Signal-to-noise ratio in NMR

To improve the signal-to-noise ratio in NMR, the data acquired under similar conditions are added. The signal gains in proportion with the number of scans, while the variance... 

Solid-state 13C NMR spectra of carbon black filled, uncured and sulfur-vulcanised HR were recorded at 22.6 MHz. The line broadening of the filled polymer relative to the unfilled polymer is attributed to incomplete motional narrowing of the NMR lines [53, 54] Incorporation of filler also results in a decrease in the signal-to-noise ratios in the spectra, but fundamentally it does not obscure the qualitative and quantitative nature of the spectra for the moderately cured elastomer systems. 

Inevitably when we record a FID we also record noise at the same time. Some of the noise is contributed by the amplifiers and other electronics in the spectrometer, but the major contributor is the thermal noise from the coil used to detect the signal. Reducing the noise contributed by these two sources is largely a technical matter which will not concern use here. NMR is not a sensitive technique, so we need to take any steps we can to improve the signal-to-noise ratio in the spectrum. We will see that there are some manipulations we can perform on the FID which will give us some improvement in the signal-to-noise ratio (SNR). 

Because NMR is a relatively insensitive technique, a NMR experiment is usually repeated several times (multi-scan accumulation) with the data from successive experiments accumulated in the computer memory in order to improve the signal-to-noise ratio in the final spectrum. It is a common misconception that the statistical noise is destructively superimposed to a constant level the signal intensity of the real resonance signals is directly proportional to the number of scans n while the random noise is directly proportional to the square root number of scans. 

Apodization is the process of multiplying the FID prior to Fourier transformation by a mathematical function. The type of mathematical or window function applied depends upon the enhancement required the signal-to-noise ratio in a spectrum can be improved by applying an exponential window function to a noisy FID whilst the resolution can be improved by reducing the signal linewidth using a Lorentz-Gauss function. ID WIN-NMR has a variety of window functions, abbreviated to wdw function, such as exponential (EM), shifted sine-bell (SINE) and sine-bell squared (QSINE). Each window function has its own particular parameters associated with it LB for EM function, SSB for sine functions etc. 

CPZ base was present in D2O mostly as microcrystallites, as indicated by the necessity of ultrasonication to obtain a sufficient signal-to-noise ratio for NMR measurements. In the solid, then, the base side chain appears to be C -Cp gauche, Cp-Cy trans, as for CPZ HCl in D2O. Ragg and co-workers maintain that all promazines exhibit the above side-chain conformation in aqueous solution, differing in their view from others, and that the tricyclic system stabilizes the observed... 

CW ENDOR is considered in this section pulse ENDOR is dealt with below. The instrumentation is usually based around a computer-controlled CW ESR spectrometer and is commercially available. A radiofrequency coil, capable of handling up to 1 kW poweg is used to introduce the NMR frequencies. The coil is contained in a special resonant cavity. Various modulation strategies are employed to improve the signal-to-noise ratio. In order to carry out the ENDOR experiment, the spectrometer is set for a given line in the ESR spectrum. The microwave power is increased to just beyond the saturation level, and then the selected NMR frequency is swept. Two NMR transitions are observed at the frequencies ... 

Since the nuclei obtain their polarization from the spins, it is the proton spin-lattice relaxation time (T ) which determines the repetition rate of the CP experiment. This circumvents the problem of the long C values normally found in solids. In addition, the C signal shows an enhancement in its intensity, which can be as large as yn/yc = 4. The CP experiment results in both a time-saving and an improvement in the signal-to-noise ratio in the C NMR spectrscopy of solid samples. 

---