Pulse sequence spin-flip method

There are several types of heteronuclear 2D J-resolved experiments (i) the gated decoupler method, (ii) the spin-flip method, (iii) the selective spin-flip method, (iv) the semi-selective spin-flip method, and (v) the use of polarization transfer, e.g., INEPT. In all these experiments, the pulse sequence results in the generation of spin echoes which are modulated during the evolution period by coupling frequencies. 

The Spin-Flip Method. In an earlier section it was shown that if a 180° pulse is applied to both and nuclei at time t after a 90° pulse, then during the subsequent period the two magnetization vectors of a CH system do not refocus but continue to diverge [see Figure 5.6(c)]. The spin-flip method utilizes the same principle. The pulse sequence and its effect on magnetization vectors of a system are shown in Figure 5.44. A 90°... 

Figure 5.45. (A) The selective spin-flip method for recording heteronuclear 2D J-resolved spectra in which the signals are modulated by long-range coupling constants. The pulse sequence is shown above while its effect on the magnetization vectors is shown below it. The selective 180° pulse results in elimination of the large one-bond coupling constants, Jch ( ) j -Methylcellobioside with the torsional angles (j) and ij/ indicated.

D J-Resolved Spectra with INEPT. Another method used to obtain heteronuclear 2D J-resolved spectra utilizes polarization transfer techniques (INEPT) and is comparable to the spin-flip method. The pulse sequence used is... 

Pulsed field gradient spin echo NMR is generally the method of choice for diffusion measurements on liquid samples (see Section 5.1 in this chapter). With modem high field instruments, however, severe problems can arise when it is applied to samples with very high proton concentrations because of the presence of radiation damping. Connel et al reduced the problems by a suitable choice of experimental parameters, in particular the use of modified stimulated echo pulse sequences with a reduced flip angle for the first pulse. 

A recently introduced method called SWIFT (SWeep Imaging with Fourier Transform) is a fundamentally different approach to MRI which is particularly well suited to imaging objects with extremely fast spin-spin relaxation rates." The method exploits a frequency-swept excitation pulse and virtually simultaneous signal acquisition in a time-shared mode. The experimental steps needed to properly implement HSn pulses in SWIFT and compact expressions are presented to estimate the amplitude and flip angle of the HSn pulses, as well as the relative energy deposited by the SWIFT sequence. 

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