I would like to thank Steve Huhn, Robert Peterson, Michael Strain Geoffrey Akien and Paul White for responding to my inquiry and providing some suggestions.
So apparently my iteration steps were not setup properly and eventually caught up to me. Here is a summary of notes pointed out to me for setting up 2H gradshim
- Select a sample that does not produce convection problems. Ideally a DMSO or D2O sample. The sucrose standard might not have enough deuterium signal and a doped water sample might not be using the optimized flip angle due to the different T1 relaxation. These can make the shim maps to be noisier. I used a DMSO sample that I made for Diffusion calibrations.
- Using the gradshim1d2h sequence, look at the gradient profile which should look like something between a Gaussian and a rectangle in profile. Should be continuous and not have big spikes, gaps or offset from center.
- The window size of the iteration steps corresponds to the part of the sample (in the Z direction) that it's trying to shim. Longer samples can use larger window sizes.
- When using multiple iterations in gradhsim, you should start with a small window size and limited number of shims and increase both in later iterations.
- Highest window size that should ever be used is 28 for very long samples (< 50 mm). Smaller window sizes for shorter samples (i.e. Shigemi tubes could be around 16 or less).
- Optimize window sizes by finding the point where a higher window size makes the line shape worse. Do one iteration (say using highz) with a small window size (say 18), and observe the effect on the lock level or the spectrum. Then do one iteration with window size 20, and observe the effect. If the shims improve up to a window size of 22, but get worse at 24, conclude that 22 is the maximum window size that can use for that sample.
- Larger Z orders can handle larger windows: Z6 using 22-24ish, Z5 14-20, Z4 14-20
- A suggested iteration steps setting was [Step #1: z1 and z2 window 16; Step #2: z1, z2, z3, window 18; Step #3 z1 z2 z3 z4, window 20; Step #4 highz, window 24]
- If there was a hardware issue, it might be seen with the 2H flip angle. Check calibrations. For the Fourier probe it is around 60us (mine was ok).
Thanks again AMMRLers.
------------------------------------------
Joel A. Tang, Ph.D
Core Facility Manager - NMR,
Department of Chemistry
Johns Hopkins University
tel: 410-516-7456
nmr.chm.jhu.edu
From: Joel Tang [mailto:joel.tang_at_jhu.edu]
Sent: Monday, June 06, 2016 11:51 AM
To: ammrl_at_ammrl.org
Subject: AMMRL: Gradshim window setting
Hello everyone,
I have a question regarding gradient shimming with Topspin 3.5. I have a Fourier 300 system that is only able to use gradient shimming (tosphim will not work because of the design of the system). A while back gradient shimming was working fine and then suddenly the shims were not being pulled in properly. I would be able to manually shim the spectrum to come close to the specs, but once I execute 1D2H gradshim, all the peaks split and broaden. I am using two iteration steps: Step #1 highz4 window: 24, Step #2 midz window:20. I looked inside the probe and the quartz inserts are still intact and the NMR tubes are not warped or scratched. I tried remaking the shimmap following the standard protocol in "Gradient Shimming: Principles and Practical Aspects" manual from Bruker. I tried with a GdCl3.6H2O doped D2O sample and the sucrose standard sample because I read somewhere that using the lineshape sample as suggested in the manual for 1D2H setup is not good due to convection. Both gradient profiles look good and are not noisy. However, gradshim would still split/broaden the peak. I then decreased the window size to 20 and 15 respectively and gradshim appears to work properly; peaks are not split and line shape improved.
So my question is, what would typically be the cause for a need to change the window size in gradient shimming? Is there a gradient issue that I should be looking for? Preliminary tests with manual shimming saw all shims responsive, however there might have been one or two mid order off axis shims that required large changes (100+) to see some response (sorry don't recall which ones right now). Are there other hardware issues that could cause this problem?
Thanks,
Joel
------------------------------------------
Core Facility Manager - NMR,
Department of Chemistry
Johns Hopkins University
tel: 410-516-7456
nmr.chm.jhu.edu
Received on Wed Jun 08 2016 - 08:56:33 MST