Nuclear magnetic resonance insensitivity

A relatively insensitive technique requiring > 5 mg of sample for proton nuclear magnetic resonance (NMR) and > 20 mg for carbon-13 NMR... 

The application of analytical methods to speciation measurements in complicated systems has remained rather limited, despite the considerable technological progress during the past 25 years. The characterisation methods (e.g. spectroscopy, nuclear magnetic resonance) are often limited to the study of isolated compounds at relatively high concentrations. They, therefore, necessitate the prior employment of sophisticated separation and pre-concentration methods which introduce severe risks of perturbation. The trace analysis methods are often insensitive to the chemical form of the elements measured (e.g. atomic absorption, neutron activation). Those which possess sufficient element specificity (e.g. electron spin resonance, fluorescence, voltammetry) still require significant development before their full potential can be realised. 

Nuclear magnetic resonance (NMR) spectroscopy, arguably the most powerful technique in chemistry, is a relatively insensitive method, but in the short time interval between the appearance of the first and second editions of the Encyclopedia of Analytical Sciences, considerable improvements have been made. Consequently, the range of applications that now become possible is greatly expanded as is illustrated here for the two isotopes of hydrogen, tritium ( H) and deuterium ( H) further developments are also anticipated. 

Several methods are available for the determination of total aluminum in biological and other materials. Chemical and physicochemical methods are in most practical situations insensitive and inaccurate X-ray fluorescence is specific but lacks sensitivity neutron activation analysis is complex and subject to interferences, although it is a very sensitive technique. Nuclear magnetic resonance spectroscopy is not very sensitive but useful to get information on speciation [33]. Graphite furnace atomic absorption spectrometry (GFAAS) is the most widely used technique and can produce reliable results, provided that the matrix effects are recognized and corrected. Savory and Wills [19] reviewed chemical and physicochemical methods for the determination of aluminum in biological materials, e.g. X-ray fluorescence, neutron activation analysis, atomic emission spectrometry, flame emission, inductively coupled plasma emission spectroscopy, and AAS. 

The measurement of internal metabolic fluxes is more difficult. The direct measurement of intracellular fluxes is possible with in vivo nuclear magnetic resonance (NMR) spectroscopy. However, the inherent insensitivity of NMR limits its applicability. An improvement over this approach can be found with isotopic tracer techniques [8]. In isotope tracer methods the cells to be studied are provided with a substrate specifically labeled with a detectable isotope (usually or C). The incorporation of label into cellular material and by-products is governed by the fluxes through the biochemical pathways. The quantity and distribution of label is measured and combined with knowledge of the metabolic network to estimate some of the intracellular fluxes. The choices of substrate labeling patterns, as well as which by-products to measure, are guided by careful analysis of the assumed biochemical network. These experiments are usually performed at isotopic steady state so that the flow of isotope into each atom of a metaboHte equals the flux out. For the nth atom of the fcth metabolite the flux balance is [9] ... 

The advantage of nuclear magnetic resonance (NMR) lies in the fact that measurements occur in aqueous solutions, i.e. the most natural form if one considers biological materials. NMR is widely used for structural characterizations of carbohydrates even though the requirements for homogeneous probes and relative insensitivity can pose limitations. The flexibility of carbohydrates sets another factor of limitation and therefore structural and dynamic properties must be determined in a combination of different methods [112]. Conformations of bound carbohydrates... 

A major consequence of the introduction of pulse (FT) NMR spectroscopy has been ready access to C data - limited prior to 1970 by factors which render this magnetic nucleus relatively insensitive to continuous wave methods of recording NMR spectra (1 % natural abundance, and low value of the nuclear magnetic moment compared with that of a proton). C-NMR spectra are generally much simpler than corresponding H spectra. When run under conditions where all couplings to protons are removed (by simultaneous wide-band irradiation of proton resonances), a C-NMR spectrum consists of a series of sharp lines, each of which corresponds to the resonance of a nucleus (or nuclei) of specific magnetic environment. Further, since the chemical shift spread of C nuclei (0-200 ppm) is about 20-times that of protons. 

Because NMR is a very insensitive experiment in terms of being able to detect numbers of atoms (due to the small size of nuclear magnetic moments) compared with many other spectroscopic techniques such as electron paramagnetic resonance, the prime job of the receiver is to amplify and detect the signal without introducing distortion or additional electrical noise (the static in the radio analogy). A comprehensive review of receiver design has been written by Hoult (1978). 

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