NMR probes applications

NMR microscopy systems are built as add-ons to NMR spectrometers. The general technology of an NMR spectrometer is not the subject of this chapter, only the parts that are of particular importance in NMR microscopy applications, such as the magnets, the shim systems, the gradient systems, the NMR microscopy probes and the gradient amplifiers, will be described in some detail. 

The application of NMR to the study of chemical reactions has been expanded to a wide range of experimental conditions, including high pressure and temperatures. In 1993, Funahashi et al. [16] reported the construction of a high pressure 3H NMR probe for stopped-flow measurements at pressures <200 MPa. In the last decade, commercial flow NMR instrumentation and probes have been developed. Currently there are commercially available NMR probes for pressures of 0.1-35 MPa and temperatures of 270-350 K (Bruker) and 0.1-3.0 MPa and 270-400 K (Varian). As reported recently, such probes can be used to perform quantitative studies of complicated reacting multicomponent mixtures [17]. 

Graether SP, DeVries JS, McDonald R, Rakovszky ML, Sykes BD (2006) A H-l/F-19 minicoil NMR probe for solid-state NMR application to 5-fluoroindoles. J Magn Reson 178 65-71... 

Measuring the pressure dependence of the exchange rate constant leads to activation volumes, AV and this technique has become a major tool for the mechanistic identification of solvent exchange mechanisms (8,16,17). In the last 25 years high-pressure, high-resolution NMR probes were developed which allow the application of all NMR techniques described to pressures up to several hundreds of mega Pascals (18). 

Smaller diameter probes reduce sample volumes from 500 to 600 pi typical with a 5 mm probe down to 120-160 pi with a 3 mm tube. By reducing the sample volume, the relative concentration of the sample can be correspondingly increased for non-solubility limited samples. This dramatically reduces data acquisition times when more abundant samples are available or sample quantity requirements when dealing with scarce samples. At present, the smallest commercially available NMR tubes have a diameter of 1.0 mm and allow the acquisition of heteronuclear shift correlation experiments on samples as small as 1 pg of material, for example in the case of the small drug molecule, ibu-profen [5]. In addition to conventional tube-based NMR probes, there are also a number of other types of small volume NMR probes and flow probes commercially available [6]. Here again, the primary application of these probes is the reduction of sample requirements to facilitate the structural characterization of mass limited samples. Overall, many probe options are available to optimize the NMR hardware configuration for the type and amount of sample, its solubility, the nucleus to be detected as well as the type and number of experiments to be run. 

Of particular importance with the use of LC-NMR as an experimental technique is that it is suited for only a limited number of applications in reference to structure elucidation. As will be discussed in greater detail, the sensitivity issues that arise between the amount of compound one is able to load onto a particular chromatographic stationary phase, and hence elute into the flow-cell of an LC-NMR probe, limit what type of structural analysis that can be performed. It is this author s current opinion that most complete structure elucidations of unknown molecular entities are not amenable to LC-NMR. In these... 

The main, and most crucial, part of the experimental set-up is the probe, which has to be pressure-and temperature-stable at the same time and must have a flow cell with an inlet and an outlet to supply and take away the samples. For the development of SFC-NMR probes, one has to keep in mind the aim of the application, i.e. the structure elucidation of unknown compounds. For the investigation of new, unknown substances, the resolution of the probe has to... 

Currently, on-line LC-NMR is just at the starting point for dramatic miniaturization and parallelization. Prototypes of NMR probes with remarkable sensitivity values are currently available in 2002, although the ease of operation of the combined separation-detection system is not guaranteed. Because the design of the current analytical LC-NMR probes is so simple, they can be used in many real-world applications. However, current research areas such as proteomics and metabolomics necessitate the utmost sensitivity, using very small amounts of sample, which can only be achieved by employing capillary probes. 

In 1989, the first applications of multidimensional NMR were applied to humic substances (Buddrus et al., 1989).This study involved the application of 13C detected J-resolved (J-Res) spectroscopy. The study was successful in that it showed multidimensional NMR was applicable to the study of humic substances. However, in 1989 the lack of various modern experiments and the corresponding hardware (mainly probes fitted with pulse field gradients) made applying NMR to humic materials very challenging. In 1997, Simpson et al. demonstrated that the more sensitive inverse-detected NMR experiments were applicable to NOM (Simpson et al., 1997). In this manuscript COSY,TOCSY and HMQC were applied (Simpson et al.,... 

This receptor shows a remarkable selectivity for Mg2+ over Ca2+ under physiological conditions and has found applications in 19F NMR probes and ratiometric fluorescent sensors based on wavelength shifts.[62] In high concentrations, however, both Ca2+ and Mg2+ can be bound. The similarity of fluorescence enhancements with both ions is the result of essentially identical conformational changes produced upon complexation. Each ion-bound state effectively decouples the amine substituent from the oxybenzene unit, so that PET is similarly suppressed. This means that the charge density difference between the two cations is of secondary importance in these conformationally switchable systems. 

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