COSY experiment with a double quantum filter: (DQF COSY)
The COSY sequence with double quantum filter affords some advantages compared to the conventional cosy. The diagonal spots and the correlation spots are not phase shifted to 90° and they have the same antiphase structure (Fig. 21). It is thus possible to phase as a whole all the peaks of the spectrum in positive or in negative absorption allowing thus an easier identification of the peaks whose correlations are the closest to the diagonal.
These diagonal peaks correspond to singlet signals and are thus eliminated. Unfortunately the sensitivity is divided by 2.
The molecule which is used as an example is the saccharose (Fig. 29).
We may notice on the spectrum 7 (DQF COSY), a strong decrease of the singlet of the proton F1 which is located at 3.68 ppm compared to the spectrum 3.