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Hello, dear NMR-comrades! I work on Bruker AVANCE II 400 MHz NMR-spectrometer and today I had some troubles with wobbing. It was necessary to record 13C solid state spectrum of reduced graphite oxide (RGO) using direct pulses, I mean there was no cross-polarization (BTW, RGO has some conductivity unlike graphite oxide). After insertion of the sample I couldn't tune and match it. I took the sample off and inserted another one but still had a problem with wobb. What was wrong? Could such effects occur due to conductivity of the sample? Thanks.
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Any sample which changes the dielectric constant within the coil will change the proper settings for Tune and Match. Highly conductive samples can change the dielectric constant so much that the Tune and Match adjustments have insufficient range to make the needed correction. A more diluted sample can reduce the range of adjustment required. |
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Any sample which changes the dielectric constant within the coil will change the proper settings for Tune and Match. Highly conductive samples can change the dielectric constant so much that the Tune and Match adjustments have insufficient range to make the needed correction. A more diluted sample can reduce the range of adjustment required. |
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hi alex when u apply a changing mag field along a conducting sample, it generates currents to oppose this changes. It shifts a lot the behavior of the induction coil, ie of the system. Another thing, the mas rotation also represents a large change in magn field along the sample. It will also generate a lot of currents opposing this change in the sample and even break the rotation. See http://www.youtube.com/watch?v=Qy8AGTbmNd4 as a demo. |
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So the KBr/Adamantane sample also cannot tune and match? That could be a problem with the probe if so. If you are able to tune and match and get a spectrum on KBr/Adamantane, I assume RGO have very few protons? That could explain the lack of CP! If you can wobb KBr/Adamantane but cannot wobb your sample then it is likely to be a sample effect such as conductivity and you should do what Tony suggested -- dilute the sample with another solid. |
Conductivity could explain this. If it is not too bad, you can expect to find the tuning dip at lower frequency and very broad, and you may need to begin with a large adjustment of the matching. If that doesn't work, you could try diluting the sample by physical mixing with a carbon-free solid. - Tony Bielecki (Jul 04 '12 at 07:07)
Let's back up a step first. It's not clear from you post. Does the probe tune OK with no sample or with a standard sample like KBr? If not, possisbly there is a problem with the probe. - Kirk Marat (Jul 05 '12 at 09:18)
I tried that time KBr/Adamantane sample, but the result was the same. - Alexander Abakumov (Jul 06 '12 at 11:48)
If you have another probe, even a liquids probe, see if it will tune and match with wobb. If this other probe doesn't tune/match, then you may have a problem in the console. - Kirk Marat (Jul 09 '12 at 09:42)
Luquid's probe tunes/matches perfectly - Alexander Abakumov (Jul 09 '12 at 11:37)