Slight mistuning of a cryogenic probe significantly perturbs the water 1H precession frequency.

A shift of the water proton precession frequency is described that can introduce errors in chemical shifts derived using the water signal as the chemical shift reference. This shift, f(s), arises as a consequence of radiation damping when the water proton and detector circuit resonance frequencies differ. Herein it is shown that experimental values of f(s), measured as a function of detector circuit tuning offset for 500 and 900 MHz cryogenic probes, are in good agreement with theory. Of importance is the fact that even a small degree of mistuning, which does not significantly impact the performance of a pulse sequence, introduces chemical shift errors of +/-0.03 ppm, that negatively impact many types of experiments. A simple remedy that attenuates the frequency shift is presented.