TAILIEUCHUNG - Understanding NMR Spectroscopy phần 10

Hãy để chúng tôi giả sử rằng chúng ta muốn tạo ra gấp đôi lượng tử như cho thấy điều ngược lại CTP. Như đã được ghi nhận, xung đầu tiên chỉ có thể tạo ra p = ± 1 và 180 ° xung chỉ gây ra một sự thay đổi trong các dấu hiệu của sự gắn kết theo thứ tự. | by the gradient at the edges of the sample yBg rmax is of the order of 1. b The rate of dephasing is proportional to the zero-quantum frequency in the absence of a gradient Qk- t 2 . c The gradient must be switched on and off adiabatically. d The zero-quantum coherences may also be dephased using the inherent inhomogeneity of the radio-frequency field produced by typical NMR probes but in such a case the optimum dephasing rate is obtained by spin locking off-resonance so that tan-1 . Qk i 54 . e Dephasing in an inhomogeneous B1 field can be accelerated by the use of special composite pulse sequences. The combination of spin-locking with a gradient pulse allows the implementation of essentially perfect purging pulses. Such a pulse could be used in a two-dimensional TOCSY experiment whose pulse sequence is shown below as a . a b DIPSI _ y _ I mI I I f1 I Tm ----------Ũ------O__ g---------------- ỊỊ--------- GA GA GA G2 Pulse sequences using purging pulses which comprise a period of spin locking with a magnetic field gradient. The field gradient must be switched on and off in an adiabatic manner. In this experiment the period of isotropic mixing transfers in-phase magnetization say along x between coupled spins giving rise to cross-peaks which are absorptive and in-phase in both dimensions. However the mixing sequence also both transfers and generates anti-phase magnetization along y which gives rise to undesirable dispersive anti-phase contributions in the spectrum. In sequence a these anti-phase contributions are eliminated by the use of a purging pulse as described here. Of course at the same time all magnetization other than x is also eliminated giving a near perfect TOCSY spectrum without the need for phase cycling or other difference measures. These purging pulses can be used to generate pure z-magnetization without contamination from zero-quantum coherence by following them with a 90 y pulse as is shown in the NOESY sequence b . Zero-quantum coherences .

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