Calibrations pulsed field gradients

In many cases, we find that our sample is concentrated enough to obviate the need to collect more than one scan per tj time increment, yet the minimum phase cycle dictates the collection of at least four scans per h time increment. Fortunately, the use of pulsed field gradients allows the collection of COSY spectra with just one scan per tj time increment through a process called coherence selection. The gradient-selected COSY (gCOSY) experiment often allows us to collect a 2-D ff- ff gCOSY experiment with 256 FlDs (256 tj time increments) in 6 minutes. Another favorable feature of the gCOSY experiment is that it is tolerant of poorly calibrated pulses. 

Figure 3. High resolution proton NMR spectra of cheese, obtained by application of a Hahn spin echo pulse sequence with and without field gradient pulses. Measurements were performed on a Bruker MSL-300 spectrometer, operating at 300 MHz. The field gradient unit used with this spectrometer was home-built and the strength was calibrated to 0.25 T/m, using a 1-octanol sample for which the diffusion coefficient is known at several temperatures.

Selected Examples. - Pulsed magnetic field gradient (PFG) NMR is today a routine method for the determination of self-diffusion coefficients. However, a remaining goal is the improvement of the precision of the method. The best procedure for the determination of accurate diffusion coefficients by PFG NMR is a calibration with a sample of precisely known D value. Thus Holz et al presented temperature-dependent self-diffusion coefficients of water and six selected molecular liquids. The gained accurate self-diffusion data are suited for an elaborate check of theoretical approaches in the physics of molecular liquids. Price et al examined the translational diffusion... 

The application of the zero-quantum dephasing element has been illustrated in earlier chapters within sequences such as TOCSY (Section 5.7.4) and NOESY (Section 8.7.2), so here we consider only the steps necessary for the calibration of the element itself. For the dephasing to perform correctly, one needs to match the frequency profile of the swept pulse with the frequency spread imposed on the spectrum by the simultaneous field gradient, and the calibration sequence of Fig. 10.40 is designed to achieve this. 

Randall EW (1997) A convenient method for calibration of the pulse length in high field gradients using Hahn echoes. Solid State NMR 8 179-183. 

A new pulse technique for counteracting RF inhomogeneity at high fields is reported. The theory behind the method is described in addition to spoiled gradient echo experimental data at 3 T for 3 D brain imaging. The images obtained yield a reduction of the standard deviation of the sine of the flip angle by a factor of up to 15 around the desired value, compared to when a standard square pulse calibrated by the scanner is used. 

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