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Tuesday, April 20, 2010

Background Suppression in Liquids

High resolution NMR probes for liquids may contain parts near the coil consiting of the nuclei being observed. The parts give rise to background signals which can severely affect the NMR data. When observing11B, there is a background signal from boron containing parts near the coil and also the borosilicate glass in the NMR tube containing the sample.

Cory and Ritchey* introduced a very simple, clever method to suppress background signals in 1988. Their method uses a composite pulse, consisting of a 90° and two 180° pulses with appropriate phase cycling, in place of a conventional 90° pulse. The phase cycled composite pulse is essentially a 90° pulse for all spins inside the coil and 0° for all spins outside of the coil. An example of its implementation is shown in the figure below. The bottom traces show the 11B [1H] NMR spectra for a dilute sample of NaBH4 and a "real" synthetic sample on the left and right, respectively. One can see an enormous background signal from both the NMR probe and the NMR tube. In the case of the "real" synthetic sample, the information from the spectrum is difficult or impossible to recover. The top traces show similar spectra acquired using the composite pulse. The only background signal remaining is that from the portion of the NMR tube inside the coil. This pulse sequence (without proton decoupling) is in the Bruker pulse program library called "zgbs". It is not exclusive to 11B.

D.G. Cory and W.M. Ritchey. Journal of Magnetic Resonance, 80, 128 (1988).

5 comments:

Daniel said...

I have tried this experiment but I still get some background signal. May be the Z-restored spin-echo 13C experiment of Xia et al, Magn. Reson. Chem. 2008; 46: 432–435 could be a solution for 11B?

best regards

Glenn Facey said...

Daniel,

Thank you for your comment. I don't believe it is possible to get rid of the 11B signal from the portion of the NMR tube inside the coil. As far as the probe is concerned, this is part of the sample (and therefore "should" be observed).

Glenn

Lora said...

Hi,

I have not tried the sequence yet, but I have been using quartz NMR tubes and still see the same background peak. Do you think the tubes are becoming contaminated over time? I usually process the spectra in the software using a backward LP and the a baseline correction... it seems to work relatively well. I am looking forward to trying out the sequence you describe though! Thank you!

Glenn Facey said...

Lora,

Thank you for the question. If you are using quartz NMR tubes, then the background may be due to the probe inserts near the coil.

Glenn

Anonymous said...

Hello,

Just tried zg x zgbs sequences for 11B in a 600MHz AVANCE III with a BBFO probe.
A probe spectrum (no sample, no tube) showed a reduction of ~100% of the probe background signal after phase and baseline correction.
I also tried this with an empty Wilmad glass tube and it reduced ~70% of the glass signal.

Amazing results from an easy setup. =)

Thank you!!