Wednesday, April 30, 2008
23Na Background in NMR Tubes.
When we measure NMR spectra, we want to observe the NMR spectrum of our sample only. However, NMR probes, tubes (or MAS rotors) must be made of real materials, so we should expect some NMR background signals for the materials present inside (or near) the coil. The user should be aware of the presence of these signals. Instrument companies expend a great deal of effort to minimize the number and intensity of background signals in their NMR probes (this is one of the reasons they are so expensive). The figure below shows three 23Na NMR spectra taken on a high resolution NMR spectrometer. The top trace is the spectrum of an aqueous solution of NaCl, the middle trace is that of an empty NMR tube and the lower trace is that of an empty NMR probe. One can see that although the NMR probe is free of a 23Na background signal, the NMR tube is not.
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2 comments:
Hi Glenn. Just out of curiosity what field was that spectrum acquired at? I wonder how sharp the 23Na signal would become at higher magnetic fields.
Mayor,
The spectrum was acquired on a 300 MHz spectrometer. Although I have not measured it, I think the background signal may become slightly narrower at higherr fields due to the smaller second order quadrupolar interaction. However, a signaficant broadening mechanism here is chemical shift dispersion due to the amorphous natute of the glass. The broadening from this source would be invariant to field.
Glenn
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