A window function (also known as apodization function or convolution) is applied to an FID to emphasize a region of the FID over another. The general goal is to improve the SNR (Signal-to-noise) ratio. The early stages of processing such as applying the “correct” window function can affect the time spent on an elucidation especially when dealing with peaks having a low SNR ratio.
I use typically the Exponential window function for both 1H and 13C NMR data. Recently, I have noticed that the TRAF function for resolution enhancement to be doing a better job. The TRAF function comes from Daniel Traficante (Concepts in Magnetic Resonance 2000, 12, 83-101).
The four 13C NMR spectra, shown below, are from the same FID of cholestenone in CDCl3. The functions applied to the Top (purple), Middle Top (red), Middle Bottom (green) and Bottom (blue) spectra are no window function, TRAF 8 Hz, Exponential 2Hz, and Exponential 8 Hz, respectively. The 13C signal for C=O at 206 ppm is clearly visible over the noise for the TRAF function. In addition, there is a significant improvement in the baseline for the TRAF spectrum.
TIP: Processing of an FID is just as important as peak picking. Try different window functions to see which function works best for your data.