Selective detection of MQC

A number of methods have been developed to enable MQ spectra of various orders to be observed selectively. Generally, a non-selective sequence (such as that in Fig. 2) is used to generate all orders of coherence in the spin system, then some type of filtering is used to remove or cancel unwanted coherence so that the detected signal reflects only the desired order(s) of coherence. 

The simplest approach to selective detection of MQ spectra involves phasecycling in the basic 3-pulse sequence (in Fig. 2) to allow selection of the 

One limitation of the phase-cycling method of selective detection is the expanding length of the required phase cycle as the order N increases. The number of steps in the phase cycle is 4N so for large N, this dictates that the phase cycle becomes prohibitively long and, in practice, this limits the number of increments that can be obtained in f and hence ultimately the digital resolution of the MQ spectra which can be obtained. 

Spectra obtained using magnetic field gradients to selectively filter the desired order of coherence are typically much cleaner than those obtained using phase cycling for MQ filtration, since the sequence does not rely on 

The phase properties of MQC transfer echoes have also been employed to selectively detect desired orders of MQ spectra. Using the basic sequence (Fig. 2), an additional (fixed) period d is inserted in the MQ pulse sequence, prior to the mixing pulse of the MQ pulse sequence (Fig. 10). MQC evolves beyond the normal tx period and coherence transfer echoes of the TV-quantum coherences maximise in the detection period at a time TVJ.35,41,42 The echoes of different coherence orders maximise at different times in the detection period and the spectra of different orders of coherence can be obtained by sampling the FID in the appropriate window in the detection period. 

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