We started having shimming problems with CDCl3 samples recently. With nt=8, the gradient profile is erratic, sometimes very tall with good S/N, sometimes very noisy and short, and sometimes the second profile is much taller than the first.
We found that none of these problems show up with nt=1. Regardless of solvent, consistent profiles are obtained when the experiment is repeated with single scans (d3=0, array nt=1,1,1,1,...). With more than one scan (even nt=2), the results are erratic. We also tried using the higher gain value (26) that we use for CDCl3 on other solvents, with pw decreased to avoid overflow, and found that gain setting is not a factor.
Has anyone seen this problem before? I would appreciate any suggestions for further tests, or ideas about where the problem lies if it is a hardware problem. I'm not even sure if gradient shimming uses the broadband receiver or the lock receiver. Does anyone have a Mercury-Plus 400 console laying around waiting for the junk pile?
Many Thanks,
Neil
Received on Fri Mar 03 2017 - 08:12:38 MST