5.4 Choosing the appropriate spinning speed

 

The correct spinning speed depends strongly on the sample under study. A minimum spinning speed is recommended, which depends on the field strength of the magnet:

 

300 MHz 2400 Hz spinning

400 MHz 3200 Hz spinning

500 MHz 4000 Hz spinning

600 MHz 4800 Hz spinning

These spinning speeds are chosen to prevent spinning sidebands to fall into regions of interest for typical proton samples.

For some samples, such as for instance in biological tissues, higher spin rates may be beneficial. Keep in mind, however, that this may result in irreversible changes in the sample due to the breakdown of cellular structures and that the rotation may lead to significant sample heating (as much as 10°C).

 

In the case of very fast spinning it is a good practice to acquire spectra at lower spinning speeds both before and after the fast spinning and compare those spectra to ensure that no changes have occurred in the sample.

 

Figure 5.5 displays proton spectra of a plant leaf, acquired at spinning speeds of 4 kHz, 10 kHz, 19 kHz and 6 kHz. The spectrum at 6 kHz is obtained after spinning at 19 kHz and shows that no irreversible changes have taken place.

 

Figure 5.5. Proton spectra of a plant leaf

Effect of spinning speed on 400 MHz HR-MAS proton spectra of a plant leaf, cut and flushed in water. The spectra are displayed with absolute intensity scaling. The spectrum obtained at 6 kHz is acquired after spinning at 19 kHz and shows that no irreversible changes due to spinning have taken place.