Basics of NMR
Once the instrument settings and the experimental parameters have been optimized, the acquisition is carried out in the best conditions. The resulting signal can be processed: it is necessary to proceed to the step of adjusting the data processing parameters of the acquired NMR signal.
- Zero filling
- Apodization if necessary (sensitivity, resolution)
- Fourier transform
- Phasing if possible
- Baseline correction
In general TD is not a power of 2, which means that the FFT cannot be applied (restriction of the use of the Cooley-Tuckey algorithm). We therefore choose to add a number of points (identically null) to the TD points sampling the FID in order to achieve the required condition by reaching the first power of 2 according to the value of TD. These points do not disturb the FID if and only if AQ has been properly selected so as not to truncate the acquired signal. These identically null points are not acquired, so they do not increase the duration of the experiment. On the other hand, since the spectrum consists of SI points > TD, they mechanically increase its resolution.
The higher the SI, the better the digital resolution. On the other hand, the size of the file increases accordingly. Moreover, if the experimental conditions do not allow for good magnetic homogeneity, increasing SI to observe better quality signals will be unsuccessful.
Apodization or filtering functions: sensitivity windows
When the signal-to-noise ratio is too low and we cannot increase the number of scans or the experiment duration, it is possible to apodize the signal in sensitivity, which consists in multiplying the FID by a sensitivity window such as an exponentially decreasing function, with argument LB > 0. An increase in sensitivity is then obtained on the NMR spectrum at the expense of its resolution.
Apodization to enhance sensitivity is common for 13C NMR signals, for example.
An apodized spectrum can no longer give quantitative information. This is because during the multiplication of the FID by the function, the relative weights of each acquisition point are disturbed. They no longer account for the quantitative information that they carry intrinsically and which results in the signal integrals.
Apodization or filtering functions: Resolution windows
When the resolution is too low and it is not feasible to increase it due to the constraint of experiment time, it is possible to apodize the signal in resolution, i.e. by multiplying the FID by a resolution window such as a Gaussian. An increase in resolution is then obtained on the NMR spectrum at the expense of its sensitivity. It is therefore essential to have obtained a sufficient signal-to-noise ratio beforehand in order to attempt the apodization in resolution.
Resolution apodization is usual for weakly resolved signals, in 1H NMR for example.
An apodized spectrum can no longer give quantitative information. This is because during the multiplication of the FID by the resolution window, the relative weights of each point of acquisition are disturbed. They no longer account for the quantitative information that they carry intrinsically and which results in the signal integrals.
When the apodization in resolution is too violent, the observed signals may lose their significance, reveal baseline distortions or generate apodization artifacts.