What to do if your sample does not dissolve? Can you still record a 1H or 13C spectrum with a high resolution?
Here are your options:
(i)
Have you checked all solvents? Sometimes it pays to try solvents beyond CDCl3, DMSO-d6 and D2O. For instance polypropylene will dissolve at high temperatures in o-dichlorobenzene.
(ii)
You can do solid-state NMR of your dry sample. This is certainly an option, however you should keep in mind that the property of a molecule in a solid is often inherently more complicated than that of a molecule in solution. Effects such as the crystallinity of the sample will have an influence on the NMR spectrum. For instance: the 13C solid-state MAS NMR spectrum of Polyethylene (CH2-CH2)n shows two peaks: one corresponding the amorphous and one for the crystalline components. Furthermore, the quantum-chemical interactions between nuclei are more complicated in the solid than they are in the solution where many of the orientational dependent interactions are described by average values. Spinning the sample at the magic angle of 54.74 degree with respect to the magnetic field will remove many of the angular dependences leading to high resolution-type spectra in the so call MAS technique. This works well for 13C NMR spectra, high resolution 1H spectra of solids can often only be measured with an extreme experimental effort (ultra-high magnetic fields and MAS spinning frequencies in combination with special pulse sequences).
(iii)
Cases, of semi-solid samples are quite common. This would be for instance samples, which can be swelled but not dissolved. Other examples include intact biological tissue. For these samples it is possible to measure high resolution 1H spectra under conditions of slow-medium Magic Angle Spinning. This is done in a special High Resolution MAS probe, which has the option of a lock and a gradient. This probe allows the convenience of automatic gradient shimming (Bruker calls it topshim). Most experiments, which are possible for solutions can be conducted with semi-solid samples in a HR MAS probe. I see great promise of this technique for NMR metabolomics studies of tissue samples from biopsies.
HR-MAS experiments require the addition of some deuterated solvent and a special sample cell, which is inserted into a regular MAS rotor.
Thanks to Dr. Paravastu the GT NMR center operates a HR-MAS probe on our 500 MHz AV3-HD instrument (yoda).