DFS, RAPT and HS
Small gyromagnetic ratios, low natural abundance, large quadrupole moments, and in some situations combinations of the above exist in many of the half-integer quadrupolar nuclei. These conditions make such nuclei difficult to analyze. As such pulse sequences have been developed to enhance the central transition of such half-integer nuclei in the solid state to make their study simpler.
Double-frequency sweep (DFS), rotor-assisted population transfer (RAPT), and hyperbolic secant (HS) experiments function by changing the equilibrium populations of the nuclear spin energy levels in an effort to enhance the central transition. In short using either saturation (RAPT) or inversion techniques (DFS and HS) the Boltzmann populations of the satellite transitions are altered to enhance the central transition.
With DFS the enhancement is experimentally accomplished by simultaneously sweeping both the high and low frequency sides of the central transition. This way the energy level populations of the satellite transitions are simultaneously inverted by time dependent amplitude modulated rf.
The HS technique alters the energy levels population with a broadband adiabatic inversion pulse. Unlike the DFS the HS pulse is created using both amplitude and phase modulation.
In the case of RAPT a phase alternating pulse train is applied under MAS conditions. This has the effect of sweeping the frequency of the satellite transitions through the resonance condition. This creates sidebands which are then used to saturate the populations of the energy levels associated with the satellite transitions. In turn an enhancement is experienced by the central transition.
RAPT experiments may appear easiest to setup but they are actually quite challenging on the spectrometer. When large QCC are present short delays and pulse lengths that are required for the pulse sequence to be effective. At this point DFS offers the best signal enhancement of stationary samples (up to 2.5 times for spin 3/2 nuclei) and HS is the choice for MAS experiments (2.7 times for spin 3/2 nuclei).Concepts Magn. Reson. A (2005), 26A, 47-61. DOI 10.1002/cmr.a.20037
enhancement in MAS spectra of half-integer quadrupole nuclei, DFS (double
frequency sweep), RAPT (rotor assisted population transfer), compared with
a SP (single pulse) experiment. Performed by S. Steuernagel (Bruker) with
the 2.5 mm MAS probe.