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 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.
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.
4 comments:
which is the explanation for that to occur? I mean, the bad spectrum quality while locking a weak signal
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.
Paulaner,
Thank you for your question. Please see Stan's comment for an excellent explanation.
Glenn
Stan,
Thank you very musch for your explanation.
Glenn
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