The rotors most commonly used for MAS are made of zirconium oxide (Zirconia). Rotors can be either filled entirely with sample or they can be used with rotor inserts. Inserts are provided to improve shimming and RF in homogeneity, and are useful when the amount of sample is limited. The maximum spinning rate for 4 mm Zirconia rotors is about 15 kHz, but is limited to lower speeds when using the spacers. A full rotor has a sample volume of about 80 ml; with a spherical insert the volume is reduced to about 25 l. Numbered rotors are available to easily distinguish between different samples.
The function of the rotor cap is twofold: firstly, to close the rotors, and secondly, to provide the driving of the rotor. There are several types of caps available. The standard caps are made of Kel-F, which can be used in a temperature range from 10°C to +50°C. This material will shrink at lower temperatures and soften at more elevated temperatures. However, for a more extended VT range (-30°C to +70°C) caps made from macor or boron nitride can be used. Those materials have temperature coefficients similar to Zirconia.
Figure
4.1. Zirconia rotor with Kel-F cap, upper spacer, cylinder head screw,
and sealing grub screw to provide a spherical sample volume
The most common type of rotor spacers provide a sample volume that is approximately spherical in order to improve the shimming of the probe (see figure 4.1 and figure 4.2. Appendix B is an example of an instruction sheet showing how to handle this type of insert. For other inserts please refer to the accompanying instructions provided with the spacers.
Figure 4.2. Schematic drawing of the rotor with spherical insert.