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Hy everyone, I'm having a strange problem when reading the NMR spectrum of a small aromatic molecule in deuterium oxide: all the resonances of the solute molecule are very distorted, as if caused by poor shimming. For example, they show multiple maxima, and are not symmetric. But the problem is that the HDO resonance has a perfectly symmetric Lorentzian lineshape. Could it be a shimming problem, even if the water resonance is perfect?

asked Jul 04 '13 at 06:50

Ruggero's gravatar image


2 Answers:
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yes, I observed such a behaviour before. I guess you are shimming on the lock signal of the solvent? This is the reason why your solvent signal is perfect. But I noticed that the perfect shimming parameters for the solvent not necessarily are the perfect parameters for your solute. In many cases they coincide, but sometimes they don't.

So I recommend that you shim while observing the signals of the solute in the spectrum instead of using the lock signal. On a Bruker machine you would use the "gs" mode to continuously acquire spectra. For this to work well, you should use a small d1 time and maybe a small acquisition time, so that your spectrum is updated rapidly. I found that with repetition times of <1s it is relatively easy to shim that way, as you have a more or less direct response.

After doing this, you will see that your solute peaks look nice while your solvent peak looks distorted.


answered Jul 05 '13 at 09:32

Pascal%20Fricke's gravatar image

Pascal Fricke

updated Jul 05 '13 at 09:59

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There are a couple of obvious reasons why you may see this behaviour:

1) temperature gradient - on older probes at high or low temp, or in cryoprobes. In a cryoprobe there is a small temperature gradient across the sample, due to the cooling effect of the outside of the sample cavity (which is rather cold, in spite of the vacuum space between it and the coil, because the coil is at something like 25K). The effect in cryoprobes is usually rather reprobucible, and the temperature gradient is rather linear. A linear temp gradient looks like a linear chemical shift gradient of the water - but that is exactly what an offset of the z-shim looks like. So as long as you shim on the water, this effect will be compensated on the water signal, but since the shift gradient doesn't apply to the solute peaks, they look like the z-shim is misset. What you can do is a Pascal indicates - shim as normal, and then adjust Z shim in GS mode while observing the lineshape of a sharp signal (for example use a sample of DSS or TSP in D2O). Note the number of units you have to change z by.

On your real samples, it should be possible to shim normally, then just adjust the Z shim by this number of units, and the solute signals should look correct.

2) concentration gradient in the sample (due to poor mixing). This could be really severe, depending on how badly mixed the sample is. In this case the correction you need to apply may be more than just the Z shim.


Have you added extra solvent to the sample at any point? This is how I usually end up with a concentration gradient. If so, try shaking the sample to mix it more.

If not, what probe are you using? If a cryoprobe, then on Bruker I would expect a z-shim correction of about 15 units (depends on probe, VT gas flow and so on).

There are other possibilities, but these seem likr the most likely...


answered Jul 09 '13 at 07:34

Pete%20Gierth's gravatar image

Pete Gierth

Dear Pete, thank you for the answer. Actually, I find it doubtoful to be a temperature gradient problem, since I am working at 300K and with a 5mm BBI 1H-B Z-GRD Bruker probe. I've also tried to mix the sample tube more, but without success... Have you other suggestionts? - Ruggero (Jul 15 '13 at 03:22)

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