Nuclear magnetic resonance spectroscopy sensitivity

In the case of heterogeneous polymers the experimental methods need to be refined. In order to analyze those polymers it is necessary to determine a set of functions / (M), which describe the distribution for each kind of heterogeneity i This could be the mass distributions of the blocks in a diblock copolymer. The standard SEC methods fail here and one needs to refine the method, e.g., by performing liquid chromatography at the critical point of adsorption [59] or combine SEC with methods, which are, for instance, sensitive to the chemical structure, e.g., high-pressure liquid chromatography (HPLC), infrared (IR), or nuclear magnetic resonance spectroscopy (NMR) [57],... 

Probably, one of the most valuable advances in this field has dealt with the first chemoenzymatic synthesis of the stable isotope-enriched heparin from a uniformly double labelled 13C, 15N /V-acetylheparosan from E. coli K5. Heteronuclear, multidimensional nuclear magnetic resonance spectroscopy was employed to analyze the chemical composition and solution conformation of N-acety 1 hcparosan, the precursors, and heparin. Isotopic enrichment was found to provide well-resolved 13C spectra with the high sensitivity required for conformational studies of these biomolecules. Stable isotope-labelled heparin was indistinguishable from heparin derived from animal tissues and might be employed as a novel tool for studying the interaction of heparin with different receptors.30... 

To determine the nature of the silicon moieties in a polymer, clearly the easiest method would be a technique that provides a direct observation of the silicon atom and meaningful, interpretable information on the atom. Nuclear magnetic resonance spectroscopy tuned to the Si isotope ( Si NMR) is a tool of this nature it can directly probe the state of the silicon atom, and with it one can often readily determine the extent to which Si-O-Si crosslinks (fi-om silanol condensation), have formed. One can observe spectra of silicon-containing compounds either dissolved in a solvent or in the solid state. Liquid-state Si NMR, while the most sensitive, cannot be used quantitatively on heterogeneous systems such a latex formulations. Therefore, one must separate the liquid and solid portions of the latex (without heat, which would promote hydrolysis and condensation) and use the solid residue for the Si NMR experiments. 

The structure and identity of such compounds that are of practical relevance as com-plexing agents may be elucidated unequivocally by both one-dimensional and two-dimensional nuclear magnetic resonance spectroscopy of the isotopes H-l, C-13, and P-31. Sufficiently high concentrations also render possible their quantitative analysis [87-91]. However, because of the low sensitivity, especially of the phosphorus nucleus, problems are encountered with the limits of detection in practical applications. 

Nuclear magnetic resonance spectroscopy provides the most conclusive evidence of both identity and purity [90] but few laboratories are equipped with such a resource and even fewer researchers with the experience to interpret the resulting data. Gas chromatography can be used to assess the chiral purity of derivatives, and mass spectrometry (MS) is a particularly sensitive and accurate measure of product purity. Use of electrospray ioni-... 

Rovnyak D, Hoch JC, Stem AS, Wagner G (2004) Resolution and sensitivity of high field nuclear magnetic resonance spectroscopy. J Biomol NMR 30 1-10... 

Nuclear magnetic resonance is sensitive to the certain nuclei such as H and C. With the help of NMR spectroscopy, these nuclei can be used to identify specific compounds, such as water or polymers. Thus, time resolved measurements using the one pulse H NMR technique allow following the water ingress into the c/p samples (Fig. 3). 

Capillary column gas chromatography (GC)/mass spectrometry (MS) has also been used to achieve more difficult separations and to perform the structural analysis of molecules, and laboratory automation technologies, including robotics, have become a powerful trend in both analytical chemistry and small molecule synthesis. On the other hand, liquid chromatography (LC)/MS is more suitable for biomedical applications than GC/MS because of the heat sensitivity exhibited by almost all biomolecules. More recent advances in protein studies have resulted from combining various mass spectrometers with a variety of LC methods, and improvements in the sensitivity of nuclear magnetic resonance spectroscopy (NMR) now allow direct connection of this powerful methodology with LC. Finally, the online purification of biomolecules by LC has been achieved with the development of chip electrophoresis (microfluidics). 

Lerner L, Bax A (1986) Sensitivity-enhanced two-dimensional heteronuclear relayed coherence transfer NMR spectroscopy. J Magn Reson 69 375-380 Leupin W, Wagner G, Denny WA, Wiithrich K (1987) Assignment of the carbon-13 nuclear magnetic resonance spectrum of a short DNA-duplex with proton-detected two-dimensional heteronuclear correlation spectroscopy. Nucl Acid Res 15 267-275 Levy GC, Lichter RL (1979) Nitrogen-15 nuclear magnetic resonance spectroscopy. John Wiley, New York... 

Solid state nuclear magnetic resonance spectroscopy (NMR), e.g. [107-109]. This technique is sensitive to the local environment of certain nuclei, their mobility and orientation [108]. It provides information about the heterogeneity of polymer blends to c. 5 nm or less (spin diffusion experiments) or c. 0.3 nm in cross-polarization experiments, from which the direct (averaged) distance between two types of nuclei in a sample can be determined [107,108]. Motions of moleuclar groups in a polymer chain can be analyzed and correlations with dispersion areas in the mechanical spectra may be possible [109]. Solid state NMR is not a standard technique at the present time but it is becoming increasingly important. 

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