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we have p Gz p Gz

In Section 7.9, we saw how phase cycling can be used to remove the 13C coherence that comes from the original 13 C z magnetization (Sz), so that only the coherence transferred from 1H z magnetization (Iz) is observed. This is a subtraction process that requires more than one scan to accomplish. With gradients we can do it in one scan alone:

The pathway Sz ^ Sx is unaffected by the first gradient (Fig. 8.26) because z magnetization does not precess, so the 13 C SQC (Sx) is only "twisted" by the second gradient and arrives at the FID in a coherence helix that adds to zero over the whole sample. There is no need to subtract it out—it never reaches the receiver. We can add up the "twists" imparted by the two gradients using the fact that coherence order (p) equals zero for z magnetization:

13 C z magnetization 13 C SQC

Because the sum is not equal to zero, we end up with twisted coherence and no signal in the receiver. We call this a "gradient-selected" experiment because the gradients are being used to specifically refocus coherence in the desired coherence transfer pathway (1H SQC ^ 13C SQC) and to reject all others. In Chapter 10, we will develop the idea of coherence order in a more precise manner, and we will see that coherence order can be either positive or negative.

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