Friday, August 8, 2008
Setting the Magic Angle with Glycine
One of the most precise ways of setting the magic angle is to maximize the number of rotational echos in the FID of a suitable spin I = n/2 quadrupolar nucleus (n =3, 5, 7 ....). When setting up for 13C CPMAS, one usually uses the 79Br resonance of KBr as the resonance frequency of 79Br is very close to that of 13C. An alternative method of setting the magic angle is to use the 13C carbonyl resonance of glycine. This has the advantage in that the glycine can also be used to set the Hartman Hahn matching condition and to check the decoupling power. The width of the carbonyl resonance is very sensitive to the setting of the magic angle. The angle can easily be adjusted and set properly while maximizing the duration of the signal in the FID interactively. The figure below shows the 13C CPMAS FID and spectrum for glycine on- and off-angle with digital filtering such that the methylene resonance is outside of the spectral width. The spectra were collected at 11.7 Tesla using a spinning speed of 12 kHz. When the angle is mis-set, one can see that the line shape for the resonance is a miniature version of the powder pattern observed in the absence of magic angle spinning.