the sample inside a dry-box or using a Schlenk line. Such NMR tubes are available form various commercial vendors. They are a bit expensive, but using them saves you a lot of time and potential frustration.
Good luck with your work and best wishes, Frank Edelmann
Serum cap for the NMR tubes is another alternative . Remove the air inside the tube in a Glove box and cover it with the serum cap. Of course, the entire operation can be done with Schlenk line as Prof.Edelman has suggested.
I doubt that serum caps in usual materials (rubber...) could be better than normal plastic caps fitting tightly NMR tubes. Rubber(s) absorb a lot of organic solvents and are more sensitive to corrosive compouds.
Dear Salem Baroudi I fully agree with Jean-François Gal in that serum caps or normal plastic caps are most likely not suitable for your purposes. If your samples are highly volatile but not really air-sensitive, I would suggest to use NMR tubes with a stable screw cap. If the samples are also air-sensitive, J Young NMR tubes with an inlet for vacuum / inert gas (as suggested by Lewis Copsey) are certainly the best choice.
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