2D Apodization/Weighting Function

The Apodization or Weighting Function is the mathematical function that the FID is multiplied by before Fourier Transform. These are applied in both dimensions in 2D experiments.

The Apodization functions improve data quality by removing non-random noise created by FID truncation and potentially increase signal to random noise or resolution by weighting different parts of the FID.

The Apodization menu is located under the Processing tab then Apodization.

1) To accomplish 2D Apodization, first choose the dimension to apodize. This is located at the top of the Apodization window, unlike Zero-Fill/LP where you select F2/F1 from a different menu. Normally, choose F2 first, the dimension detected.

2) The optimum apodization varies depending how the data were acquired- gCOSY is acquired in magnitude mode, so not phase-sensitive, gHMBC is often acquired this way also, while almost all other 2D experiments are acquired in phase-sensitive mode (so the spectra have positive/negative peaks and noise). On a Varian, there is an acquisition parameter- phase- where phase = 1,2 is phase sensitive in t1 while phase = 1 is not. In general, phase sensitive experiments such as TOCSY, NOESY, ROESY, HSQC, HMQC the optimum apodization is similar to a cosine in both dimensions, for the gCOSY and non-phase sensitive gHMBC, a sine like function is better. For the phase sensitive gHMBC, a cosine like function is better in F1 and a sine like funtion in F2.

3) Specific suggestions for each experiment are included in the documents for each:
gCOSY , gHSQC , gHMBC , gHMBC Phase Sensitive , NOESY/ROESY/TOCSY

4) For any experiment other than one with specific instructions, assume the NOESY/ROESY directions are optimal.

5) For Phase sensitive experiments (everything except gCOSY, gHMBC), the optimum function is a cosine like function. The more weight that is given to the end of the FID, the sharper the resonances will be, but with the possibility of creating truncation artifacts or negative tails on the resonance, especially near the diagonal in NOESY/ROESY/TOCSY. The more weight that is given to the beginning of the FID, the broader the resonances will be. This might give more signal to noise. For line sharpness, cosine function is sharper than cosine squared, but cosine might lead to truncation artifacts.

6) To determine the quality of the apodization, look for side bands on the resonances, resolution between resonances and overall signal-to-noise. Adjusting the apodization can improve data quality.