Cross polarization techniques

The problems involved in quantitative analysis using NMR spectroscopy, have been discussed by several authors and it is evident that it still causes a lot of problems as especially pointed out by Hays55 in his excellent review on the subject. Thus in liquid state NMR spectroscopy the quantitative estimation of atoms and groups involves the use of normal analytical method. In the case of solid state NMR spectroscopy, however, the application of the cross-polarization technique results in signal enhancements and allows repetition rates faster than those allowed by the carbon C-13 Tl. Therefore, the distortion of relative spectral intensities must always be considered a possibility, and hence quantitative spectra will not always be obtained. 

Fig. 7. 13CNMb spectrum at 15.1 MHz of the carboxyl carbon in PHBA at 27 °C and 357 °C obtained by cross-polarization techniques. Note the change of width and shape of the CSA pattern... 

Two recent studies have examined the NMR spectra of coal macerals and lithotypes respectively. Retcofsky and VanderHardt (12) reported the aromaticities of the vitrinite, exinite, micrinite, and fusinite from Hershaw hvAb coal using non-spinning cross-polarization techniques. The fa values of 0.85, 0.66, 0.85, and 0.93 -0.96 for these macerals demonstrate clear variations between the materials at a given rank. Gerstein et. al. (13) used carbon-13 CP/MAS proton combined rotation and multiple pulse spectroscopy (CRAMPS) to examine Iowa vitrain (Star coal) and a Virginia vitrain (Pocahontas 4 coal) with aromaticities of 0.71 and 0.86 respectively. 

In order to study the molecular aggregation during the volume relaxation of network epoxies, CP/MAS carbon-13 (natural abundance) NMR was utilized. The Hartman-Hahn cross-polarization technique 129) was used with a cross contact time of 1 mi llisecond for transfer of proton polarization to carbon nuclei. The protondecoupling was achieved at the radio frequency of 56.4 MHz. Carbon-13 14.2 MHz spectra were measured in a 1.4 Tesla magnetic field. Room temperature (23 °C) experiments were performed at 54.7° MAS at 1 KHz. The spinner was constructed using an Andrew-type rotor driven by compressed air. 

OH groups (or coordinated water molecules) connected to the outer or iimer surface of the porous structure of a zeolite or in lattice defects can be detected by the - 29sj cross-polarization technique. Strong intensity enhancements are observed in the CP spectra for the resonances of (TO)3SiOH or (TO)2Si(OH)2 but not of Si(OT)4 environments (2). 

Solid state 13C-NMR spectra of humic substances are usually obtained by cross polarization techniques (15). In this method, relaxation occurs at almost the proton T rate, rather than the carbon Ti rate which can be slower by a factor of 100. Hence, in principle, the problems of long carbon T. s in solution are overcome by obtaining the spectrum in the solid state. However, solid state Tis are... 

There are several other problems associated with quantitation using cross polarization techniques. In determining the aromaticity of humic substances, cross polarization is achieved by applying r.f. magnetic fields at the resonant frequency of both 13C and IH nuclei. By adjusting their relative intensities, contact between the two populations is established and cross polarization occurs between protons and carbons. However, the decay of polarization of the population of nuclei occurs at a rate determined by a time constant T. (H) which is Independent of the rate of cross polarization. 

The cross-polarization technique described above is the basic one most often used in practice [Mid]. Several variations of it exist [Levi, Tegl, Tekl], and other techniques... 

When the nucleus under study has poor abundance, such as for carbon-13, the cross polarization technique is used. This uses a complex pulse sequence to transfer the polarization from a nucleus with high abundance (normally hydrogen, but phosphorus and aluminium are also used) to the second nucleus, enhancing its signal. 

The correlation between theoretical and experimental shielding anisotropies has been most important for 13 C shielding results. Several previously discussed experimental techniques including the liquid crystal and cross-polarization techniques have recently supplied a large amount of data. The theoretical approach, in progressing from diatomics to small polyatomics has also placed strong emphasis on carbon-containing molecules. 

Quantitative analysis is a very important component of pharmaceutical analysis. In a number of the publications describing the use of solid-state NMR, the majority of the work has dealt with qualitative studies with brief references to the possibility of quantitative analysis. Under proper data acquisition conditions, solid-state NMR is a quantitative technique that typically provides sufficient selectivity and sensitivity. An excellent guide to the utilization of magic angle spinning and cross-polarization techniques for quantitative solid-state NMR data acquisition has been outlined by Harris (34). 

---