Improving Resolution
Pathway Selective Pulses and Line Narrowing
Pathway selective pulses (PSPs) were introduced by the PI's group to selectively excite a spin system if a particular coherence or evolution pathway was available in the spin system. One particular PSP we developed was based on the reverse INEPT sequence (upper left) where coherence on say 13C is transferred into coherence on 1H spin. In this case, a PSP was developed (upper right) that would selectively excite 1H spins only if they were in a molecule with a 13C nuclei. While PSPs have the potential to speed up acquisition of multidimensional experiments or edit spectra based on evolution pathways, one of our findings was that after application of a PSP, the remaining signal had better resolution. In the figure (bottom), the regular spectrum (red) vs. the spectrum after a PSP (green) acquired on a 600 MHz Varian spectrometer are shown. While the PSP generated lower S/N, there was a marked improvement in resolution. We have developed a theory to describe the line narrowing, and future questions we are trying to address are how to optimize the line narrowing while minimizing reduction in S/N, applications of PSPs to solids to improve resolution in solid-state NMR.
Localizing Signals using the Gibbs phenomenon
Our group recently demonstrated that spectral localization can occur near localized near frequencies in the spectrum where the spectrum is either sharp or non-smooth. We had attributed this localization due to the Gibbs phenomenon, which results from decomposing the spectrum that isn't smooth everywhere in terms of basis functions that are (e.g., cosines and sines of a Fourier series). In the presence of magnetic field gradients, this spectral localization corresponds to spatial localization (as shown in the right). We have demonstrated this behavior in a variety of systems, and future directions involve exploiting and controlling this effect to localize signals in specific voxels for MRI applications and to explore this phenomenon in other contexts such as in quantum scarring.
