Set the instrument to normal ¹H NMR operation and load a pulse program as shown above. You have to set:

td	4 k
sw	500 Hz
o1	on resonance of CHCl₃ signal
p1	30° ¹H transmitter pulse
d1	5 s
d2	1 s–1 μs, to be varied
g1	pulsed field gradient, sinusoidal shape on 100 points, duration = 1 ms, strength to be varied
ns	1

First record a normal ¹H NMR spectrum without a gradient, then use a sinusoidal-shaped gradient with approximately 0.1 T/m field strength using d2 = 1 s. The signal should have identical intensity to that of the normal ¹H NMR spectrum. Reduce d2 until the intensity drops significantly. At this point reduce the gradient strength and change the gradient shape to observe the influence of these parameters. For all further gradient experiments use as ring-down delay the shortest possible delay d2 for which the signal is not yet significantly attenuated.

5. Processing

Use standard ¹H processing as described in Experiment 3.1 with an exponential window (lb = 2 Hz).

6. Result

The figure shows a series of signals obtained on an AMX-500 spectrometer with a BGU (10 A) gradient unit and an inverse gradient probe-head with self-shielded gradients. A ring-down delay of 50–100 μs seems to be appropriate for the configuration used. When a rectangular gradient was used, the decrease of the signal was already visible with d2 = 100 μs.

7. Comments

Pulsed field gradients can be generated by gradient coils that are mounted on the shim system or within the probe-head. Recent instruments use self-shielded gradient coils [3], where in the outer part of the assembly a field opposing the one in the inner part is generated. With this design the ring-down problem is greatly reduced. In general it is better to have the coils mounted within the probe-head; however, this may cause difficulties for work at very low temperature. The necessary ring-down delay is very dependent on the gradient shape. If gradients are used within the spin coupling evolution time a rather short ring-down is not necessary, since the spin coupling period is usually longer than the sum of gradient pulse duration and ring-down delay.