Triple-pulse excitation: An efficient way for suppressing background signals and eliminating radio-frequency acoustic ringing in direct polarization NMR experiments
作者：Fenfen Wang; Sanath K. Ramakrishna; Pingchuan Sun; Riqiang Fu*
關(guān)鍵字：Direct polarization; Single-pulse excitation; Triple-pulse excitation; Acoustic ringing; Background suppression
論文來(lái)源：期刊
具體來(lái)源：The Journal of Magnetic Resonance, 2021, accepted
發(fā)表時(shí)間：2021年
Direct polarization using a single pulse is the simplest excitation
scheme in nuclear magnetic resonance (NMR) experiments, capable of quantifying
various compositions in many materials applications. However, this single-pulse
excitation generally gives rise to NMR spectra with a severely distorted
baseline due to the background signals arising from probe components and/or due
to the radiofrequency (RF) acoustic ringing, especially in low-γ nuclei and
wide-line NMR. In this work, a triple-pulse excitation scheme is proposed to
simultaneously suppress the background signals and eliminate the RF acoustic
ringing. The acoustic ringing is cancelled through subtraction in any two
consecutive scans by alternating the receiver phase while keeping the phase of
the pulse right before acquisition the same. While the triple-pulse scheme generates
an additional flip-angle dependent scaling to the traditional single-pulse
excitation profile in such a way that the scaling is one when the flip-angle is
~ 90° but becomes almost zero when the flip-angle is very small. Therefore, the
background signals arising from the materials outside the sample coil experiencing
a very small fraction of the RF flip-angles can be effectively suppressed.
Various samples containing 1H and quadrupolar nuclei (17O, 25Mg, and 23Na) have been used to demonstrate the
effectiveness of this newly proposed triple-pulse excitation in terms of
suppressing the background signals and eliminating the acoustic ringing
effects.