Sample: Butter + D2O.

Mix thoroughly with a spatula half a teaspoon of common butter with 10 drops of D.O. Transfer this smear into the rotor of the HR-MAS probe-head and spin the sample at 8 kHz. Control the temperature of the probe-head by passing room-temperature air through it. Before the experiment, the magic-angle adjustment of the probe-head (see Exp. 14.2) should have been checked using KBr. Tune the probe-head and then lock and shim the sample on D₂O. You have to set:

td: 32k sw: 20 ppm o1: middle of ¹H NMR spectrum [5 ppm] p1: 90° ¹H transmitter pulse [10 μs, 5 dB] dl: Is ns: 1

5. Processing

Use standard 1D processing for ¹H NMR spectra (see Exp. 3.1).

6. Result

The figure shows spectra obtained on a DRX-600 spectrometer equipped with an HRMAS probe-head using a standard 4 mm zircon rotor. In a the static spectrum is shown, with the typical broad lines of such semi-solids, whereas in b the result with spinning at 8 kHz is given. Under these conditions the typical line-width obtained in this sample was 4 Hz, which is a remarkable difference compared with the spectrum shown in a.

7. Comments

Shimming of these probe-heads is not very straightforward. Due to the magic angle of the insert the usual z shim gradients transform into other directions. Instead of z, z” and z’, you have to use mainly x, xz and xz’. Note that these probe-heads are not built to accept high r.f. power as typically used for other solid-state applications described in this chapter. Only standard high-resolution r.f. techniques can be used, or example, no cross-polarization technique should be tried.

Also record for comparison the ¹³C NMR spectra in the static mode and with 8 kHz spinning. For ¹³C NMR the difference will not be as dramatic, because in natural abundance the dipolar interactions between the ¹³C spins are not predominant.