NMR is now recognized as one of the most powerful spectroscopic tools. The four parameters usually determined in NMR are: Chemical Shifts, Coupling Constants, Spin-Lattice, and Spin-Spin Relaxation Times. Whilst the last three are easily measured in Hz or seconds, the first is subject to some difficulties since chemical shifts are a function of the applied polarizing magnetic field B<sub>0</sub>, and their measured values in frequency units are given by ν = γ/2π.B<sub>0</sub>, where γ is the gyro-magnetic ratio characteristic of a given isotope. Consequently, the experimental values are a function of each instrument, since it is impossible to build instruments with a fixed B<sub>0</sub> and with precision and stability of about 0.1 Hz at 600 MHz, the highest field used at present. Consequently the measured values cannot be compared between laboratories.

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