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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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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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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 ! |
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)