In conventional high resolution spectroscopy with gradients, the gradient field most commonly used is aligned with the main magnetic field direction, a so-called z-gradient. Although the sample is usually stationary in gradient spectroscopy, for reasons discussed later, it is in principle possible to rotate the sample, since the isoplanes of the gradient field are perpendicular to the rotation axis (see figure 6.1). This means that a rotating spin will always experience the same magnetic field strength.
Figure 6.1. Use of a z-gradient in MAS spectroscopy
If a z-gradient would be applied to a sample spinning at the magic angle, the rotating spin would pass through different isoplanes, since the axis of rotation is no longer parallel to the direction of the magnetic field gradient ( figure 6.1). This would result in a partial averaging of the gradient field as experienced by the spins, and would necessitate the need to synchronize one's experiments with the spinner rotation (which may be difficult or even impossible due to the finite rise and fall times of the pulsed gradient fields).
A better solution is to implement gradient spectroscopy experiments in a MAS probe in such a way that the gradient should introduce a resonance that is not temporally modulated. This may be accomplished with a time-independent gradient field where the gradient is oriented such that the z-component of the magnetic field increases along the axis of the spinner and the z-component of the magnetic field gradient is uniform in the planes perpendicular to the spinner axis (see figure 6.2).
Figure 6.2. Gradient field is directed along the magic angle spinner axis
The HR-MAS probe is optionally equipped with a magic angle gradient 12 . The gradient coil design is compatible with commonly used MAS stators (for widebore and standard bore probes) and does not interfere with the automatic sample ejection and insertion capabilities. Gradient strengths of up to 45 G/cm (at 10A) can be achieved.