Fast amplitude modulation technique

In this review, we shall introduce some basic concepts about sohd-state NMR of half-integer quadrupolar nuclei and discuss the most useful and promising methods presently available to study them. These include older methods, such as Double Rotation (DOR) and Dynamic Angle Spinning (DAS), and novel techniques including MQMAS, Quadrupolar Phase Adjusted Spinning Sidebands QPASS, SateUite Transition (ST) MAS and Inverse-STMAS NMR, and Fast Amplitude Modulation (FAM). We also discuss several techniques based on dipolar interactions between quadrupolar and spin-1/2 nuclei, such as Cross-Polarization (CP) MQMAS, MQ Heteronuclear Correlation Spectroscopy... 

Since the introduction of MQMAS, much effort has been expended on developing methods to enhance the signal of the CT, and thus to extend the utility of the technique. A common signal enhancement technique is fast amplitude modulation (FAM). Shaped pulses, including the HS pulses introduced above,as well as p)psi78.208 been used for signal enhancement in MQMAS experiments. Kwak et have combined RAPT with the... 

The aluminium species in activated zeolites have been studied using NMR spectroscopy. Aluminium and iron in the framework of zeolites have been analysed quantitatively by Al NMR spectroscopy. The templating of high-silica zeolites by THF has been studied by C NMR spectroscopy. O NMR spectroscopy has been used to study the atomic multipoles in Fl-form aluminosilicate frameworks. A comparison of fast amplitude modulation shifted-echo pulse sequences for O 3Q- and 5Q-MAS NMR experiments has been carried out for oxygen sites in zeolites. By using a combination of the Monte-Carlo technique with Si NMR spectroscopy, cation sites in zeolites have been located. " The growth of zeolites on kaolin microspheres has been investigated by NMR spectroscopy. F1 MAS spin echo NMR spectroscopy,... 

Rocha et al. have reviewed recent advances in NMR spectroscopy of quadrupolar nuclei with half-integer spins. In particular, B, O, Na, A1, Ga, Nb MQ MAS applications have been considered. A range of related techniques, such as satellite transition (ST) MAS, inverse-STMAS NMR, fast amplitude modulation, and techniques based on the dipolar interactions between quadrupolar and spin-1/2 nuclei (CP MQ MAS and MQ HETCOR) and the recently introduced /-coupling based experiments (/-HMQC), have also been reviewed. 

It is possible to monitor relaxation times by modulating the microwave amplitude at a rate close to the inverse spin-lattice relaxation time, Tj (Misra, 2004). This is due to the fact that if the microwave amplitude is changed faster than that which the spins can follow as governed by the electron SLR time, the EPR signal is determined predominantly by T. This modulation technique permits measurement of relaxation times in the interval between the values that are slow enough to be measured by the recovery techniques and the values that are too fast to have any significant impact on continuous wave (cw) lineshape. These latter relaxation times can be as short as 10 s, which is substantially shorter than those that can be measured by the saturation/inversion recovery techniques. 

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