UNEXPECTED INSTABILITY ON NMR PARAMETERS

NMR chemical shift and relaxation time (T₁ and T₂) are found to vary by up to 10% over a period of several hours after the sample has been inserted into the magnet under stable environmental conditions.

The diffusivity is compatible with a dissolved gas diffusing in or out of solution over a similar time period after its solubility suddenly changed. In the case of D₂O the solubility of oxygen is inferred to decrease by 1.5% in a magnetic field of 11.75 Tesla.

The effects have been observed for dissolved xenon and dissolved air.

There are consequences for accurate NMR measurements. The amount of time in the magnet prior to acquisition of the spectrum should be reported.

Figure. Longitudinal relaxation time (T₁) of xenon (~23 atm) in CD₃OD showing a large change over a period of half a day before stabilizing.