Spin-lattice relaxation calibration

Fig. 9. Magnetic field dependence of spin-lattice relaxation rates of acetone embedded in packed samples of calibrated porous glass beads with pore diameter of 75 A for several temperatures. The solid lines are best fits to the theory (66).

The longitudinal relaxation also called spin-lattice relaxation is induced by a quasi-resonant exchange which occurs between the Zeeman energy of the spin system and the thermal energy of molecules which carry nuclear magnetic dipoles. The condition of quasi-resonant exchange of energy is used to calibrate the time scale of relaxation of molecular... 

Lastly, it is commonly assumed that the relaxation rate of all solute atoms is consistent, which is not the case for multiple compounds in a single sample (e.g. metabonomics). Calibrations must be applied to any standard database for identification based on relative rates of relaxation, and therefore intensity of peaks. Different functional groups in the same molecule can have different spin-lattice relaxation rates as would more flexible regions of large molecules. For extremely accurate measurements the calibration of signal loss is also frequency dependent (i.e. distance to carrier position is proportional to intensity variation) due to the profile of the hard pulses. 

The SPEMAS technique has the advantage to be quantitative if the recycle delay is well calibrated as a function of the spin-lattice relaxation times (TO previously measured on each signal. However, CP experiment is not much applied for quantitative aspects because the signal intensities are not only dependant of the total nuclei concentrations but also of their dynamics. In order to get quantitative reliable data fi om CPMAS data, a series of spectra have been recorded in order to exactly determine the match of optimum cross polarization, the proton decoupling power, the pulse width and the delay times. These parameter calibrations are essential before further analyses even if they take a long time. Concerning the spin-echo experiment, the low intensity of signals for few transients excluded precise parameter calibration. Anjnvay, measurements of spin-spin relaxations times (T2) is necessary for reliable quantification of spin-echo spectra. 

For all of the applications outlined above, and many others besides, it is desirable to use NMR parameters which possess an intrinsic temperature dependence in order to measure directly the sample temperature. These measurements can either be performed as a pre-experiment calibration procedure using identical data acquisition parameters as for the actual experiment, or as an in situ measurement using the actual sample. Temperature-dependent NMR parameters include spin lattice (Ti) and spin-spin T2 relaxation times, chemical shifts, dipolar and scalar couplings, molecular diffusion coefficients and net equilibrium polarization. Dependent upon the particular application, each of these parameters has been utilized as an NMR thermometer . 

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