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. Suitable samples are BaClO 3 H 2 O ( 1 H observe) and KBr ( 79 Br observe). For both samples, to adjust the angle the sample has to be spun at about 5 to 6 kHz. Note that either sample can be used to set the angle, irrespective of what nucleus will be observed in the HR-MAS experiments.
For the proton sample a spectrum is acquired and Fourier transformed. The magic angle setting is then slightly changed by turning the micrometer screw at the bottom of the probe. Take another spectrum and repeat the procedure until the magic angle setting is optimal (see figure 5.1).
Figure 5.1. Proton NMR spectra of BaClO3H2O Obtained off the magic angle (top) and obtained with the correct angle setting (bottom). Experimental parameters: ns = 1, swh = 125kHz, pulse program zg, p1 = 3 us, pl1 = 0 dB, d1 = 5s, spinning rate 6000 Hz.
The 79 Br resonance frequency is very close to the 13 C 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 T 1 . 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 ( figure 5.2). 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 ( figure 5.3).
Figure 5.2.79 Br FID's KBr. Experimental parameters for acquisition: pulse program = zg, swh = 100 kHz, td =2 k, d1 = 30 ms, p1 = 2 us, pl1 = 0 dB, spinning rate 6000 Hz.
Figure 5.3.79 Br NMR spectra corresponding to the FID's shown in figure 5.2.