Dear members,
I have noticed the following problem while observing 29Si CPMAS on a
Bruker ASX 200 with a 7 mm triple resonance MAS probe (in double
resonance mode, i.e. no Y frequency insert).
1 scan spectrum has noise bursts in 3 or 4 places.
Many scan spectrum has reduced noise bursts in same places (noise does
not average to zero). Therefore noise not random.
When all power is turned way down (80 dB attenuation) noise is
unaffected. Therefore, problem not due to arcing.
When receiver cable is removed from receiver there is no noise bursts -
just clean receiver noise. Therefore, problem not in receiver.
When rotor is stopped, problem persists. Therefore not due to spinning instability.
All cables replaced - problem persists. Therefore problem not due to bad cables.
When proton cable disconnected from probe, problem persists. Therefore, problem
not with proton channel of probe.
When 29Si cable disconnected from probe, noise disappears. Therefore,
problem due to X channel of probe.
When 29Si channel of probe is detuned, noise disappears.
The problem does not occur at other frequencies.
What do you think this problem could be and how can I solve it?
I have run 29Si CPMAS spectra very often in the past and have never seen the problem before.
I know of no new rf noise sources at this frequency.
I took the probe apart and cleaned it but that didn't help.
There are a number of grounding springs to the probe case broken off.
Could that be it?
Dr. Glenn A. Facey
NMR Facility Manager
University of Ottawa
ph (613) 562 5800 ext 6077
fax (613) 562 5170
gafacey_at_science.uottawa.ca
www.science.uottawa.ca/nmr
Received on Mon Nov 03 2003 - 16:00:21 MST