Sensitivity-enhancement techniques

As mentioned earher, the main problem that hinders the application of Zn SSNMR spectroscopy is its inherently low sensitivity. Several approaches deahng with sensitivity issue are briefly discussed later. 

Very recently, the WURST-QCPMG sequence was developed to achieve uniform excitation of quadrupolar nuclei across very wide bandwidth using frequency-sweep chirped pulses [86]. The WURST (wideband uniform-rate smooth truncation) pulses provide broadband excitation. Integrating the WURST pulses with QCPMG results in the WURST-QCPMG method, which has proven to be a highly efficient approach for acquiring ultra-wideHne NMR spectra [74]. 

CP is one of the most useful SSNMR techniques for signal enhancement. It involves transferring magnetization from abundant sensitive spins (usually proton) to dilute insensitive spins. For example, C and 

CP is mediated by the heteronuclear dipolar interaction between proton and the target spin. Since the direct dipolar interaction strongly depends on 

The recent renaissance of dynamic nuclear polarization (DNP) provides an extremely promising approach for sensitivity enhancement [87—89]. It borrows the large Boltzmann polarization from unpaired electron spins of free radicals to boost the NMR signal. DNP experiments have been performed on quadrupolar nuclei such as [90], N [91], [92,93], 

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Sensitivity Enhancement Techniques by Adiabatic Population Transfer. 134... 

Many of the characterization techniques described in this chapter require ambient or vacuum conditions, which may or may not be translatable to operational conditions. In situ or in opemndo characterization avoids such issues and can provide insight and information under more realistic conditions. Such approaches are becoming more common in X-ray adsorption spectroscopy (XAS) methods ofXANES and EXAFS, in NMR and in transmission electron microscopy where environmental instruments and cells are becoming common. In situ MAS NMR has been used to characterize reaction intermediates, organic deposits, surface complexes and the nature of transition state and reaction pathways. The formation of alkoxy species on zeolites upon adsorption of olefins or alcohols have been observed by C in situ and ex situ NMR [253]. Sensitivity enhancement techniques play an important role in the progress of this area. In operando infrared and RAMAN is becoming more widely used. In situ RAMAN spectroscopy has been used to online monitor synthesis of zeolites in pressurized reactors [254]. Such techniques will become commonplace. 

From the viewpoint of polymer applications, the full exploitation of the combined resolution/sensitivity enhancement techniques to obtain "high-resolution" spectra of rare-spin nuclei in solids requires variable temperature spinning capability. In this paper, we describe briefly a spinner assembly suitable for routine operation over a wide range of temperature at the full complement of spinning angles and report - C spectral data at low temperature on several polymers, including fluoropolymers. In addition, variable temperature spin-lattice and rotating frame relaxation times are reported for isotactic poly(propylene). 

Several sensitivity-enhancing techniques were applied simultaneously in the Radionuclide Aerosol Sampler Analyzer (RASA) (Miley et al., 1998). This system employs a Ge detector with about twice the efficiency of contemporary manual systems (90 vs. 40% relative efficiency) and twice the airflow of manual systems (24,000 vs. 12,000 m3 per day). A simple layering mechanism provides a large filter (0.25 m2) during sampling and a moderately small filter volume ( 400 cm2 and 0.5-cm thick) for measurement. 

Several of the sensitivity-enhancement techniques that have been successfully applied to MQMAS are also suitable for STMAS. For example, Amoureux and co-workers have incorporated SPAM with DQF-STMAS. Other techniques, such as multiplex phase cycling, " that have been used successfully with MQMAS, may also be applicable to STMAS. 

This chapter has summarized the tremendous progresses made over the past two decades in developing Zn SSNMR spectroscopy. The increased popularity and feasibility of Zn SSNMR can be credited, in large part, to several factors such as (1) the increased accessibiUty of ultrahigh-magnetic-field NMR instruments, (2) the development of sensitivity-enhancement techniques for quadrupolar nuclei such as... 

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