The MLEV scheme is a composite pulse sequence highly advisable for broadband decoupling purposes. In addition, the rapid inversion of spins over its wide bandwith provides a good homonuclear Hartmann-Hahn match in the rotating frame for all resonances and induces magnetization transfer between coupled spins. This is the basis for the use of MLEV sequences as a isotropic mixing in TOCSY experiments (see The TOCSY block).

Easy implementation on AVANCE spectrometers.

 The basic and inverted elements of the MLEV scheme ( 81JMR502-43 , 82JMR345-50 , 83JMR313-53 , 82JMR328-47 , and 83JMR56-51 ) is

although the first descriptions used the element 90_x240_y90_x. The most simple scheme is the MLEV-4 (RRrr):

However, the most used scheme is the MLEV-16 in which the 16-step cycle RRrr rRRr rrRR RrrR is repeated indefinitely. A more sophisticated MLEV-64 supercycle did not provide a significant improvement.

The MLEV scheme is usually used in TOCSY experiments as a MLEV-17 sequence consisting of the MLEV-16 scheme with an extra 60º_x pulse added at the end to invert possible phase errors. In this way, MLEV-17 is repeated indefinitely sandwidched between two trim pulses of 2-3 milliseconds in length, which defocus any magnetization not parallel to the x axis. Calibration of 90º and 180º pulses must be made at the required power. In general, for TOCSY applications, the power is adjusted to give a 90º pulse about 30-35 microseconds. A 10% variation in such calibrations could be tolerated.

For further experimental details, see the following tutorials:

• Tutorial: 90º ¹H observe pulse calibration.
• Tutorial: 2D TOCSY experiment.