NMR sensitivity

Several problems m basic methodologies have persisted over the 25 years since F-NMR spectroscopy was first apphed to a biochemical question Most hmiting IS that of NMR sensitivity High substrate concentrabons, relative to the naturally occumng biological levels, are required for NMR detection Although most NMR studies use millimolar and somebmes submilhmolar concenbatrons, many bio molecules exist at micromolar or lower levels... 

Electrochemical nuclear magnetic resonance (NMR) is a relatively new technique that has recently been reviewed (Babu et al., 2003). NMR has low sensitivity, and a typical high-held NMR instrument needs 10 to 10 NMR active atoms (e.g., spins), to collect good data in a reasonable time period. Since 1 cm of a single-crystal metal contains about 10 atoms, at least 1 m of surface area is needed to meet the NMR sensitivity requirement. This can be met by working with carbon-supported platinum... 

Although the feasibility of direct probe MS for the analysis of additives in complex polymeric matrices has been demonstrated (Section 6.4), application is limited, difficult and requires above-average mass-spectroscopic expertise. Direct desorption in the MS probe is usually limited to screening of volatile components. Direct multicomponent spectroscopic analysis has other hurdles to overcome (UV/VIS lack of spectral discrimination IR/R functional-group recognition only, with no discriminative power for additives with similar functionalities NIRS unsuitable for R D problems NMR sensitivity). 

The 13C NMR sensitivity can sometimes be a problem, but for the kind of samples studied here the effective concentration of monomer units is several molar which does not place excessive demands on present Fourier transform NMR spectrometers. In addition to the sensitivity of the chemical shift to structure (9), the relaxation of protonated carbons is dominated by dipole-dipole interaction with the attached proton (9). The dependence of the relaxation parameters T, or spin-lattice, and Tor spin-spin, on isotropic motional correlation time for a C-H unit is shown schematically in Figure 1. The T1 can be determined by standard pulse techniques (9), while the linewidth at half-height is often related to the T2. Another parameter which is related to the correlation time is the nuclear Overhauser enhancement factor, q. The value of this factor for 13C coupled to protons, varies from about 2 at short correlation times to 0.1 at long correlation... 

The selective modification of reactive side chains within proteins with compounds containing NMR-sensitive isotopes offers an alternative route to the labeling of proteins. The chemical modification of side chains with reagents labeled with NMR-sensitive isotopes may permit detailed structural and dynamic questions to be addressed, although perturbations of the system caused by the labeling may be of concern in cases where the chemical identity... 

When using NMR as a detector for online separations, additional consideration must be given to how the sensitivity is affected by the movement of nuclei past the detector cell. Aside from the physical hardware setup, the chromatographic and spectroscopic parameters also play a role in the quality of the resulting data. Flow rate, solvent composition, and residence and acquisition times can be optimized to provide optimal results. NMR sensitivity and chromatographic resolution tend to have an inverse relationship with respect to online LC-NMR experiments. By slowing the flow rate, more scans can be acquired for a particular analyte in the flow cell, but... 

Broadening of peaks in the flow system present yet another problem for sensitivity and can mean that even less of the analyte peak is used, even if it would appear narrow enough to be totally contained in the flow cell. Broadening of peaks during Ailing of the cell is much larger for peaks that are narrower than the volume of the cell compared with those that are larger than the cell volume [58]. This can lead to a further deterioration of LC-NMR sensitivity by a factor of 2 for narrow peaks [57]. 

In practice, if the NMR acquisition times are longer than 30 min then on-column diffusion dilutes subsequent peaks, thus reducing the sensitivity [63], Typically, LC peaks will be broadened by 30% and 50% after the flow is stopped for 5 and 10 h, respectively [74], significantly reducing the LC-NMR sensitivity. Such peaks are more efficiently analysed by repeating the chromatography. 

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