Probe Tuning

 The RF tuning is performed by centering the dip of the wobble curve on the screen through adjusting the tuning and matching rods at the bottom of the probe. The rods for the ¹H channel are labeled yellow, the rods for the ¹³C channel are labeled blue. The tuning bandwidth of the ¹³C channel is equal to a few MHz. The ²H lock channel is fixed, i.e., no external matching and tuning adjustment is necessary. Always tune the probe while the sample is spinning. When tuning the probe pay attention to the wobble curve. If modulations are superimposed on the wobble curve, this is usually a sign that the sample spinning is not stable. Adjusting the drive and bearing pressures or restarting the spinner can stabilize the spinning. If problems persist, repack the sample.

Note:

The HR-MAS probes have been designed to perform solution type experiments, while spinning the sample at the magic angle. The probes are either doubly tuned (e.g. ¹H and ¹³C) or triply tuned (e.g. ¹H, ¹³C and ¹⁵N), in addition to a ²H lock channel. All three (or four) channels are operating via a single NMR transmit/receive solenoid coil located inside the MAS turbine. The probes are capable of performing either direct or indirect detection experiments.

Probe Shimming

In MAS spectroscopy the spinner axis is at an angle  with the magnetic field direction and the distinction between the traditional on-axis and off-axis shims no longer holds. The spinning rates in MAS spectroscopy, however, are typically at least a few kilohertz, which is much larger than the magnetic field in homogeneity. As a result the amplitudes of the sidebands are small and shimming may be done with a set of shims that is cylindrically symmetric about the MAS spinner axis. Such a set of shims can be constructed from combinations of the standard laboratory frame shims, via a transformation to the tilted magic angle frame9. In the simplest implementation the MAS probe is aligned such that the spinner axis is in the laboratory xz-plane.

The effective shims on our magnet:

X, ZX, Z2X, Z4 and Z5.

To shim the HR-MAS probe, a sample of 3%CHCl3 in Acetone-d6 is suggested, using a rotor with a spherical insert. Make sure the magic angle is adjusted prior to shimming the probe. Spin the sample at a rate suggested in the following section. It is always best to shim up the probe already under the conditions used later for the real samples.

Line shape Specification:

1.2/10/30   ????

Probe Calibration

The precise setting of the magic angle is mandatory to obtain the maximum spectral resolution obtained by MAS. In order to adjust the magic angle, a sample is needed with an NMR line that is very sensitive to angle misalignment.  The ⁷⁹Br resonance frequency is very close to the ¹³C resonance frequency, thus no or just a slight tuning of the RF-circuit is necessary. Furthermore KBr provides a very sensitive signal with a very short T1. Already at moderate spin rates the satellite transition will be split into sidebands, which have to be as narrow as possible for the optimum angle setting. With a rotor completely filled with KBr, the angle setting procedure can be performed in “gs” mode on the free induction decay (FID). In “gs” mode, go to the “acqu” window, separate the real and imaginary part of the FID and adjust for an on resonance decay. This can be done by changing “o1” or adjusting the field. Make sure that the field sweep is off and remains off! If the angle is close to the magic angle, you will see rotational echoes on top of an exponential decay. Far off angle only the exponential decay is visible. Turn the micrometer screw until the rotational echoes last up to 4 ms. Write down the setting of the micrometer screw at the probe bottom for the correct angle position. Check for the optimal setting by acquiring 32 scans and maximizing the number of sidebands in the spectrum. Detailed procedure see:

http://www.emory.edu/NMR/Hall/index.htm