The RELAY block is an extension of the COSY block in which polarization of the relayed spin is transferred to a third spin by applying another 90º mixing pulse.

The process follows as:

• After a  COSY block, we obtain antiphase magnetization of this second spin (I_zS_y) with respect to the target I-spin via J_IS.
• This magnetization evolves during a delay (optimized to 1/2*J_SX) to achieve antiphase S-magnetization with respect to a third X-spin (I_zX_zS_y). A 180º placed at the midpoint of this period refocus chemical shift evolution.
• The final 90º pulse works as described in the COSY block. Antiphase X-magnetization is created  (I_zX_yS_z) as well as other components.

Multiple RELAY steps can be consecutively added to achieve step-by-step polarization transfer into a coupling spin network. However, improved results can be obtained using a TOCSY block.

Easy implementation on AVANCE spectrometers.

 See  2D RELAY and related experiments.

The standard way to implement a single-step RELAY block is:

... p1 ph1 d0 p1 ph2 d2 p2 ph3 d2 p1 ph4 ...

in which:

• p1 and p2 are the 90 and 180 pulses applied at high power level pl1 from the f1 channel.
• d2 is the first-step evolution delay optimized to an average 1/(3.2*J) value
• All phases may be specified at the end of pulse program.

 See Tutorial: 2D ¹H RELAY experiment