After many years of dedicated solids measurements I am trying to bring an Inova 400 back to measuring liquids.
In the intervening time something has happened to how the instrument is responding to locking and shimming that I do not understand. Here are the symptoms:
* After finding Z0... When I attempt to phase the lock I get an oscillatory response. Specifically, after I've found an ideal resting lock phase, when I decrement or increment the lock phase the lock level response initially climbs about 5-8 units, then dips 3-5 units then settles at a nominal response lower than it was.
* The strongest lock level response I obtain is when the instrument is unlocked. When I enabled lock on, the lock level drops about 20 dB.
* When shimming, when I get close to a reasonable shim set I observe a similar oscillatory response while adjusting the z shims. (I'm not sure if similar behavior applies to the non-spin shims currently.)
* I've been able to shim the proton line shape sample down to 0.4/8/15. Which is disappointing, but there is also a broad lump to the left of the isotropic signal. I can adjust the shims and change the shape and location of the lump but it never gets narrower than about 30 Hz and is always to the left of the 13C satellite.
* There have been a couple of times where I've tried to move the lump under the isotropic signal but at that point the shim solution is substantially worse.
Input on what might be causing this irregular lock/shim behavior would be appreciated.
Thank you,
Don
Received on Thu Aug 27 2015 - 02:40:55 MST