The general TOCSY block consists of an spin-lock period during which an isotropic mixing sequence applied along an specific axis  (WALTZ, DIPSI or MLEV) is applied. In this way, the Hartmann-Hahn consition is accomplished. This block is used to achieve transfer polarization into a coupling spin network in a improved way than the RELAY block.

This TOCSY transfer can be achieved in the z plane using a z-filtered DIPSI sequence. The process is as follows:

• The first 90º pulse converts the initial in-phase transverse magnetization of the target spin  (I_1x) in longitudinal magnetization  (I_1z).
• TOCSY transfer to the relayed spins affords longitudinal magnetization of these spins (I_2z+ I_3z + I_4z ...).
• The final 90º pulse creates the corresponding in-phase transverse magnetization of all relayed spins (I_2x+ I_3x + I_4x ...).

Easy implementation on AVANCE spectrometers.

Mixing TOCSY processes can be implemented using several pulse trains. The most accepted schemes are the MLEV-17 and  DIPSI-2 mixing sequences whose are set-up in a similiar way. See Tutorial: low-power ¹H pulse calibration for TOCSY applications.

The z-filtered DIPSI-2 block stands for:

... d12 pl1 p1 ph2 d20 pl10:f1 4 p6*3.556 ph23 p6*4.556 ph25 p6*3.222 ph23 p6*3.167 ph25 p6*0.333 ph23 p6*2.722 ph25 p6*4.167 ph23 p6*2.944 ph25 p6*4.111 ph23 p6*3.556 ph25 p6*4.556 ph23 p6*3.222 ph25 p6*3.167 ph23 p6*0.333 ph25 p6*2.722 ph23 p6*4.167 ph25 p6*2.944 ph23 p6*4.111 ph25 p6*3.556 ph25 p6*4.556 ph23 p6*3.222 ph25 p6*3.167 ph23 p6*0.333 ph25 p6*2.722 ph23 p6*4.167 ph25 p6*2.944 ph23 p6*4.111 ph25 p6*3.556 ph23 p6*4.556 ph25 p6*3.222 ph23 p6*3.167 ph25 p6*0.333 ph23 p6*2.722 ph25 p6*4.167 ph23 p6*2.944 ph25 p6*4.111 ph23 lo to 4 times l1 d21 pl1:f1 p1 ph3 ...

in which:

• d20 and d21 are the z-filter delays (2 and 3 msec, respectively).
• p6 is the 90º  ¹H pulse (in usec) applied at low power level pl10 from the f1 channel.
• p1 is the 90º  ¹H pulse (in usec) applied at high power level pl1 from the f1 channel.
• The overall mixing time is defined by the loop l1. Thus, the TOCSY mixing time can be calculated from the equation:

                ((p6*115.112) * l1)

However, for simplicity, the user define a desired mixing time using the d9 command and then, the following relationships inserted at the beginning of the pulse program calculates the required l1 value.

"FACTOR1=(d9/(p6*115.112))/2+0.5" "l1=FACTOR1*2"

• All phases may be specified at the end of pulse program.

 Mixing TOCSY processes can be implemented using several pulse trains whose are set-up in a similiar way. For instance, see the MLEV block for TOCSY applications.

See  2D TOCSY and related experiments.