The direct measurement of 13C 90° pulses in solids under MAS conditions by the conventional method suffers from the very low inherent sensitivity of 13C and is very time consuming due to the typically long 13C T1's. These problems can be at least partially overcome by using 1H - 13C cross polarization which has a potential four-fold sensitivity gain and also a time advantage as the repetition rate depends on the 1H T1 rather than the 13C T1, the former typically being less than the latter by a factor of ten. The 90° pulses are measured by carrying out the usual cross polarization contact which leaves the 13C magnetization along the -y axis. The contact is followed by a 13C -x phased pulse (φ-x) which rotates the magnetization towards the z axis in the -yz plane. The acquisition follows with high power 1H decoupling. The sequence is illustrated in the figure below.
When φ-x = 0°, one observes the usual positively phased CP spectrum. As φ-x is increased the signal decreases until φ-x = 90° at which point the 13C magnetization is on the z axis and a null signal is observed. As φ-x is increased further, +y magnetization is created and a negative signal is observed until φ-x = 180° at which point the magnetization is on the y axis and a maximum negative signal is observed, etc..... The 90° pulse can be read directly from the first null or 1/3 of the second null at 270°. The vector diagrams and a typical measurement (where φ-x was increased from 0.5 µsec to 20 µsec in 0.5 µsec steps) are illustrated in the figure below.