I'd like to relay my experience with a magnet quench we had at Scripps last
week and my excitement over how nicely the system came back up.
My main reason for writing is to give my justly deserved praise to
Bruker's triple axis gradient shimming method, so if you don't want
to hear what might sound like propaganda (it isn't), please delete
this message now. But as Rich Shoemaker always does, I'd like to
give due credit when it's made my life easier.
---We have a DRX600 that was installed last September. On Wednesday 5/28, one of our postdocs "attached" a magnetic 160 liter nitrogen dewar to the Magnex 14T magnet. (this is the 1st time in almost 3 years I've been here that I've seen one of these non-stainless-steel ones...) After four of us pulled off the dewar (which was on aluminum dolly with wheels), the magnet slow quenched over about 4 hours.
I topped it off with helium at hand, and by next morning, the boiloff rate had come down considerably, and I was hopeful that there was no major damage to the coil or anything.
By Monday morning, Magnex engineer Jeff Seward was in town (fortuitously scheduled to do another 600 install nearby in town), and on Wednesday he energized it to 600.8MHz and parked it there.
Yesterday Jeff entered the cryoshim values he used from last September, and he couldn't tell how good the water lineshape looked because He's not used to looking at the signal apart from his own rig. But the water was 500 Hz at 1/2 height, and 1kHz at the base, which seemed awefully broad to me, since I recall one of our 500's being less than 100Hz water after cryoshimming.
Overnight I ran a gradient field mapping routine to try and remap the homogeneity profile again, but while speaking with Bruker's Tim Kelly this morning, I was reminded that the field profile need not be redone since the upper and shim stacks were not removed and the fieldmap that I already have is still good for the present shimcoils in that particular orientation.
Skeptically, I pressed "RUN" on my gradient shimming routine (w/o going on further with field mapping but using the old field map), and to my amazement, I saw the water go from 500Hz to 10Hz within 3 iterations of gradient shimming. I put in a sucrose water suppression sample and after 3 iterations and touching up z1, I had BETTER water suppression than we had at install time (last September), and the anomeric is split down to the baseline.
This was truly impressive, and my jaws literally dropped for a while.
Back in September, when the system was first installed, the triple-axis gradient shimming also made the install almost a no-brainer. Right after cryoshimming, Tim Kelly did the field map, gradient shimmed on sucrose, then put in a .1% chloroform sample, and within 5 minutes of handshimming, he'd had 4hz/8hz lineshape (.55%/.11%).
I was just surprised at how the whole thing still worked even without having to remap the field profile after the cryoshims were touched (although it makes sense now).
So if you have a lab that's prone to a lot of magnet quenches, I highly recommend the triple-axis gradient shimming setup on your spectrometers.
(by the way, we also have a Nalorac 8mm z-axis gradient probe, which I haven't tried yet, but I expect to have a bit more problem shimming this one because I can only do the axial autoshimming on such probes.)
I'd like to express my thanks and acknowledge the help of Jeff Seward of Magnex for ably bring up the magnet for the 3rd time for our lab and setting the field at exactly where it should be, and thanks also to Tim Kelly of Bruker for pointing out the gradient shimming thing.
John Chung
************************************************************* Manager, NMR Laboratories (619)784-7453 (Office) Dept. of Molecular Biology, MB2 784-7455 (Lab) The Scripps Research Institute 784-9822 (Fax) 10666 N. Torrey Pines Rd. email: chung@scripps.edu La Jolla, CA 92037 http://www.scripps.edu/~chung *************************************************************