Dear members,
I have a Gemini 200 spectrometer (circa 1989) still running (and until
now running well). Perhaps some of you can share some wisdom with me.
During the last liquid helium fill the filling turret was blocked with
frozen air. In order to get the tube out, I had to carefully run warm
helium gas down the tube until I could pull it free. I then ran helium
gas slowly down the turret until I could feel no more ice in the turret
with a flutter tube. I then proceeded to fill the magnet without
incident. After the fill, I now see HUGE "sidebands" spaced at 60 Hz
all in random phase. These are present whether the sample is spinning
or not and whether or not one observes protons or carbon. They are
present when all but the observe cable are disconnected and are present
when all sources of gas are disconnected from the probe. They are not
present when a scan is collected on an empty probe - just clean receiver
noise. The boil off rates for both helium and nitrogen were normal both
before and after the fill.
I have seen a similar problem on another magnet of mine after helium
fills. On that magnet (a 500 wide bore) the problem disappears
approximately 1 day after the fill or when the helium is vented directly
to the atmosphere without a one-way valve. I have always attributed
this problem to an oscillation in the one-way valve.
On my R2D2 magnet of the Gemini 200, I have collected a scan while both
of the helium turrets were completely open and still observed these huge
"sidebands"
Does anyone have any idea what this problem might be and how I can
resolve it?
Cheers,
Glenn
Dr. Glenn A. Facey
NMR Facility Manger,
Department of Chemistry, University of Ottawa
10 Marie Curie, Ottawa, Ontario, K1N 6N5
CANADA
ph. (613) 562-5800 ext 6077
fax. (613) 562-5170
email. gafacey_at_uottawa.ca
web. www.science.uottawa.ca/nmr
Received on Tue May 29 2007 - 10:03:20 MST