Sensitivity of the NMR Experiment

300 Daltons with a 400-600 MHz NMR spectrometer, the minimal sample amount is in the nanogram range for and two-dimensional NMR spectra, microgram amounts are necessary. 

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The "decrease of the spin temperature means an increase of population difference between the upper and lower energy spin states and consequently an increased sensitivity of the NMR experiment. From Equation (25), the temperature of dilute spins has been lowered by a factor 7x/y1 h, that is, V4 when X = 13C. This means an increased sensitivity of the FID resonance experiment equal to about 4 for the 13C nuclei. Because the X signal is created from the magnetization of dilute nuclei, the repetition time of NMR experiment depends on the spin-lattice relaxation time of the abundant spin species, protons, which is usually much shorter than the spin-lattice relaxation times of the dilute nuclei. This, a further advantage of cross polarization, delay between two scans can be very short, even in the order of few tens of milliseconds. 

The sensitivity of the NMR experiment is strongly dependent on the magnetic field used for the experiment, which precludes measurements at low field strengths with high sensitivity. A solution to this problem is to make the magnetic field time-dependent either by moving the sample from... 

As a matter of fact, the onset of mobility, as observed from the t /i and Tip (13C) measurements performed on the CHOH-CH2-O and CH2-N groups, occurs in the upper part of this range. This result is consistent with previous assignment of the p transition to motional processes of the HPE sequence. Moreover, the parallel behaviour of the CHOH - CH2 - O and CH2 - N groups, observed in NMR, shows that the crosslink points are involved in the motional processes. However, the temperature of the NMR onset of mobility indicates that the sensitivity of the NMR experiments probes the cooperative motions rather than the isolated ones. This conclusion is in agreement with results obtained by 2H NMR [67]. 

The sensitivity of the NMR experiment is concentration-dependent, and thus at a given amount of sample, molecules with a lower molecular weight (MW) give a more intensive signal. On the other hand, larger molecules usually possess shorter relaxation times which allow more spectra to be accumulated per time increment by using shorter relaxation delays. The work published so far has focused mainly on the characterisation and identification of small molecules up to an MW of ca. 700. However, LC-NMR has been successfully applied to... 

The sensitivity of the NMR experiment depends on the frequency v it follows, to some extent, the 1 /v law of decreasing noise, and therefore higher sensitivity, as the Larmor frequency v and the magnetic field H0 are increased. 

If the approximate magnitude of coupling constants /( Sn, H) is known, polarization transfer pulse sequences such as INEPT or DEPT ° can be successfully applied in order to enhance the sensitivity of the NMR experiments. These techniques, if applied correctly with optimized setting of the spectrometer, normally give Sn NMR signals for which the line widths are mainly governed by natural transverse relaxation processes (see Figs 1 and 2). 

If the extracellular Rb+ concentration changes with time, the solution is more complicated, but we will not discuss this here. If the NMR experiment allows sufficient time resolution, we can mirror radioactive tracer experiments and use the data acquired in the linear part of the exponential curve, i.e. where t < 1 /k, as this eliminates any need to model the passive efflux part of the transport process correctly. For cellular studies at high field this is possible, but for perfused organ studies at lower field the sensitivity of the NMR experiment does not permit this, and we are forced to acquire data over a longer time period and fit equation (85) to the resulting spectral areas. 

Generally the sensitivity of the NMR experiment, excluding population difference, is a function of the gyromagnetic ratio of the observed nucleus increasing the sensitivity of H detection. 

The latter approach increased the sensitivity of the NMR experiment enormously, up to [y( H)/ (X)] = 128, and these experiments (e.g. HMQC, HSQC, HMBC) are now standard (Fig. 2) in all modern NMR spectrometers. A further gain in the performance could finally be achieved by the application of pulsed gradients for these methods (Fig. 3). Other methods of inverse detection or selective excitation can become important in the future. ... 

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