Cross polarization quantitative measure

High-resolution 13C NMR studies have been conducted on intact cuticles from limes, suberized cell walls from potatoes, and insoluble residues that remain after chemical depolymerization treatments of these materials. Identification and quantitation of the major functional moieties in cutin and suberin have been accomplished with cross-polarization magic-angle spinning as well as direct polarization methods. Evidence for polyester crosslinks and details of the interactions among polyester, wax, and cell-wall components have come from a variety of spin-relaxation measurements. Structural models for these protective plant biopolymers have been evaluated in light of the NMR results. 

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 method of checking whether all protons in a sample have Tj (H) s long enough for carbons to cross polarize, is to measure the T. (H) s Independently of carbon and directly through protons. Data of thls sort is presented in Table III. The shortest Tj (H) can be as small as 0.15 ms. Clearly these protons will not be Sble to transfer polarization quantitatively to carbons. Whether these protons are associated with organic as well as inorganic matter has, however, yet to be established. This is important because it is only the organically bound protons which may influence aromaticity measurements. 

Both spin-lattice (motional) and spin-spin processes contribute to TjpCC). Experimental cross-polarization transfer rates from protons in the local dipolar field to carbons in an applied rf field can be used to determine the relative contributions quantitatively. This measurement also requires a determination of the proton local field. Methods for making both measurements have been developed in the last few years [1,2]. For polystyrenes, the spin-lattice contribution to TjpCCO s is by far the larger. This means that the TipCCVs can be interpreted in terms of rotational motions in the low-to-mid-kHz frequency range. 

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 SPEMAS technique has the advantage to be quantitative if the recycle delay is well calibrated as a function of the spin-lattice relaxation times (TO previously measured on each signal. However, CP experiment is not much applied for quantitative aspects because the signal intensities are not only dependant of the total nuclei concentrations but also of their dynamics. In order to get quantitative reliable data fi om CPMAS data, a series of spectra have been recorded in order to exactly determine the match of optimum cross polarization, the proton decoupling power, the pulse width and the delay times. These parameter calibrations are essential before further analyses even if they take a long time. Concerning the spin-echo experiment, the low intensity of signals for few transients excluded precise parameter calibration. Anjnvay, measurements of spin-spin relaxations times (T2) is necessary for reliable quantification of spin-echo spectra. 

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. 

Liquid crystal detectors were developed for the visualization of pesticides. Compounds having liquid crystalline properties around room temperature were incorporated into a porous foil, which was pressed onto the developed plates for several minutes. The compounds on the TLC plates were transferred into the foil where the liquid crystalline structure was disturbed by the pesticides incorporated. Light transmittance changed in the spots of pesticides, which were measured between crossed polarizers to give quantitative evaluation of the spot areas (177). 

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