Weighting Functions (Apodization Functions)
A simple yet effective manipulation of the FID consists in multiplying it with suitable functions. They are called weights when the purpose is to improve sensitivity at the expense of resolution (or resolution at the expense of sensitivity). They are called apodizations when they compensate for a truncated FID. In the latter case, especially when zero-filling is applied, the apodization smoothes the final spectrum, which would be affected, otherwise, by small negative artifacts at the sides of true peaks. There is not a clear-cut division between the two kind of functions, which are better considered cumulatively as "weights".
If you give them the (little) importance they actually deserve, weighting functions are extremely easy to learn and to master. You may already have noticed that iNMR only offers 4 of them. Normally in 1D spectroscopy you will either use no function, or the exponential and/or the gaussian. iNMR changes the rule in 1D spectroscopy: weights are better applied after FT, not before! (This is the impression the user gets, actually weighting is still performed in the traditional way, but very behind the scenes; if you are curious, check the option “time domain”).
Weighting in Multidimensional Spectroscopy
In this case weighting is a necessity, because FIDs (along the direct dimension) or interferograms (along indirect dimensions) are heavily truncated. Here we furnish some starting points for absolute beginners who need just an immediate start. We can distinguish between:
- Phase Insensitive Spectra. If sensitivity is satisfactory use a Plain Sine Bell (shift = 0) along all dimensions. Increase the shift when the signal/noise ratio is very low. This counter-intuitive weight zeroes antisymmetric components of the spectrum, mainly dispersion signals (signals out of phase by 90°). In other words, you filter out what you cannot phase.
- Phase Sensitive Spectra. Use a Cosine Bell along the direct dimension (that is, a sine bell with shift = 90°). Use a Squared Cosine (a squared sine bell with shift = 90°) along indirect dimensions.