Thanks for all the responses. We tweaked the VT gas flow and the
temperature of the cooling bath prior to the probe, and this stabilized the
temperature enough to make the user happy.
Several different causes of shimming drift with temperature were suggested:
The effect of the higher temperature on the sample could cause drifts in
shimming. The sample could be evaporating at the higher temperature which
lowers volume of sample (Z2 shim) or causes condensate on sides of NMR tube.
Droplets can form a concentration gradient in the sample, or can contract
the sample meniscus on one side. It was suggest using about 0.2 mL more
sample than normal and positioning the sample so the top is farther from the
sensitive region of the probe or use floating plug. In aqueous solution,
gas bubbles can form, so degassing the sample was suggested.
Another suggestion was to run dummy scans to make sure the system is at
equilibrium and then touch up the z-shims, before collecting data.
Other possible causes were related to the RT-shims and/or probe taking
longer to come to temperature equilibrium. It was suggested that it could
take 30 minutes to several hours for the RT-shims to equilibrate depending
on the VT and body air flow and the AC stability of the room temperature.
Spinning the sample also helps move the VT exhaust away from the probe, so
VT gas flow may need to be adjusted for non-spinning experiments.
There are instrument problems that can cause this effect. The vacuum in the
VT-dewar on the probe could be bad causing the probe electronics to warm up.
RT-shims could be getting too warm and degrading. Also the cryoshims should
be ruled out.
-----Original Message-----
> From: Stephanie Mabry [mailto:mabry_at_umbc.edu]
> Sent: Monday, April 12, 2004 10:57 AM
> To: AMMRL
> Subject: Temperature stability and shimming
Dear AMMRL:
Would a temperature fluctuation of 0.1-0.2 degree be expected to cause a
visible change in shimming?
One of my users is running experiments at 323K and has complained that the
shimming (i.e. lineshape) degrades over several hours. He attributes this
to poor temperature stability. I had considered +/- 0.1 to be good for
temperature control, especially since the temperature reading is only to the
tenth of a degree, but would appreciate any insight from the group.
Thanks,
Stephanie
-------------------------------------------
Stephanie Mabry, Ph.D., Instrument Manager
University of Maryland, Baltimore County
Dept. of Chemistry and Biochemistry
1000 Hilltop Circle
Baltimore, MD 21250
office (410)455-1031
fax (410)455-2608
mabry_at_umbc.edu
http://userpages.umbc.edu/~mabry
Received on Fri May 07 2004 - 16:29:22 MST