Cross polarization reverse

Therefore, local dissolution and recrystallization seem to play an important role in the gas uptake mechanism in these type of sensor materials. The coordination of SO2 to the platinum center (and the reverse reaction) is therefore likely to take place in temporarily and very locally formed solutes in the crystalline material, whereas the overall material remains crystalline. The full reversibility of the solid-state reaction was, furthermore, demonstrated with time-resolved solid-state infrared spectroscopy (observation at the metal-bound SO2 vibration, vs= 1072 cm-1), even after several repeated cycles. Exposure of crystalline samples of 26 alternat-ingly to an atmosphere of SO2 and air did show no loss in signal intensities, e.g. due to the formation of amorphous powder. The release of SO2 from a crystal of 27 was also observed using optical cross-polarization microscopy. A colourless zone (indicative of 26) is growing from the periphery of the crystal whereas the orange colour (indicative for 27) in the core of the crystal diminishes (see Figure 9). 

Recently, Asbrook and Wimperis[89] proposed a novel approach for combining SQ cross-polarization and MQMAS the SQ coherences created in the CP step are transferred directly to 3Q coherences, rather than via a population state. This results in a pulse sequence with a minimum number of coherence-transfer steps. Two types of experiments have been proposed, using either a z-filter or a reversed split-ti, where pure-phase hneshapes are recorded via acquisition of the whole-echo. In addition, the authors showed that the sensitivity of both experiments is considerably improved by incorporating FAM pulses in the SQ to MQ excitation step. 

Structural function of HPO ions in bone mineral and their content are of great biological interest. Concentration of these ions decreases as bone apatite matures with age [50,51 ]. In order to analyse scarce HPO " ions in the presence of abundant PO ions, one has to resort to a special CP technique proposed by Zumbulyadis [52] and X. Wu et al. [53], and apphed to bone mineral by Y. Wu et al. [37]. The technique is a modified version of conventional CP (Fig. 14a) and is called differential cross-polarization (DCP). The proton spin-lock is divided into two periods, f and t. During the first period, polarization is transferred, as usual, from H to P. The phase of the H spin-lock pulse is then reversed, which forces polarization to transfer backwards (depolarisation), from P to H. The DCP experiment is performed with arbitrarily chosen, constant f and constant or variable t. The variable experiment begins with CP peaks of... 

Reversed phase packings Non-polar Reversed phase silicas with n-alkylsilyl groups, reversed phase silicas with hydro-phobic polymer coatings, hydrophobic cross-linked organic polymers, porous carbon Aqueous/organic mobile phases... 

PISEMA experiment utilizes the evolution under the H-A dipolar coupling to acquire the signal in the channel. Therefore, this new sequence eliminates the sensitivity-loss due to the reverse cross-polarization step and could be used to develop a variety of H-detected multi-dimensional solid-state NMR experiments. 

Knowledge of the electrooptic behavior of the FLCPs is of the utmost importance for display device applications. One relevant parameter in this respect is the response time. As for the spontaneous polarization, the determination of the response time requires a uniformly aligned sample. The test cell is placed between crossed polarizers so that one tilt direction is parallel to the direction of one polarizer. The electrooptic effect is achieved by applying an external electric field across the cell, which switches the side chains from one tilt direction to the other as the field is reversed. A photodiode measures the attenuation of a laser beam when the cell is switched between the two states. Generally, the electrooptical response time is defined as the time corresponding to a change in the light intensity from 10 to 90% when the polarity of the applied field is reversed ( 10-9o)-... 

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