The COSY block is the most simple mixing process consisting of a single 90º pulse to transfer polarization between J-coupled homonuclear spins.

The process follows as:

• Before the pulse, antiphase magnetization of the spin 1 (I_yS_z) with respect to a second spin via J_IS is required.
• The 90º pulse creates antiphase magnetization of this second spin (I_zS_y). This magnetization can be detected (see COSY experiment) or can evolve during a delay to refocus the antiphase component or to achieve antiphase magnetization with respect to a third spin (see RELAY block).

The COSY block can also be applied to heteronuclear ¹H-X spin systems by applying to simultaneous 90º pulses to ¹H and X (see INEPT block). In this case, an intermediate I_zS_z state is created that can be used for several purposes.

Easy implementation on AVANCE spectrometers.

 See  2D COSY and related experiments.

The standard way to implement a COSY block is:

... p1 ph1 d0 p0 ph2 ...

in which:

• p0 is the mixing COSY pulse (in usec) applied at high power level pl1 from the f1 channel.
• All phases may be specified at the end of pulse program.