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Wednesday, March 28, 2018

Appropriate Choice of Presaturation Time

Presaturation is one of the most common methods of solvent suppression.  A long selective low power pulse is applied at the solvent frequency followed by a hard non-selective read pulse (or composite pulse).  Aside from a well-shimmed homogeneous magnet, there are two important parameters required for effective presaturation: saturation power and saturation time.  The selection of saturation power was addressed in a previous post.  With a properly selected saturation power, the appropriate choice for the saturation time depends on the relaxation properties of the solvent and the B1 field inhomogeneity of the probe.  To determine an appropriate saturation time experimentally, one can run a pseudo 2D saturation pulse sequence like the one in the figure below.
This sequence uses a recycle delay, D1, which is the sum of the incremented presaturation time and a resting delay.  Each FID is Fourier transformed but no Fourier transform is done with respect to the incremented presaturation time.  The result of this sequence for a plant extract dissolved in H2O/D2O on a 300 MHz spectrometer using a saturation power of 38.4 Hz (54 dB) and a recycle time, D1, of 5 seconds is shown in the figure below.
A partial proton spectrum is displayed on the horizontal axis with the saturation time on the vertical axis, incremented in 50 msec steps.  Several selected 1D spectra are shown on the right with the vertical scale adjusted such that the water signal at 4.7 ppm is at full-scale. Clearly, resonances of the plant extract at 5.08 ppm, 4.51 ppm and 4.48 ppm are independent of the saturation time, whereas the water signal decays as a function of saturation time.  For saturation times between 0 and ~1.3 sec, the intensity of the water signal follows a decaying sinusoidal curve with positive or negative phases depending on the duration of the saturation pulse.  For the saturation power used in this measurement, the 90° pulse is 6.5 msec therefore the trend observed is not the primary 1H nutation curve but results from sampling the primary nutation curve in 50 msec increments. Since the Nyquist sampling condition is not met with sampling intervals of 50 msec, one observes an aliased nutation curve with a much lower frequency. The overall decay is due to relaxation and B1 inhomogeneity. After ~1.3 seconds, the water signal is saturated and the data are invariant for longer presatutation times.  These data suggest that the minimum saturation time should be set >1.3 sec.

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