Resolution During a Liquid Nitrogen Refill

Just as it is not advisable to collect high resolution NMR data during (and shortly after) a liquid helium refill, it is similarly not advisable to collect data during a liquid nitrogen fill. The figure below illustrates this point. A 300 MHz NMR spectrometer was set up to collect an NMR spectrum every minute before, during and after a liquid nitrogen refill. The resolution deteriorates as soon as the one-way valves are removed from the nitrogen cryostat. One can see that the resolution during the fill is much worse than before the fill. Even 60 minutes after the fill, the magnet has not fully recovered the resolution attained before the fill. The last spectrum of the series was collected after re-shimming the magnet and is comparable to the spectrum collected before the fill. The resolution profile during a fill may be different for every magnet. For this particular magnet, it is advisable not to collect high resolution data for at least 90 minutes after a nitrogen refill.

Nitrogen Pressure and Spectral Resolution

Super-conducting NMR magnets are cooled with liquid helium. The boil off rate of the liquid helium is minimized by surrounding it with a high vacuum and a liquid nitrogen cryostat. The liquid nitrogen cryostat may either be vented directly to the atmosphere or it may be maintained at a pressure slightly above atmospheric pressure with the use of pressure relief valves. NMR users should be aware that the homogeneity of an NMR magnet is affected drastically by the pressure in the liquid nitrogen cryostat. The left-hand panel of the figure below shows a well resolved 400 MHz ¹H NMR spectrum of the methyl triplet of ethylbenzene in a well shimmed magnet where the nitrogen cryostat was maintained slightly above atmospheric pressure with pressure relief valves for several days. The spectrum in the right-hand panel was run under identical conditions after the cap to the nitrogen fill port was removed, relieving the pressure in the nitrogen cryostat.