Sensitivity in NMR

Summarize the common methods for enhancing sensitivity in NMR spectroscopy. 

LC-NMR can be operated continuously ( on-flow ) or discontinuously ( stopped-flow ). The optimum flow-rate in continuous-flow NMR is a compromise between best resolution and sensitivity. The sensitivity in NMR measurements has been increased significantly by ... 

The main difficulty in l3C NMR is the low natural abundance of the carbon-13 nucleus (1.108%) and its low gyromagnetic ratio y, which yields a much smaller Boltzmann exponent 2yp0B0ikT than that of protons. Low natural abundance and small gyro-magnetic ratio are the reasons why 13C NMR is much less sensitive (1.59%) than H NMR (100%). A common measure of sensitivity in NMR is the signal to noise of a reference sample, e.g. 1% ethyl benzene in deuteriochloroform. Several methods are available for improving the signal noise in 13C NMR. 

One of the mayor drawbacks is that only volatile and temperature-resistant compounds can be investigated. Gases are magnetized faster than liquids, because they have shorter spin-lattice relaxation times (T ), due to an effective spin rotation mechanism. Therefore, pulse repetition times in flow experiments can be in the range of 1 s and some dozen transients can be accumulated per separated peak. Nevertheless, the sample amounts used nowadays in capillary GC are far from the detection limit of NMR spectroscopy, and therefore the sensitivity is low or insufficient, due to the small number of gas molecules per volume at atmospheric pressure in the NMR flow cell. In addition, high-boiling components (> 100 °C) are not easy to handle in NMR flow probes and can condense on colder parts of the apparatus, thus reducing their sensitivity in NMR spectroscopy. 

Increasing the NMR Signal with Hyperpolarization. A very promising avenue for increasing sensitivity in NMR and MRI is to increase the signal from the molecules being detected. The low radio-frequency energy used for NMR means that... 

Protein concentrations in solution are often limited by solubility and aggregation (and, of course, protein availability) to a maximum of 0.5-5 mM. Today most NMR structural studies are performed at 1-2 mM protein concentrations, although it can be expected that new technologies leading to increased sensitivity in NMR experiments (increased field strength, improved probe design, cryo-probes etc.) will lower the concentration requirements to the 100-mM range within the next few years. 

Increased sensitivity in NMR spectra due to the use of cryogenic probes makes it possible to record triple-resonance experiments at natural abundance in the uniformly N-labelled samples. Such systems have an advantage of negU-gible effect of couplings, which was used by Kupce et to design a... 

The detection and characterization of a productive acyl intermediate by NMR during catalysis with natural substrates at ambient temperatures is not possible at present, because there is an inherent lack of sensitivity in NMR spectroscopy. Therefore, the lifetimes of these intermediates must be extended into the domain of the NMR experiment One approach is to utilize low-temperature cryoenzymological techniques to extend the lifetimes of such intermediates. 

As a measure of sensitivity in NMR spectroscopy, the signal-to-noise ratio is a common and convenient scale. Many NMR spectroscopists are used to a... 

The natural abundance of C is only 1.1%. Furthermore, its sensitivity in NMR spectroscopy (a measure of the energy difference between a spin aligned with or against an applied magnetic field) is only 1.6% that of H. What are the relative signal intensities expected for the H-NMR and C-NMR spectra of the same sample of Si(CHj)4 ... 

Elements of nuclear magnetic resonance speciroscopy Pulsed NMR Fourier transfomi speciroscopy Basis of the Fourier uansromi NMR experimem Experimental coasideraiions for the FT NMR experiment Improved sensitivity in NMR Nuclear spin relaxation... 

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