Many pulse sequences employ pulsed field gradients for coherence selection thereby minimizing or eliminating the need for phase cycling. The routine use of pulsed field gradients has dramatically reduced the data collection times needed for many
2D experiments and therefore increased the throughput and productivity of NMR spectrometers. The field gradient coils in modern high resolution NMR probes surround the
rf coils and are powered by an amplifier in the NMR spectrometer console. When a pulsed field gradient (typically 1 -2 msec in duration) is applied, the sample is no longer in a homogeneous magnetic field. When the gradient is turned off, the system must recover from the disturbance. This recovery is not instantaneous. Pulse sequences typically have delays of 50 - 200 μsec following a gradient pulse to allow for recovery of field homogeneity. The time for recovery after a gradient pulse depends on the design of the NMR probe, the strength and shape of the gradient pulse as well as the shielding between the gradient coils and the shim coils. One can measure the time required for recovery by applying a gradient pulse, and then collecting an NMR spectrum after a variable delay. In the figures below, the gradient recovery time was measured using a console equipped with gradients of maximum strength 50 G/cm, and a narrow bore 500 MHz broadband probe adapted to fit in a wide bore magnet. The duration of the gradient pulses was set to 1 msec. The first figure below shows the proton NMR data for a sample of doped 1% H
2O in D
2O with a line width of approximately 4 Hz with short term recovery times from 1 to 20 µsec.
The top trace shows the data for a rectangular gradient at 100 % strength. The middle trace shows the results for a rectangular gradient of 50% of full strength and the bottom trace shows the data using a sine bell shaped gradient pulse of 100 % strength. One can see that for the rectangular gradients the full intensity of the line is recovered in as little as 10 µsec. When the sine bell shaped gradient pulse is used, the full intensity of the line is recovered in less than 1 microsecond. This faster recovery is the result of the gradual rise and fall of the gradient strength in the sine bell shaped pulse.
A 4 Hz line is not a very sensitive gauge for the measurement of recovery times, so the experiments were repeated for a sample with a line of ~0.3 Hz in width where the line shape could be examined in detail at longer recovery times. The second figure shows the proton NMR data for a sample of 1% CHCl
3 in acetone-
d6 with a line width of approximately 0.3 Hz with long term recovery times from 50 to 800 msec.
The top trace shows the data for a rectangular gradient at 100 % strength. The middle trace shows the results for a rectangular gradient of 50% of full strength and the bottom trace shows the data using a sine shaped gradient pulse of 100 % strength. One can see that in all cases a reasonable line shape is recovered in ~ 400 msec. The shape of the gradient pulse does not seem to influence the time required to recover a good line shape.
3 comments:
How much line broadening are you using when you process these data? Standard manufacturer's parameters usually come with 10 Hz which completely masks all problems. Another important thing also apart from amplitude recovery is phase recovery. What do you see when you expand vertically the bases of the peaks of the 4Hz D2O spectrum?
I also find your second set of figures very hard to believe. Have you tried the experiment with a gradient intensity of 0%? I wonder if you are observing something else instead of gradient recovery. My problem is that it looks exactly the same no matter what.
Anonymous,
Thank you for your comments and questions. In the first figure, I used 1 Hz of line broadening. There are phase errors in the data at the earlier times which are evident in the figure. In the second figure, the distortions in the spectra are the result of the instabilities of the lock signal while recovering after the gradient. There are no distortions if the spectra are run unlocked on the time scale of the figure. Also, there are no distortions if the sample is locked and the amplitude of the gradient is zero.
Glenn
Thanks for the clarification on the first figure. 1Hz line broadening is OK for a 4 Hz line width sample. It is the 10 Hz that does not make sense. Another useful test in such a series of experiments is to record a final spectrum with a 1 sec delay. Sometimes you see that this last one has a bit more intensity due to some very weak eddy currents.
However indeed what you observe in the second figure is not related to gradient recovery but instead to lock over-sensitivity. You can most certainly get better results if you adjust the lock channel time constants, averaging etc.
Another interesting comparison is between a narrow bore and a wide bore magnet. Using the same probe you can usually see better results on the wide bore magnet. The reason is that most of the eddy currents are induced on the metal (usually copper or bronze) of the magnet bore, so if it is further away as on a wide bore system they are weaker.
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