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 B0, and their measured values in frequency units are given by ν = γ/2π.B0, 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 B0 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.
P. Granger, "Multinuclear NMR Referencing," Appl. Spectrosc. 42, 1-3 (1988)