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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.

Tuesday, March 20, 2018

Field Dependence of a Simple Spin System

With the recent re-emergence of low-field NMR spectrometers at proton frequencies of 40, 60, 80 and 100 MHz, many younger NMR users (who have grown up with high-field spectrometers) are encountering more and more second-order spectra. These spectra are observed when the frequency difference between signals is comparable to the coupling between them.  On a 600 MHz spectrometer, 1 ppm in a 1H spectrum = 600 Hz while on a 60 MHz spectrometer, 1 ppm in a 1H spectrum is only 60 Hz.  Unlike frequency differences between signals (in Hz) which depend on the field strength, the coupling between signals (in Hz) is field invariant. Easily interpreted first-order spectra on high-field instruments can be information rich but much more complicated second-order spectra on low-field instruments.  The figure below shows simulated 1H NMR spectra of a fictitious isolated ethyl group as a function of field strength.  The difference in chemical shift between the -CH3 and -CH2- signals is 0.5 ppm and the 3JH-H  coupling constant is 10 Hz.  The spectra are plotted on a ppm scale on the left and on a Hz scale on the right.  At higher fields, one immediately recognizes the familiar triplet and quartet.  At lower fields, the spectra are much more complicated.  The signals are closer to one another (in Hz) and therefore have more second-order character as the frequency difference between signals becomes comparable to the coupling between them.