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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.

5 comments:

Antonius Adamius said...

Was the one way helium valve faulty? Or did you find a work-around?
Thank you

Glenn Facey said...

Antonius,

I don't think the one-way valve is defective as I have seen it with others as well. I think that the higher boil-off immediately after a helium fill is sufficient to make the valve vibrate. As a work-around, I just connect a long latex hose to the open helium vent for a day or two after the fill after which I replace the one-way valve.

Glenn

Anonymous said...

Bruker is trying to sell me a one-way valve that can stay on during a fill (pn z58140)is it worth the extra cost or am I going to see the same artifacts?

Glenn Facey said...

Anonymous,

I do not know the answer.

Glenn

Hugo Azurmendi said...

About the Bruker valve (Z58140) I think are great for helium filling and I can't give any downside (I've have them on 3/4/5/6/ and 700 magnets at VT and FDA). I don't know about artifacts, all I can say is I haven't notice them (we do wait some time before running anything, but haven't done any careful comparison).