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
channel are labeled yellow, the rods for the
channel are labeled blue. The tuning bandwidth of the
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.
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
or triply tuned (e.g. 1H,
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.
Doubly tuned (1H
and 13C) probe is available at Emory
In MAS spectroscopy the spinner axis is at an angle
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
X, ZX, Z2X, Z4 and Z5.
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
300 MHz 2400 Hz spinning
400 MHz 3200 Hz spinning
500 MHz 4000 Hz spinning
600 MHz 4800 Hz spinning
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:
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: