Chapter 2. CP/MAS Solids Operation
25
VNMR 6.1C User Guide: Solid-State NMR
01-999162-00 C0402
always checking the spinning quality of the empty rotor before packing it with the sample
material. Discard damaged rotors. Worn rotor caps can cause imbalance. Changing caps or
rotating them between rotors sometimes cures these problems.
If a packed rotor does not spin properly at first, inspect the sample to see if it has been
disturbed. If part of the sample broke loose and was thrown out of the rotor, repacking the
sample in the rotor might be the solution. Sometimes loose material balances itself if it is
kept in the rotor and spun below its vibration speed for a few minutes. If the sample seems
intact on the surface and the rotor is not balanced, it is likely that the sample is not
homogeneous or not evenly packed. The only solution is to remove all the sample and
repack the rotor. With inhomogeneous materials, this repacking may have to be tried more
than once. In the case of a machined plug, the plug could have a void in it or it fits too
loosely in the rotor cavity.
Probe Adjustments for Improved Spinning
Increased bearing pressure often stabilizes samples that do not spin well. This adjustment
must be made at low speed and then ramped up once the rotor spinning is stable.
Using the Rotor Speed Controller
The rotor speed controller for Varian MAS probes can be operated with spinning speed
regulation or with a specific airflow setting to control the spinning speed of a sample in a
magic angle spinning (MAS) probe. Alternatively, the air flow can be set to a maximum
(65535) and the drive-pressure regulator on the pneumatics/tachometer box can be used for
manual control the spinning speed.
If the rotor speed controller has not been calibrated, follow the calibration procedures in the
Pneumatics and Tachometer Box Installation manual before continuing.
Chemmagnetics CP/MAS probes use a different speed controller and are not compatible
with this accessory.
Starting the Spinner Speed Controller
1.
Enter
spinner
in the VNMR
input window to open the Spinner
Control window (see
2.
Click on the High speed spinner
(solids style) control button.
3.
Click on Turn spinner off.
4.
If Set spinner airflow instead of
speed is engaged (button pressed
in and red), click on the button to
disengage and set the spinner in
regulation or closed-loop mode.
5.
Make sure the drive is set to 0.
6.
Set the bearing pressure to 0.
7.
Using your fingers, insert an end-cap into the rotor to be spun. Rotate the end cap
while pushing it into the rotor. Make sure the end cap is fully seated into the rotor.
8.
Carefully place the rotor with the end cap into the stator.
Figure 3. Spinner Speed Control Window