I have a compound which is only stable in concentrated sulfuric acid and I'd like to get a NMR spectrum so I can deduce the structure. I use a 300 MHz Bruker instrument and the software is topspin.
In such a case, I would not rely on automation but rather do all steps of the measurement manually. There is no real need to do a special set-up. You have to find the lock signal (adjust the lock system for another solvent, D2O for instance, with the "lopo" command, center the signal on the lock screen by changing the Field value, then switch on the lock system with the appropriate button on the BSMS panel), to shim by hand, and then probably correct the tuning and matching settings of your probe because they will differ from settings appropriate for organic solvents. Then you should be able to record a spectrum as ususal.
Maybe you can seek advice from a person who has learned operating such a machine without all the automated features and computer aids - that would help a lot since it is difficult to describe these procedures with words only.
Excuse me Dragos Florin Flesariu for my suggestion. I think your problem is mainly due to H/D exchange, i.e. that you are making deuterated sulfuric acid and maybe also to that your lock frequency changes. Instead of adding a little bit of D2O to the solution, in your case I would suggest you to use an internal capillary filled with a different D-rich solvent (d6-DMSO, d6-acetone, d6-benzene, etc) to lock. I am curious whether this will solve your problem. I am an early-stage researcher, so this is just a hypothetical suggestion ..
There are three good answers here. If you are after a quick 1D proton experiment then you can run unlocked (Clemens' and Wolfgang's suggestions). If you need a longer experiment then you may need to lock the spectrometer (depending on the drift of your magnet). You can do this by adding D2O to your solution (as long as this doesn't affect the stability of your material) or use an external lock as suggested by Debanjan).
Running unlocked may give you a problem with referencing, in which case I would recommend adding some TSP as a reference. ( https://en.wikipedia.org/wiki/Trimethylsilylpropanoic_acid )
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