Dear All,
I am working with a varian 500 MHz NMR spectrometer with a direct detection C-H-N probe. Sometimes, after I lock and shim a sample, the lock signal suddenly drops while starting the acquisition. I was wondering if someone could tell me what possible problems I am facing.
Wish you all the bests.
Zahra
If it happens while acquiring HSQC spectra, then it could be of a decoupler Channel issue.
what kind of experiments are you running? is the experiment successful? For example there are some experiments that are conducted with gradient shim, and the lock is lost. e.g. noesy roesy tocsy ecc. in that case is normal. If you are talking about a simple proton experiment I think is not normal.
Thank you all,
Dear Adriano, I am just running simple proton experiment. I lock and shim and run fid scan to check the shim and everything is ok. Then I go to acquisition tabe and click on « Go» and suddenly the lock signal drops down almost 90 percent or even more. What could be possibly wrong?
Thanks again for your kind response.
It might have to do with the deuterated solvent in use. Try another solvent and see if problem persist.
is it possible that you have autogain and when you run the experiments it cut down the signal?
Don't let the spectrometer hear you ask "what could possible go wrong?".
After shimming turn the lock off and see if the lock value changes. Re-lock if it changes. Sometimes the z1-z4 shims can affect the z0 lock value, so the z0 may need some touching up.
On top of some of the other answers already given, especially looking into any potential solvent issues, if all else fails you may need to diagnose the health of the lock transceiver board with your administrator.
Thank you all for the valuable information and answers.
Dear Darrell, unfortunately, there seems to be a problem with the instrument rather than solvents or anything else. How could I diagnose the health of the lock transceiver board? To give you more information, I think something changes the lock tuning and also the hydrogen tuning drastically.
Before digging into your console, try running with a lower lock power. Too much lock power can make the signal jumpy. A little bit of noise in the lock signal from bumping up the gain is fine, but too much power can make the signal go all over the place. I remember reading about this in NMR Spectroscopy Explained by Jacobsen if you want some more background.
Depending on your software, you might be able to see something wrong with the lock signal. I was able to suspect an issue with a lock transceiver once using VNMRJ software for Agilent because the baseline of the lock signal was below the screen threshold. I don't know if that translates over to Bruker or Jeol software, though.
The easiest way to diagnose bad boards is to swap them with a good board and see if it works. Procedures and layout of consoles are very brand and type dependent, so my answer can't get more specific than that. You'll have to dig into your service manual for the details.
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