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Monday, November 12, 2007

Testing a Magnet for Field Drift

One can test a magnet for drift by measuring a spectrum unlocked with multiple scans using a very long recycle delay. The entire spectrum will shift between scans according to the magnet drift, therefore you should have as many multiples of the spectrum as the number of scans. In the figure below, the drift rate for a 500 MHz wide bore magnet was measured. A 16 scan 1H spectrum was acquired for CHCl3 in acetone-d6 without locking. The recycle delay was 1 hour. The resonance shifts to lower frequency as a function of time. One can also see that the shim currents (either the cryoshims or room temperature shims) have drifted over the course of the measurement accounting for the broadening and multiplicity at later times.



3 comments:

Anonymous said...

Hi there, I have a significant loss of spectrum quality (when locked) for our 500 (like you show here without locking). I dont have autoshim on during acquisition (you dont want this for 2D?). Anyway, the older 400 keeps its lock level perfectly constant without any autoshimming. What is your thoughts on this? (Its all bruker instr.)
Thanks!

Glenn Facey said...

Anonymous,

Here are a few ideas.

You may be having a problem with one of your room temperature shim colis or have a bad connection. You could try shutting the console down, disconnect the shim cables from both the console and the shim set, cleaning the connections, and re-establishing them.

You may be having problems with a cryoshim in which case you will need to call a service engineer.

Your sample could be evaporating over the course of your measurement causing resolution to deteriorate.

Your sample could be changing chemically over the course of your measurement.

The temperature of your sample may be changing over the course of your measurement due to RF heating. Are you using long spin locks or high duty-cycle decoupling?

I hope one of these ideas may help.

Glenn

Anonymous said...

The most common reason of homogeneity drift on a settled (not recently installed) magnet is the bad starting homogeneity of the main coil. If your main coil is not homogeneous enough and requires large values of current in the superconducting shims in order to be corrected then when it drifts the supercon shims will be over-correcting resulting in bad lineshape. The scale of your spectra is ppm, actually fractions of ppm, so any minor change will show, especially on very narrow lines like chloroform's.
Homogeneity drift is far more important than main field drift as it is extremely difficult to correct without compromising the spectrum quality. However it is always played down my manufacturers. I have seen magnets with <1 Hz/hour main field drift that were unusable as the line broadened to a few hundred Hertz in a few hours and also magnets that drift faster than a Porsche on the Autobahn that maintain perfect lineshape over days. Since lock can compensate very efficiently for drifts up to almost 100Hz/hour I would go for a driftsing homogenous magnet without thought instead of a stable wobbly one.