Rotor size

Keywords Magic-Angle Spinning NMR Sample rotation Rotor size Selectivity Sensitivity Spectral resolution... 

The recently introduced rotation sweep spectroscopy [11] is based on the realization that decreased rotor size provides acceleration proportional to the inverse cube of the rotor diameter... 

The new potential of MAS comes mainly from a reduction in rotor size. The fastest rotors (70 kHz rotation frequency) currently hold 4 pL of sample, while ca. 10 pL samples can be rotated up to 50 kHz. If the filling factor is maintained, down-scaling leads to better mass sensitivity. On the one hand, increased mass sensitivity is welcome in cases where the availability of the sample is a problem. 

Reduction of the rotor size opens up several hitherto unexplored avenues for study of the principles of MAS NMR and for practical applications. Exploitation of these new possibilities and features may sometimes reward the observant investigator with unexpected associations and nodes of seemingly unrelated regularities. 

W2 has been reported as 2.9, 3.9, and 6.8 KJ/mol for three solvents, n-nonane, cyclohexane, and i-propanol, respectively [25]. Considerably smaller W2 than that of pyrene excimer formation, and the large deviation from Stokes - Einstein flow shown by the small value of x is probably relevant to the small rotor size. This chromophore will therefore be suitable to detect a small free volume change in highly viscous media. 

Dopant uptake is much higher in this case, y = 0.85, which implies a Cs -Cs spacing of Si 3.9 A. The authors point out, in a comparison with the structures found in doped polyacetylene, that the occurrence of either three-fold or four-fold column structures depends on the size ratio of the projected polymer chain (Totor dimension ) and the ion. In polyacetylene, Na ions leads to three-fold columns as in PPV, but for doping the columns in polyacetylene are formed by four chains, in contrast to the result for PPV. This is because the rotor size of a polyacetylene chain is smaller than that of PPV (Table 1.7). Rubidium doping of PPV represents an intermediate case, for which, apparently, neither three-fold nor four-fold... 

Figure 3.4 Relative sensitivities measured for various rotor sizes. Reproduced from Ref. [30] with permission, copyright 2007 John Wiley Sons, Ltd.

Clearly, the requirement of a MAS rate well above the CSA frequency range is quite restrictive for values of A/csa above approximately 20 kHz. Although there are solid-state NMR probes capable of providing MAS rates of this magnitude (or even higher), they are quite expensive and rotor sizes are sometimes prohibitively small [27]. The commonly used NMR probes with rotor diameters down to 4 mm typically spin at no more than 18 kHz and are, thus, not quite appropriate for use in NMR spectrometers with Bq above approximately 9.4 T when C NMR experiments with solid carbon materials are intended (unless TOSS or related methods are employed). Also, as mentioned in Section 2.3.2, use of fast MAS rates can bring serious problems to the efficiency of the CP process and consequently to a quantitative analysis of carbon materials using this method [56]. 

Comparison of tangential and intermeshing rotor size to give equivalent batch size... 

Plant capacity is a function of feed size distribution and Hberation. Separators can accept a size range as wide as 50—1000 p.m. Capacities are typically 1000 2500 kg/(h-m) based on rotor length which could be up to 3 m and have dia 150—250 mm. The feed should be as dry as possible because moisture interferes seriously with separation. Heaters are usually provided before the feed enters the charged field. Final cleaning is often conducted in electrostatic-type separators. Electrostatic shape separation, a newer form of ion bombardment separation, involves separation of particles based on shape and density without consideration to conductivities (37). 

Mech nic l Impact Mills. The mechanical types include cmshers, hammer mills, pin disk mills, turbine mills, and mills with air classifiers. Impact Crusher. Feed material is introduced through a feed opening onto a rotor moving at between 25 and 50 m/s (Fig. 11). The initial impact by the rotor causes some size reduction, and the material is accelerated up to the speed of the rotor and flung against the impact plates, where further size reduction occurs. It is possible to wear-protect these units quite well, so that abrasive materials can be handled. The final end particle size can be varied by the inclusion of an outlet grid to vary the residence time in the machine. 

Mechanical Mills with Mir Classifiers. To improve the end fineness and achieve a sharper topsize cutoff point, many mechanical impact mills are fitted with integral air classifiers (Fig. 13). These can be driven separately from the mill rotor or share a common drive. The material to be ground is introduced into the mill section of the machine, where impact size reduction takes place. The airflow through the machine carries the partially ground product to the air classifier, which is usually some form of rotating turbine. The speed of rotation determines which particle size is internally recycled for further grinding and which is allowed to exit the machine with the airflow. Machines are available up to 375 kW and can achieve products with essentially all material <20 fim. 

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