Nuclear magnetic resonance general principles

GC and GC-MS (see Chapter 2), are ideal for the separation and characterization of individual molecular species. Characterization generally relies on the principle of chemotaxonomy, where the presence of a specific compound or distribution of compounds in the ancient sample is matched with its presence in a contemporary authentic substance. The use of such 6molecular markers is not without its problems, since many compounds are widely distributed in a range of materials, and the composition of ancient samples may have been altered significantly during preparation, use and subsequent burial. Other spectroscopic techniques offer valuable complementary information. For example, infrared (IR) spectroscopy and 13C nuclear magnetic resonance (NMR) spectroscopy have also been applied. 

Methods used to obtain conformational information and establish secondary, tertiary, and quaternary structures involve electron microscopy, x-ray diffraction, refractive index, nuclear magnetic resonance, infrared radiation, optical rotation, and anisotropy, as well as a variety of rheological procedures and molecular weight measurements. Extrapolation of solid state conformations to likely solution conformations has also helped. The general principles of macromolecules in solution has been reviewed by Morawetz (17), and investigative methods are discussed by Bovey (18). Several workers have recently reexamined the conformations of the backbone chain of xylans (19, 20, 21). Evidence seems to favor a left-handed chain chirality with the chains entwined perhaps in a two fold screw axis. Solution conformations are more disordered than those in crystallites (22). However, even with the disorder-... 

Laser-induced fluorescence (LIF) has also been utilized as a highly sensitive detection principle for CE [48-51]. However, while the LIF detector is now able to achieve zeptomole (10 21) detection limits, conventional derivatization techniques are inefficient at these exceptional levels [52]. Also, CE has successfully been coupled with mass spectrometry (MS) [53], nuclear magnetic resonance (NMR) [54, 55], near-infrared fluorescence (NIRF) [56, 57], radiometric [58], flame photometric [59], absorption imaging [60], and electrochemical (conductivity, amperometric, and potentiometry) [61-63] detectors. A general overview of the main detection methods is shown is Table 1 [64]. 

This article treats the benefits, possibilities and drawbacks of supercritical fluid chromatography (SFC) and supercritical fluid extraction (SFE) coupled to nuclear magnetic resonance spectroscopy. After a general overview and consideration of the motivation for such techniques, the design of high-pressure flow probes, as well as the principle experimental set-ups, are described. By means of several applications and comparison to HPLC-NMR, the utility of these hyphenated techniques is demonstrated. 

In general, any analytical equipment or procedure used in the field of natural products chemistry and environmental engineering is also helpful in aroma analysis 64,65 The history and principles of such art are described in detail elsewhere and will not be featured here. Gas chromatography (GC), GC-mass spectrometry (MS), and nuclear magnetic resonance (NMR) are the most frequently used techniques along with rather specialized setups such as proton transfer reaction-mass spectrometry66 (PTR-MS) used in retronasal aroma analysis (see Chapters 9.02, 9.06, 9.10-9.11). 

We have written this book with two main purposes in mind. One aim is to acquaint chemists and biochemists with the general principles of crystal structure analysis so that they can critically appraise articles in the crystallographic literature, and extract, with a reasonable comprehension of the precision of the results of the experiment, any structural information they are interested in. The second aim is to make the reader aware of the vast amount of structural information that has resulted from this method of analysis, to inform him or her of how to access the results in the most useful way, and to indicate the manner by which these types of data have enhanced our understanding of chemistry and biochemistry. It is our aim to place the method in context with other methods of structure analysis, such as solution studies (nuclear magnetic resonance and infrared analyses) and molecular modelling. 

Nuclear magnetic resonance (NMR) spectroscopy is an invaluable instrumental technique for the structural determination of all flavonoids including 3-deoxy-anthocyanins. As well as providing information on the chemical environment of each proton or carbon nucleus in the molecule, the technique can be employed to determine linkages among nearby nuclei, often enabling a complete structure to be assembled. The reader is referred to Refs. 44 and 45 for details of the principles of NMR and general interpretation of NMR spectra. 

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