Nuclear magnetic resonance spectroscopic analysis, systems

Infrared (IR) spectroscopy was the first modern spectroscopic method which became available to chemists for use in the identification of the structure of organic compounds. Not only is IR spectroscopy useful in determining which functional groups are present in a molecule, but also with more careful analysis of the spectrum, additional structural details can be obtained. For example, it is possible to determine whether an alkene is cis or trans. With the advent of nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy became used to a lesser extent in structural identification. This is because NMR spectra typically are more easily interpreted than are IR spectra. However, there was a renewed interest in IR spectroscopy in the late 1970s for the identification of highly unstable molecules. Concurrent with this renewed interest were advances in computational chemistry which allowed, for the first time, the actual computation of IR spectra of a molecular system with reasonable accuracy. This chapter describes how the confluence of a new experimental technique with that of improved computational methods led to a major advance in the structural identification of highly unstable molecules and reactive intermediates. 

Improvements in column technology, detector sensitivity and the development of new detection systems, have made possible the routine separation of picomole quantities of nucleic acid components in complex physiological matrices. The very sensitivity of most LC systems, however, which is invaluable in the analysis of biological samples, is often the limiting factor because of inadequate or ambiguous identification methods. Although tremendous advances have been made in the on-line combination of HPLC with spectroscopic techniques [e.g., mass spectrometry, Fourier transform infrared (FT/IR), nuclear magnetic resonance], their application has not become routine in most biochemical and biomedical laboratories. 

To evaluate the importance of GC, we must distinguish between the two roles the method plays. First, GC is a tool for performing separations. In this role, GC methods are unsurpassed when applied to complex organic, metal-organic, and biochemical systems made up of volatile species or species that can be derivatized to yield volatile substances. The second role that GC plays is in the completion of an analysis. In this role, retention times or volumes are used for qualitative identification, and peak heights or peak areas provide quantitative information. For qualitative purposes, GC is much more limited than most of the spectroscopic methods considered in earlier chapters. Thus, an important trend in the field has been in the direction of combining the remarkable separation capabilities of GC with the superior identification properties of such instruments as mass, IR, and nuclear magnetic resonance spectrometers (see Section 27B-4). 

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