Quantitative analysis, cross-polarization

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. 

To understand how the CP/MAS experiment can be applied to quantitative analysis, the magnetization transfer and relaxation processes, which directly affect the signal intensity, must be considered. Cross polarization is mediated by H-13C dipolar interactions, so the magnetization buildup will occur at different rates for different types of carbons. In general, the cross polarization rate (Tcp1) increases with the degree of protonation.21 22 That is, TcP is generally shorter for methylene and methine carbons than for quaternary carbons, Fig. 12.6. Motional modulation... 

Clearly, in order to correctly apply CP pulse sequences for quantitative analysis (or even qualitative analysis), many relaxation processes (Tic, T pH, Till. TCp) must be considered and spectral acquisition parameters appropriately set. While a CP spectrum may be obtained when Tic > 7) 11 3TlpH Tqp, a quantitative CP spectrum requires that the recycle delay is sufficient (on the order of Tm) for the protons to be uniformly relaxed at the beginning of the contact time,25 all proton magnetization spin locked in the rotating frame decays at the same rate (T ph), and the contact time is sufficient to allow complete cross polarization (at least 5 times the longest TCp)26 Except when relative peak intensities are constant and appear to be correct, single contact time measurements should be avoided. Instead, 13C spectra and relaxation times should be measured and complete magnetization curves analyzed. 

A 13C nuclear magnetic resonance (NMR) study in solution and in the solid state has been reported for three thermochromic spirooxazines, 29-31.32 From a quantitative analysis of I3C cross-polarization/magic-angle-spinning (CP-MAS) NMR spectra shielding anisotropies were estimated and a correlation of these characteristics with thermochromic activity was suggested. 

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). 

Garroway et al.(l ) have recently pointed out that the relative intensities of resonance lines of CP/MAS C NMR spectra are proportional to the number of carbons, when the time constant T y for H-13c cross polarization is much shorter than the spin-lattice relaxation times and tJp in the rotation frames of 1h and 13c nuclei, respectively. Since this condition has been found to be fulfilled under our experimental condition(15), such a quantitative analysis as shown in the text can be reasonably carried out for cellulose samples. 

It is worth stressing that cross-polarization kinetics and relaxation processes (mainly proton relaxation) govern C resonance intensities of CP MAS NMR spectra, and therefore the intensities measured in routine experiments cannot be used for quantitative analysis. In order to obtain quantitatively reliable results a series of spectra should be recorded as a function of varied cross-polarization time and repetition time (to determine... 

The cross-polarization technique is very important from the technological point of view. Without this technique solid-state high-resolution C NMR might not have been developed. However, it has no inherent physical significance. Furthermore, it must be noted that CP efficiency is not necessarily the same for the individual carbon nuclei since it depends on their molecular mobility. Hence sometimes one has to carry out the NMR measurement without CP for a quantitative analysis. 

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