Thursday, May 22, 2008
Artifacts due to One-Way Helium Valve Oscillation
Many NMR magnets use a one-way valve on the helium exhaust to permit the helium gas to vent but not allow air to be sucked into the cryostat. Some of these valves have a mechanical oscillation when helium gas is vented. This oscillation can be seen as an artifact in NMR spectra and is more of a problem immediately after a liquid helium fill. This is illustrated in the figure below for the 500 MHz 1H NMR spectrum of the singlet in ethyl acetate. All three spectra were acquired non-spinning with 1 scan.The left-hand panel shows an artifact-free spectrum acquired immediately before a liquid helium fill with the one-way valve in place. The middle spectrum was acquired immediately after a liquid helium fill with the one-way valve in place. The oscillation "sidebands" are approximately 10 times higher than the 13C satellites. For this particular valve on this magnet, the artifacts are 11.5 Hz from the from the NMR signal and disappear slowly over the course of approximately 24 hours. The right-hand spectrum was acquired immediately after a liquid helium fill without the one-way valve in place. Like the left-hand spectrum, it is artifact-free.