University of Ottawa NMR Facility Web Site

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Wednesday, June 24, 2015

Radiation Damping and Pulse Calibration

Radiation damping causes broadening in the NMR resonances of very strong signals (such as the 1H signal of pure water) as a result of currents induced in the coil from the strong transverse magnetization.  Radiation damping can also produce asymmetry and phase irregularities in the affected resonances.  These problems make pulse calibration by the standard nutation curve problematic when very strong signals are used for the calibration.  The left-hand panel (black) of the figure below shows the standard 1H nutation curves for 0.1% H2O in D2O (bottom) and 80% H2O in D2O (top).  In both cases, single-scan spectra with a recycle delay of 30 sec were collected and plotted horizontally.  The pulse was varied from 1 µsec to 24 µsec in steps of 1 µsec.  In the case of 0.1% H2O in D2O, the nutation curve is well behaved and one is easily able to read off the 90°, 180°, 270° and 360° pulse durations.  In the case of 80% H2O in D2O, where radiation damping is a problem, the nutation curve is not well behaved.  There are asymmetry and phase distortion problems which make it impossible to determine the 90° pulse, based on maximum signal height, with any accuracy.  Nor is it possible to determine a reliable 180° based on the first minimum.  The spectra show very little distortion in the vicinity of the second minimum so the 360° pulse can be used reliably to determine the 90° pulse.  The right-hand panel of the figure (red) shows the integrals of the corresponding nutation spectra.  The integrals for both samples behave similarly.  It is clear that even in the case of severe radiation damping, one is able to determine a well behaved nutation curve from the integrals.