Practical Aspects of Pulsed Field Gradients and Shaped Pulses

In Section 7.9, we saw how phase cycling can be used to remove the 13C coherence that comes from the original 13C z magnetization (Sz), so that only the coherence transferred from ll 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 S, S is unaffected by the first gradient (Fig. 8.26) because z magnetization does not precess, so the 13C 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  

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 (XH 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. 

8 PRACTICAL ASPECTS OF PULSED FIELD GRADIENTS AND SHAPED PULSES 

Shaped pulses are created from text files that have a line-by-line description of the amplitude and phase of each of the component rectangular pulses. These files are created by software that calculates from a mathematical shape and a frequency shift (to create the phase ramp). There are hundreds of shapes available, with names like Wurst , Sneeze , Iburp , and so on, specialized for all sorts of applications (inversion, excitation, broadband, selective, decoupling, peak suppression, band selective, etc.). The software sets the maximum RF power level of the shape at the top of the curve, so that the area under the curve will correspond to the approximately correct pulse rotation desired (90°, 180°, etc.). When an experiment is started, this list is loaded into the memory of the waveform generator (Varian) or amplitude setting unit (Bruker), and when a shaped pulse is called for in the pulse sequence, the amplitudes and phases are set in real time as the individual rectangular pulses are executed. 

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