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Friday, April 20, 2012

Weak Lock Signals and Distorted NMR Spectra

A good 2H lock signal with a high signal-to-noise ratio is a real advantage for maintaining a stable magnetic field for long data acquisitions and also for shimming the magnet using the lock signal.  Sometimes, however it is desirable to run NMR spectra for samples with only a very small quantity of deuterated solvent and therefore a very weak lock signal.  Such may be the case when one is monitoring a chemical reaction by removing aliquots and adding a drop or two of a deuterated solvent to help with magnet shimming using the 2H lock signal.  Although one may be able to shim a magnet using a very weak lock signal (with difficulty), running the spectrum locked may not be a good idea.  Running a spectrum while locked on a very weak lock signal can lead to distortions in the spectrum.  It is often better to use the weak lock signal to shim the magnet as best you can and then run the spectrum unlocked.  This is demonstrated in the figure below. 
The figure shows two single scan 1H NMR spectra of a sample of acetone (one drop) in CCl4 with a drop of CDCl3.  The spectrum on the left was acquired using the 2H lock and the one on the right was acquired unlocked.  One can clearly see the distortion in the 1H spectrum caused by locking on a very weak 2H signal.

12 comments:

paulaner said...

which is the explanation for that to occur? I mean, the bad spectrum quality while locking a weak signal

Stan Sykora said...

Yes, Glenn, nice entry. People often forget that the lock system is just an electronic circuit. It takes as input the 2D NMR signal, compares it to a fixed desired frequency and generates an output "error" signal driving an additional B0-correction coil located in the shims assembly. If the input is low quality (bad S/N) or if the circuit is defective, or oscillates, or otherwise misbehaves, it will actively produce field noise!!! On supercons, where inherent noise is very low, it is used primarily to compensate long-term drifts and environmental noise arising from the mains (lighting and other currents in the room). But it is a mixed blessing since it usually introduces more noise at frequencies over 100 Hz than what would be present without the lock. And that is normal operation - just imagine what happens when it is defective or when its input is inadequate.

Glenn Facey said...

Paulaner,

Thank you for your question. Please see Stan's comment for an excellent explanation.

Glenn

Glenn Facey said...

Stan,

Thank you very musch for your explanation.

Glenn

Stetty said...

Hi Glenn,

I hope my question(s) relate best to this entry of yours: We encounter difficulties to get proper spectra in CDCl3/CD3OD (let's say 3:1) mixtures in Bruker's automation - what is your advice to answer this problem on rare occations or as a routine job?

Besides maybe necessary additional manual work, can there be any physical reason that a mixture-spectrum likde this can never be as good as a spectrum in a pure solvent?

All the best,

Christian

Glenn Facey said...

Christian,
Sometimes when a solvent mixture is used or a deuterated solvent with more than one deuterium resonance is used, the spectrometer will lock on the wrong 2H resonance and the chemical shift scale on the spectrum will be displaced according to the difference in chemical shift between the the lock resonance and the other 2H resonance. This can be avoided by carefully editing the edlock table.

I'm guessing here but perhaps resolution in the spectrum of a material in a solvent mixture may suffer due to the exhange of solvent shells around the molecule of interest at a rate that would produce line broadening.

Glenn

Anonymous said...

Hi Glenn,

You can usually compensate for a noisy lock by adjusting the lock averaging time. The lock system works exactly as Stan described it but it does not feedback instantaneously. Usually the signal is averaged over a period of time, from either a few milliseconds up to a few tens of seconds in a form of rolling average so such unwanted behaviour is avoided. The averaging time can be defined with some parameters, either more or less precisely depending on the instrument manufacturer. There are cases where people were able to obtain good lock on modern systems with cryogenically cooled probes using tap water! This depends heavily on the ...tap and it is said that for some reasons the water from the taps in Toronto is particularly good for that. Have you tried with Ottawa tap water?
But in any case having a high concentration of deuterium is the best thing to do and you should blame your sample preparation if it does not work and not the instrument;-)

Glenn Facey said...

Anonymous,
Thank you for the information. You are correct. One can certainly improve the quality of the lock signal on a Bruker spectrometer with the loop adjust routine ("loopadj" command, if I recall correctly). I have seen this optimization make a remarkable improvement in spectral distortions when the lock signal is weak.
Glenn

Paul King said...

Run you run a spectrum unlocked, but have the other necessary like relaxation, will running NMR unlocked be quantitative?

Glenn Facey said...

Paul,
If you are asking if a spectrum is quantitative if the lock is not used, the answer is yes, providing that your acquisition parameters are appropriate for quantitative results. The quantitative nature of a spectrum is not dependent on whether or not a deuterium lock is used.
Glenn

Anonymous said...

Glen,

we have, recently, set up non-deuterated solvent choices for our users on our Bruker instruments. We then link those to an edited shim routine (topshim 1H lockoff o1p=x.xx selwid=0.5) with the sweep and lock off. This works well for samples where users are taking aliquots from reactions and looking for quick 1H experiments.

Craig.

Glenn Facey said...

Hi Craig,
Thanks for the comment. We have been doing this at uOttawa as well but have never set it up as solvent options from the lock table.
Glenn