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Monday, January 7, 2008

Shimming Without a Lock Signal

Many people who do liquids NMR are at a loss when asked to shim a magnet without the benefit of a deuterium lock signal. Solids NMR people must do this routinely. To shim a magnet without a lock signal, you must set up the spectrometer to observe a strong FID (usually the proton signal is used but a strong narrow signal from any nucleus will do). The instrument must be setup to acquire scans without adding up the signals ("gs" on a Bruker instrument and "fid" on a Varian instrument). While observing the FID adjust the shims until the FID is as long lived as possible while retaining an exponentially decaying envelope. If multiple signals are present, the carrier frequency may have to be changed to get an FID with the "best appearance".

5 comments:

Anonymous said...

It is possible to shim on protons (without use of lock channel) using either z-autoshimming (quicker) or even 3D autoshimming (takes longer).

Anonymous said...

Autoshimming can be done on deuterium as well with dual probe.

Anonymous said...

Greetings:

I have a question related to shimming a solids probe on the proton signal in water at 300 MHz.

For solids, what are typical linewidths that one should shoot for when shimming using protons in water as opposed to shimming using13C in adamantane?

I am asking because I have been attempting to shim on a HX-4mm MAS probe using the proton resonance in water. Thus far I can only get it down to 40 Hz which I would imagine is too large.

Thanks in advance

Glenn Facey said...

Anonymous,

If you are using pure water, I would not expect you to do much better than 40 Hz due to radiation damping for the water signal. See this post.

http://u-of-o-nmr-facility.blogspot.com/2007/10/width-of-your-water-line-radiation.html

If you use dilue H2O in D2O you should be able to shim down to a couple of Hz.

Happy shimming!!

Glenn

Anonymous said...

Thank you Glenn... I will try what you said and bring the information back. (I am sure it will work though.)