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Tuesday, November 27, 2007

How Fast Should I Spin My Solid Sample?

The choice of MAS rate depends on the interaction to be averaged by the magic angle spinning. If you want to average out the chemical shielding anisotropy then you must spin the sample at a rate comparable to or greater than the span of the chemical shift tensor expressed in Hz. Although the spans of chemical shift tensors, measured in ppm, are independent of field strength they are linearly dependent on field strength, when expressed in Hz. As a result, an appropriate spinning speed in one magnet may not be an appropriate spinning speed for the same sample in another magnet. The spectra below illustrate this point. Both are 31P CPMAS spectra of dibasic ammonium phosphate with a spinning speed of 4 kHz. The lower trace was acquired at 11,75 Tesla while the upper trace was collected at 4.7 Tesla. One can see that there are many more spinning sidebands in the spectrum acquired at higher field despite the identical spinning speeds.One would have to spin the sample at 10 kHz in an 11.75 Tesla magnet to get a spectrum comparable to the one acquired with a spinning speed of 4 kHz in a 4.7 Tesla magnet.

6 comments:

Anonymous said...

Can you please explain why you get spinning side bands and why they are increasing with the field strength?

Thank you

Glenn Facey said...

Anonymous,

Thank you for your comment. Spinning sidebands arise because of the time dependance introduced by spinning the sample in the magnet. The sidebands are the result of rotational echoes in the free induction decay. The manifold of sidebands is approximately the same shape as the NMR resonance in the absence of magic angle spinning. The number of sidebands in the higher field spectrum is greater than that in the lower field spectrum when the sample is spun at the same rate because the chemical shielding anisotropy, expressed in Hz, (defining the shape and width of the resonance in the absence of magic angle spinning) is larger at higher field than at lower field.

Glenn

anusree viswanath said...

Could you please explain the dependence of NMR line width on magnetic field strength?

Glenn Facey said...

Anusree,
Thank you for your very good question. The answer depends on what type of NMR experiment is being run and on whether the nuclide being observed is quadrupolar or spin, I=1/2.

If one is measuring a CPMAS spectrum of a spin, I=1/2 nuclide, then the line width may be determined by either the decoupling efficiency or the chemical shift distribution of a particular site. The efficiency of decoupling may be less at higher field if very fast MAS rates are required, leading to broader lines. The chemical shift distribution of a particular site is the same in ppm regardless of field strength therefore broader lines (measured in Hz) are expected at higher field.

Quadrupolar nuclides will give sharper lines at higher field as the line width (and shape) depend on the second order quadrupolar effects which scale inversely with field. Finally, spin I=1/2 nuclides bound to quadrupolar nuclei will also yield shaper lines at higher field because the dipolar coupling between the nuclei is not fully averaged by MAS and the residual coupling observed in the spectrum depends on the second order quadrupolar interaction.

There can also be effects from dynamics which will manifest themselves to different extents at different fields. There may also be effects due to the interference of MAS and decoupling or molecular dynamics and decoupling.

In summary, I guess there is no simple answer to your question but I hope that it is a bit more clear.

Glenn

Anonymous said...

Hello, Glenn,

What determines the number of sidebands? does it indicate the number of components in the sample?

Thanks,
Sharon

Glenn Facey said...

Hi Sharon,

Thank you for the question. The number of spinning sidebands depends on the spinning speed, the width of the NMR resonance (in Hz) in the absence of magic angle spinning and the skew of the chemical shielding tensor. The sideband manifold in an MAS NMR spectrum approximately mimics the line shape of the NMR resonance in the absence of MAS. For example, if the span of a chemical shift tensor with a skew = 0 was 100 kHz and an MAS NMR spectrum was measured using a spinning speed of 5 kHz, one would expect an isotropic center band with 5 spinning sidebands on either side of the isotropic line. There would be fewer sidebands if the skew of the chemical shift tensor was unequal to zero and the sideband manifold would not be symmetric about the center band.

Glenn