pulsed field gradients

Glenn Facey (GAFACEY@oreo.chem.uottawa.ca)
25 Apr 94 13:13:06 EST

We have on loan from Bruker a bottom of the line pulsed field
gradient accessory for our AMX-500. This accessory runs off the Z
shim of the magnet and uses any standard probe. The maximum gradient
strength is about 2 G/cm which makes it useful for COSY experiments
but not much else. When the unit was first installed we measured
the recovery time after a 2 msec gradient pulse to be about 3 msec
(this was done by applying the gradient pulse, waiting for a period
of time then applying the rf pulse and receiving the signal). When we
ran gradient enhanced COSY spectra using a 2 msec gradient
pulse and a 3 msec recovery time with one scan per slice we get data
sets with very low signal to noise which still contain artifacts.
A little bit of optomisation on the COSY experiment directly led me
to the following conditions which give very good results (little
artifacts and higher signal to noise):

Gradient pulse Recovery time
2 msec 1.6 msec
3 msec 1.4 msec
4 msec 1.0 msec
8 msec 0.2 msec

It is difficult to believe that the recovery time should go down as
the gradient pulse goes up. Can anyone provide a possible
explanation for this? I am wondering if the shape of the gradient
pulses, which are controlled be the preemphasis unit may have
something to do with it. I recommend to anyone receiving such a unit
to optomize their experimental conditions directly on the pulse
programs they will be using. Currently I use 3 msec gradient pulses
with a 2 msec recovery time to record my COSY data.

I am very happy with this addition to our spectrometer. It is now
used routinely in our lab and requires no fiddling around. A typical
COSY takes 4 minutes of acquisition time.

cheers:

__________________________________________________________________
| * |
| * |
| ** Dr. Glenn A. Facey |
| * * NMR Facility Manager |
| * * Department of Chemistry |
| * * University of Ottawa |
| * * Ottawa, Ontario |
| * * CANADA, K1N 6N5 |
| * * ph. (613) 564-7894 |
| * * FAX. (613) 564-6793 |
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