How is RF in-homogeneity in NMR addressed?

[Lattitude]

IN NMR, the sample is placed in the probe and is excited by the coil using RF fields. But, errors creep in due to the inhomogeneous nature of the B1 field and also the errors in the static magnetic field(which are taken care of using shim coils). In some of the research papers I read, they just describe that the pulses they design are robust to the RF in-homogeneities in the B1 field. Is it that this cannot be addressed in an analytical way? Moreover, what does one mean when one speaks about the inhomogeneity? Is it the variation in the strength of the field or the spatial variation of it or both?

 

[marcusl]

IN NMR, the sample is placed in the probe and is excited by the coil using RF fields. But, errors creep in due to the inhomogeneous nature of the B1 field and also the errors in the static magnetic field(which are taken care of using shim coils). In some of the research papers I read, they just describe that the pulses they design are robust to the RF in-homogeneities in the B1 field.

I'm not sure which pulses you are referring to. There is some compensation inherent to echo experiments, for example. A CPMG pulse sequence will refocus echoes after every 2n*(pi/2) pulse. You can convince yourself that the echo refocuses even in regions of the sample where B1 is not perfectly n*pi/2, that is, where the magnetization vector is not rotated perfectly into the transverse plane. There is a loss of SNR, however, resulting in an exponential decay of subsequent spin echoes that decays more rapidly than expected from conventional relaxation processes.

Is it that this cannot be addressed in an analytical way?

Yes, there are many analyses available. Here's one I found on the web
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiu_OO0jLvUAhVB0oMKHZNyCmcQFggnMAA&url=http://www.springer.com/cda/content/document/cda_downloaddocument/9783642163067-c1.pdf?SGWID=0-0-45-1067939-p174037661&usg=AFQjCNERqaj7rokNwBaaK6NhMQl3xWbnEA
You'll find lots of other material.

Moreover, what does one mean when one speaks about the inhomogeneity? Is it the variation in the strength of the field or the spatial variation of it or both?

Spatial. RF amplifiers and digital signal generation are pretty good these days.

 

[Lattitude]

Thank you very much for your reply. I would also like to know how much will be the influence of this RF inhomogeneity in experiments like various 1D and 2D NMR experiments. Is it that its effect can be reduced significantly reduced by some mathematical methods applied to the observed spectrum or any pulse generation mechanisms?

 

[marcusl]

This is much too broad a question. I suggest that you do some reading and research. If you have a specific question, we can attempt to answer it.

 

[Mike H]

The effect of RF inhomogeneity will depend on how you contend with the problem. If you use a pulse sequence intended to compensate for imperfections along the height of your sample (for a typical high-res solution NMR probe) such as this classic one (link to a PDF), you should be getting data from the nuclei that underwent the spin choreography you had in mind as the ones that didn't were filtered out. You could also attempt to quantify the inhomogeneity as is done in this article and attempt to correct for it, where they mention that the severity of this issue can depend on the probe (especially if one is using cryogenically cooled probes) and can be reduced by using a volume-restricted sample, among other things.