Hello Group,
Our Bruker AVIII-HD 300 with a 5mm BBFO probe with ATM is showing a subtle issue that we are not able to fully understand even after a lot of troubleshooting. Hopefully someone on the group can give me some advice/input. Basically the issue is that during 1H tuning (ATMM or ATMA) the wobb response to changes of just the "tune" value shows a significant loss/change in the match impedance. I know that some T/M coupling is normal, but compared to similar probes on our other systems (all AVIII-HDs 2x 400s and 500) the coupling of the 300 is much more significant. On the 300 a 1 MHz change in tuning has the wobb dip rise almost 50% up the display. On our other systems you lose less than 10% moving the tune more than +/- 4 MHz.
Why is this an issue: In ICON automation we are occasionally seeing the system go out on a tangent trying to tune. For about one sample out of 50, the system will spend >30mins while it spins the match value way off optimum. When we stop automation and tune manually, the starting (recorded for the solvent) T/M values are always pretty close and the sample can be tuned correctly. We have not seen any correlation to sample quality, solvent, concentration, X-nucleus value, etc. The sample will manually tune and run fine. It will also auto-tune fine given new starting T/M values.
Deleting the ATM tuning (lookup) files or forcing the motor reindexing does not reduce the frequency of the automation failures. We have done both multiple times. Frequently saving tuning values for different solvents does not help either.
What we have looked at:
Initially I thought this was a probe issue. However, this happens with a second (loaner) probe. In fact the behavior is identical. Out of curiosity I took the 300 probe to our 500 and tried tuning it on that system. The results were what I would expect for a normal probe/system and the way the 500 probe 1H channel behaves.
The Bruker system has an option to externally reference the wobb channel (preamp) with a 50 ohm load (ATMM ext50). When we tune the 300 probe on the 300 console with this option the wobb behaves like it does on the 500 console, i.e., correctly. With external referencing large tuning changes give very little loss to the impedance matching. This of course is not a solution to the automation issue since it requires you to disconnect the probe to take a reference on an external load for each sample. However, it does seem to rule out the probe.
After these observations I (and Bruker) concluded that the fault was in the preamp. However, the system is making S/N specs so I was reluctant to spend >$6k to exchange the preamp for a fault that effects only 2% of the samples run. At least I wanted to be more certain that the issue was really the preamp. Therefore, I obtained and tested a loaner 1H2H preamp module. The second preamp behaves identically to the original preamp and during 1H tuning the impedance match changes significantly with slight tuning adjustments.
So I am back where I started and looking for any advice. I am not sure if there is something specific with my system or if this is a 300 design quirk. I am wondering if there are any Bruker 300 owners out there than can let me know how their systems behave with 1H tuning on a ATM probe.
Thanks for any input,
Perry.
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Perry J. Pellechia
Director of NMR Services
University of South Carolina
Department of Chemistry and Biochemistry
<mailto:pellechia_at_sc.edu>
<http://homer.chem.sc.edu/perry>
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Received on Tue Mar 26 2019 - 07:51:04 MST