Visual Weighting
Spectroscopists improve the appearance and readability of NMR spectra by multiplying the FID with suitable functions. These functions often contain an adjustable parameter. In some cases (like in multidimensional spectroscopy) experience indicates in advance which value for the parameter will give satisfactory results. In the common case of a 1-D spectrum, however, nothing is more effective than interactive adjustment.
The exponential function represents the first choice, because it is the natural shape of an NMR signal in time domain. A negative exponential corresponds to a positive line broadening in frequency domain (with a reduction of noise). A positive exponential has the opposite effects: lines get narrower, spectral resolution increases and the noise is amplified. For the latter reason, a positive exponential is seldom used alone. It is normally accompanied by a Gaussian weight, which reduces the noise (it also broadens the signals, but not as much as the exponential).
The simultaneous application of an exponential and a Gaussian function in time domain corresponds to a shape change, from Lorentzian to Gaussian, in frequency domain. Gaussian peaks have less pronounced tails and are therefore better resolved. Anyway, what matters is not the theory, but the practice. You can apply whichever combination of weights, as long as the results are good enough for you.
How to Optimize Weighting:
- Transform the spectrum into the frequency domain.
- Select a few important peaks of your spectrum and zoom in. You can hide any intermediate empty region with the cutter tool.
- Choose Process > Visual Weighting.
- Choose Sensitivity or Resolution.
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Move the exponential slider to the right and observe how the spectrum changes.
Stop when you are satisfied.
If the peaks become too high, press the minus key. You can use all the usual navigation keys (plus, arrows, Home, End, etc.) to observe what happens in other regions of your spectrum. This is always possible on the Mac. To make it possible on Windows, click the word “interact”. -
If you are optimizing for resolution and it is never enough,
move the Gaussian slider to the left.
Apart from this case, the second slider is not needed. It is there to offer more freedom, but you can ignore it if you prefer the simplicity of a single-parameter adjustment. -
If the slider reaches the end of its range and you need more, close this dialog,
choose File > Repeat, then Process > Fourier Transform and
change the numerical value of the Exponential weight.
You can optionally add a sine bell, a cosine bell, etc.
Each time you move a slider, iNMR reloads the FID from disk, applies the new weight, transforms the spectrum and draws it again. The dialog also gives you the possibility to observe the shape of the weight alone or the weighted FID.