Exclusive correlation spectroscopy

[Image:ECOSYImage.jpg|thumb|280px|right|E COSY experiment: A and B are the two parts of the detected signal.]
Exclusive correlation spectroscopy is an NMR correlation experiment introduced by O. W. Sørensen, Christian Griesinger, Richard R. Ernst and coworkers for the accurate measurement of small J-couplings.
The idea behind the experiment is to measure an unresolved coupling with the help of a larger coupling which is resolved in a dimension orthogonal to the small coupling. Three active nuclei are needed and the pulse sequence must be able to transfer magnetization from I to S without changing the spin state of X, otherwise the ECOSY pattern will vanish.
ECOSY experiment is often used to determine the relative signs of J-couplings and to distinguish between the active coupling and the passive couplings caused by observer spins.