Minimizing Lineshape Distortions in Static Ultra-wideline Nuclear Magnetic Resonance of Half-Integer Spin Quadrupolar Nuclei
Jonas Koppe, Michael Ryan Hansen
Abstract
Chirped excitation using frequency-swept wideline uniform rate smooth truncation (WURST) pulses in combination with Carr-Purcell-Meiboom-Gill acquisition (WCPMG) is currently the state-of-the-art method for the direct observation of the central transition (CT) in static ultra-wideline nuclear magnetic resonance (NMR) of half-integer spin quadrupolar nuclei. However, CT lineshape distortions and an inefficient, large number of transmitter steps in frequency-stepped acquisition are two major drawbacks. Here, we identify three main sources for lineshape distortions occurring in WCPMG NMR spectra of the CT: (I) distortions due to inaccurate setting of the radio frequency field strength, (II) chirped-excitation artifacts, and (III) distortions due to non-selective irradiation. A new and efficient approach for the acquisition minimizing these distortions is presented using low sweep rates (R ≤ 5 kHz/μs) and sweep widths (Δ ≤ 600 kHz). We further demonstrate that such an acquisition strategy also minimizes the number of transmitter steps in ultra-wideline NMR. This is achieved from numerical simulations and theoretical analysis of the orientational dependence for the quadrupolar-perturbed Zeeman states and their transition frequencies. The theoretically derived strategies are validated experimentally, allowing us to set up guidelines for the optimum recording of wideline and ultra-wideline WCPMG NMR spectra.