Friday, October 5, 2007
The Width of Your Water Line - Radiation Damping
The width of the water resonance in a proton NMR spectrum depends critically on the amount of water present. When the concentration of H2O is very low the NMR resonance is very narrow. When the concentration is very high the width is many times greater. The reason for this is that the strong magnetization of the water signal induces currents in the NMR coil which generate magnetic fields which broaden the line. This phenomenon is called radiation damping. The width of the water line is a function of the strength of the water signal which depends on the amount of water, probe tuning, field strength, coil size etc... Radiation damping can also affect the symmetry and phase of the peak. Below are the 500 MHz proton NMR spectra of two samples of H2O / D2O with different concentrations of H2O. In both cases the magnet is well shimmed.
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26 comments:
just wanted to let you know people actually read your blog and find it useful...although I'm still skeptical about my water impurity - its very very sharp and the highest peak on my spectra. but thanks,
Grad student, uOttawa
Anonymous,
Thanks for your comment. If you are measuring a 1H NMR spectrum of a dilute (and even not so dilute) organic compound in a deuterated organic solvent, it is easily possible for the water impurity to be many times larger than the resonances of your compound and still be sharp. This is particularly true for solvents like DMSO-d6 which can often be quite wet. Remamber that it doesn't take very much water to give a large peak. Broadening due to radiation damping is usually only a problem when the water makes up a significant proportion of the sample.
Glenn
is the width linear?
Charlie,
Linear with what?
Glenn
Backlinked to this post from NMR Wiki forum! Thanks for the starter post!
-Evgeny.
I believe that your explanation is not accurate. 100% D2O will give you the 2nd graph with a sharp peak. The 0.1% of H20 that you add is completely overwhelmed by the D20 peak. This is to say that the 2nd graph bear no relation to the 1st graph.
The wide peak in the 1st graph may not be all due to Radiation Damping. H20 forms clusters of varying sizes (a water cluster with 200 molecules is quite common), and these coupled protons will show spin-splitting shift. We have large number of these shifter peaks and they are blurred into a very fat peak.
Humble Voice.
Thank you for your comment. Respectfully, I must disagree with you on both points in your comment. For the first point, 100% D2O would not give any proton signal at all. For the second point, while you may or may not be correct about the formation of clusters (I really don't know), the motion and exchange of water molecules in and out of the clusters would be very fast on the NMR time scale. In this case, I am certain that the broadening is due to radiation damping as the signal can be made sharper by detuning the NMR probe.
Glenn
Hi Glenn,
Thanks for your reply. I just did a 100% D20 sample on our 500MHz NMR, and I am seeing a peak with abundance reading of ~80 and a shift of around 2.3 ppm
I also did some water samples with 5% D2O. These give much wider peak, half-width ~9Hz, abundance reading ~7, same shift of around 2.33ppm.
I am new to NMR and I am not sure what are the settings used by the technician. How would you interpret this result? Thanks,
Chut
Humble Voice,
If it was possible to buy 100.000% D2O, you would (of course) observe no proton spectrum as there simply would be no protons in the sample. The "100%" D2O you are using is likely 99.x% and the signal you observe is HDO. It should come at approximately 4.8 ppm. Could it be that your spectrum is not referenced correctly? I am not sure what you are reporting when you claim an "abundance reading of ~80". The signal for the HDO in your 5% D2O sample may be broader due to radiation damping or shimming.
Glenn
Hi Glenn,
Thanks for the post. Pretty interesting.
What if one is working at very low field? Is this an issue for radiation damping?
Mark
Mark,
Radiation damping is not usually an issue at very low magnetic field strengths.
Glenn
Hi Glenn,
I have a sample that gives a single proton signal, and found out that when I have 90% H2O / 10% D2O there are "mountains" of peaks around the water signals, which do not show up in 100% D2O. Through reading your posts, I assume this might be a radiation damping issue? It is not about the width of the water peaks, but of the presence of wide peaks at both sides of the water signal at +/- ~1 ppm.
I tried presaturation and it did suppress the water peak, but not the others (which again, are absent when using 100% D2O). I've always worked on Bruker, but this is happening on a 500 MHz Varian and I'm only aware (by now) of the PRESAT sequence.
Mike,
What is your sample?
What field are you working on?
What is the width of the water line?
Could you be saturating the receiver?
Glenn
Just wanted to thank you for the great posts.
Your explanations are clear and very helpful.
Hi Glen,
We had a sigmoid shaped curve in inversion recovery expweriment on pure H2O. We knew that it was radiation damping. Is there any pulse sequence to remove RD effect?
best regards
Istvan Banyai of University of Debrecen, Hungary
Istvan,
I am not aware of any inversion recovery experiments designed to avoid radiation damping.
Glenn
https://arxiv.org/pdf/1408.2457.pdf
Characterization and suppression techniques for degree of radiation damping in inversion recovery measurements
Hi, first of all I want to thank for the Blog, it is really useful.
If I have a solution of a peptide (OXT) 10 mM in 85% H2O, why the few signals that I detect of the peptide have a lot of noise? it has to do with the ADC?
Thanks again. Leandro
Leandro,
If you are not suppressing the water signal, then the receiver gain must be set to accommodate the very intense water signal. This is not optimum for achieving the best signal-to-noise-ratio for your much smaller peptide signals. See this link https://u-of-o-nmr-facility.blogspot.com/2013/02/receiver-gain-and-signal-to-noise-ratio.html
Glenn
Glenn, excellent post as always.
In addition to using a inversion recovery sequence with a gradient during the variable delay it is also important to use the area and not the height of the peaks during analysis. During detection RD will be active and broaden the lines more when signal intensity is high compared to spectra obtained near the zero passage.
Clemens
Thank you so much, Glenn for all the interesting notes!.
I am actually tryng to shim a CPMAS refurbished probe starting from zeroshim. I used H2O to start and I reach, as you wrote a signal of about 50Hz (not completely symmetric) then I moved to adamantane but the 13C spectrum is very bad and it seems quite insensitive to shimming.
to tell the truth I also tried with the shim file of another analogous probe, it is a little bit better but still very bad.
Do you have any further suggestions before thinking that it is a problem of the "refurbishing"?
EMS,
In my experience, I have been able to shim commercial CPMAS probes by using only the lower order shims. You may have better luck starting with a 10% H2O/D2O solution so that radiation damping is not so much of an issue. It is extremely important not to have any bubbles in the rotor. This should produce a much sharper water resonance. Once this is done, insert a rotor with adamantane spiked with KBr. Set the magic angle with the 79Br signal of KBr. Set up a 13C CP acquisition with a spinning speed of about 2 kHz, contact time of 5 msec, recycle time of 4 seconds and acquisition time of 100 msec. With a strong CP signal, you should be able to shim it down to 5-10 Hz with the Z, X, Y, XZ, YZ, XY, X2-Y2 and Z2 shims.
Glenn
I just came across your blog page.
I am interested in understanding the size of water clusters using O17 NMR on a variety of relatively pure water sources including how treatment affects the size of these water clusters.
There is evidence to suggest that the size of the water cluster can be correlated with the width of the peak obtained eg rainwater, tap water, ionised, THz all show significant differences of 200 Hz to as low as 8 Hz.
Do you have any experience with this type of studies?
Thank you.
Unknown,
I have no experience correlating the width of the 17O resonance with water cluster size. Any changes in 17O line width are certainly not due to radiation damping. The width of the 17O resonance would depend inversely on the T2 relaxation time. The relaxation is dominated by the quadrupole interaction which in turn depends on the quadrupolar coupling constant. The quadrupolar coupling constant depends on the time averaged symmetry around the 17O which may be related to the cluster size.
Glenn
Thank you Dr Glenn for your response. Besides what I have come across in the literature about using the O17 line width, have you used or come across other methods to estimate the water cluster size?
Regds
Dr Patrick
Patrick,
No, I have not.
Glenn
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