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.

The Width of Your Water Line - Radiation Damping

The width of the water resonance in a proton NMR spectrum depends critically on the amount of water present. When the concentration of H2O is very low the NMR resonance is very narrow. When the concentration is very high the width is many times greater. The reason for this is that the strong magnetization of the water signal induces currents in the NMR coil which generate magnetic fields which broaden the line. This phenomenon is called radiation damping. The width of the water line is a function of the strength of the water signal which depends on the amount of water, probe tuning, field strength, coil size etc... Radiation damping can also affect the symmetry and phase of the peak. Below are the 500 MHz proton NMR spectra of two samples of H2O / D2O with different concentrations of H2O. In both cases the magnet is well shimmed.