Mixture Resolution in 13C CPMAS NMR

The recycle delay necessary to get the highest signal-to-noise ratio in a multi-scan ¹³C CPMAS NMR spectrum depends on the relaxation properties of the protons in the sample.  The protons in pure solid samples normally belong to a single homogeneous dipolar coupled network.  As a result, all of the protons in the coupled network have a common T₁ relaxation time.  One would expect the same behavior for a mixture of compounds only if the components were mixed at the molecular level.  If the compounds are not mixed at the molecular level, the sample consists of domains of pure materials, each of which has a common proton T₁.  If the proton T₁'s of the domains are significantly different, then one has a means of discriminating between the domains and hence the compounds of the mixture with ¹³C CPMAS NMR data.  The figure below illustrates this principle for a tablet of vitamin C ground into a powder.  The vitamin C tablet consists primarily of ascorbic acid for which the structure is shown in the figure.  The other major solid organic additives are hypromellose (hydroxypropyl methylcellulose), stearic acid (n-C₁₇H₃₅COOH), magnesium stearate and carnauba wax (a complex mixture of C₂₆ to C₃₀ acids, esters and alcohols).  When the tablet is ground up, the powder consists of ascorbic acid domains, stearic acid domains, magnesium stearate domains and carnauba wax domains.

¹³C CPMAS NMR spectra were acquired with a 30 second and a 2 second recycle delay and are shown in (a) and (b), respectively.  One can see that relative intensity of the components in the mixture depends on the recycle delay.  The proton T₁ of the ascorbic acid is obviously longer than that of the other components of the mixture.  The spectra in (c) - (e) are linear combinations of (a) and (b).  The linear combination in spectrum (c) was created such that the ascorbic acid resonances were nulled.  The resulting spectrum is that of only the organic additives. The hypromellose resonances are in the 50 ppm to 110 ppm range.  The aliphatic resonances of the stearic acid, magnesium stearate and carnauba wax overlap in the 10 ppm to 50 ppm range and appear to have similar proton T₁'s.  The linear combination in spectrum (d) was created such that the aliphatic stearic and wax resonances were nulled.  The resulting spectrum is that of ascorbic acid and the inverted spectrum of the hypromellose.  The linear combination in spectrum (e) was created such that the hypromellose resonances were nulled. The resulting spectrum is that of the ascorbic acid with the stearic acid, magnesium stearate and carnauba wax additives.  This combination allows observation of the ascorbic acid with no overlapping resonances from the additives.