The 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.

The MLEV process follows as:

• Before the TOCSY block,  in-phase transverse magnetization of the target spin (I_1x) is required.
• The magnetization is transferred to all spins belonging to the same spin network during the TOCSY block. The length of the spin-lock period allows to determine the extension of this propagation. The final result is in-phase magnetization of the relayed spins (I_2x+ I_3x + I_4x ...).

Recently, the use of adiabatic spin inversion (WURST) has been proposed for this purpose ( 98JMR361-135 ).

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 MLEV-17 block stands for:

... d12 pl10:f1 (p17 ph26) 4 (p6 ph22 p7 ph23 p6 ph22) (p6 ph24 p7 ph25 p6 ph24) (p6 ph24 p7 ph25 p6 ph24) (p6 ph22 p7 ph23 p6 ph22) (p6 ph24 p7 ph25 p6 ph24) (p6 ph24 p7 ph25 p6 ph24) (p6 ph22 p7 ph23 p6 ph22) (p6 ph22 p7 ph23 p6 ph22) (p6 ph24 p7 ph25 p6 ph24) (p6 ph22 p7 ph23 p6 ph22) (p6 ph22 p7 ph23 p6 ph22) (p6 ph24 p7 ph25 p6 ph24) (p6 ph22 p7 ph23 p6 ph22) (p6 ph22 p7 ph23 p6 ph22) (p6 ph24 p7 ph25 p6 ph24) (p6 ph24 p7 ph25 p6 ph24) (p5 ph23) lo to 4 times l1 (p17 ph26) ...

in which:

• p5, p6 and p7 are 60º, 90º and 180º ¹H pulses (in usec) applied at low power level pl10 from the f1 channel.
• p17 are the trim pulse (2-3 msec) also applied at low power level pl10 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*64) + p5) * l1) + (p17*2)

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.
 

#include <Delay.incl> ... "SCALEF=p7*2/p5+0.5" "FACTOR1=((d9-p17*2)/(p6*64+p5))/SCALEF+0.5" "l1=FACTOR1*SCALEF"

 
• 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 DIPSI-2 block for TOCSY applications.

See  2D TOCSY and related experiments.