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Dear Friends, I've recently encountered a problem with gradient shimming that has never happened to me before. I can't clearly determine what caused it but it certainly didn't involve anybody manipulating with the probe. Here's the symptom: when running the gmapz experiment, the Z-profiles (before and after gradient) look like this:
As you can see the second profile (after the gradient) is distorted at the left edge (more or less severely, depending on a sample - here it's just a tiny "step"). I assume it has something to do with the transverse shims - but I'm not really sure which of them I am supposed to modify. Maybe some of you have experienced this kind of behaviour? Here's a shimmap I've made using 10% D2O:
I wonder if this distortion influences my gradient experiments (which use only axial gradient)? 2D spectra seem to be just fine but I'm afraid of the results from diffusion ordered experiments. I'll appreciate any help! Thank you in advance! D. Kubicki |
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I am unable to see your figures however I am thinking that Improperly calibrated 2H decoupling would cause problem with gradient shimming and a sharp sensitivity drop. Thus, proper application of 2H decoupling requires tuning the 2H channel and calibrating 2H 90° pulse. may be it helps you ! |
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The look of the second profile may indicate either not enough sample in the rf coil region. Make sure that the sample is at the right depth in the spinner. As you indicate, it may be due to the transverse shims. To be quite honest, this doesn't look severe enough to cause a shimming problem and would probably not effect the gradient shimming. Don Frank Open Technologies, Inc. |
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Just saw this post for the 1st time, and see that a month has gone by since you initially submitted it. I'm wondering if you resolved the issue. I've seen this kind of stuff on our spectrometers on a couple of occasions, and at least part of the time either sample solvent issues or probe cleanliness was the case. I think.... Broken glass in the probe can also give odd 2nd spectra, although usually the 1st profile will also be serious distorted as well. The most common problem we've seen are samples that have not quite mixed well. The solvent and solute have formed bubbles or pockets of differing concentrations, making small regions of susc differences. Shaking the sample would at least sometimes clear up the problem. And I think a couple times just cleaning the probe would fix problems in the profile. Either of these possibilities for what happened here? |
Not 100% sure, but looking at your shim map, I think the probe may be slightly off center of the shim stack, or the shim stack moved from the center of the magnet (this can happen when a screw holding the shim stack loosens). - Evgeny Fadeev (Oct 12 '11 at 11:10)