Category Archives: Polarization Enhancement

Boosting sensitivity and suppressing artifacts via multi-acquisition in direct polarization NMR experiments with small flip-angle pulses

This is not directly related to DNP spectroscopy but an interesting method to increase the sensitivity specifically in solid-state NMR experiments. This method is compatible with DNP and in combination can lead to even larger sensitivity gains than just DNP.

Fu, Riqiang, and Arturo J. Hernández-Maldonado. “Boosting Sensitivity and Suppressing Artifacts via Multi-Acquisition in Direct Polarization NMR Experiments with Small Flip-Angle Pulses.” Journal of Magnetic Resonance 293 (August 1, 2018): 34–40.

A small flip-angle pulse direct polarization is the simplest method commonly used to quantify various compositions in many materials applications. This method sacrifices the sensitivity per scan in exchange for rapid repeating of data acquisition for signal accumulation. In addition, the resulting spectrum often encounters artifacts from background signals from probe components and/or from acoustic rings leading to a distorted baseline, especially in low-γ nuclei and wideline NMR. In this work, a multi-acquisition scheme is proposed to boost the sensitivity per scan and at the same time effectively suppress these artifacts. Here, an adiabatic inversion pulse is first applied in order to bring the magnetization from the +z to −z axis and then a small flip-angle pulse excitation is used before the data acquisition. Right after the first acquisition, the adiabatic inversion pulse is applied again to flip the magnetization back to the +z axis. The second data acquisition takes place after another small flip-angle pulse excitation. The difference between the two consecutive acquisitions cancels out any artifacts, while the wanted signals are accumulated. This acquisition process can be repeated many times before going into next scan. Therefore, by acquiring the signals multiple times in a single scan the sensitivity is improved. A mixture sample of flufenamic acid and 3,5-difluorobenzoic acid and a titanium silicate sample have been used to demonstrate the advantages of this newly proposed method.

Orphan Spin Polarization: A Catalyst for High-Throughput Solid-State NMR Spectroscopy of Proteins

Not an article that directly relates to DNP-NMR spectroscopy, but another interesting technique to enhance sensitivity that can be combined with DNP.

Gopinath, T. and G. Veglia, Orphan Spin Polarization: A Catalyst for High-Throughput Solid-State NMR Spectroscopy of Proteins, in Annual Reports on NMR Spectroscopy, Academic Press.

Magic-angle spinning solid-state NMR (MAS ssNMR) spectroscopy is a powerful method for structure determination of biomacromolecules that are recalcitrant to crystallization (membrane proteins and fibrils). Relatively low sensitivity and poor resolution of protein samples require long acquisition times for multidimensional ssNMR experiments. Conventional multidimensional ssNMR pulse sequences acquire one experiment at a time, which is time consuming and often discards orphan (unused) spin operators. Here, we describe our recent progress in the development of multiple acquisition ssNMR methods for protein structure determination. A family of experiments called polarization optimized experiments (POE) was designed, in which we utilized the orphan spin operators that are discarded in classical NMR experiments to recover them and acquire simultaneously multiple 2D and 3D experiments using conventional probes and spectrometers with one receiver. Three strategies namely, DUMAS, MEIOSIS, and MAeSTOSO were used for the concatenation of various 2D and 3D pulse sequences. These methods open up new avenues for reducing the acquisition time of multidimensional experiments for biomolecular ssNMR spectroscopy.

Polarization enhancement technique for nuclear quadrupole resonance detection #DNPNMR

Kim, Y.J., et al., Polarization enhancement technique for nuclear quadrupole resonance detection. Solid State Nucl Magn Reson, 2014. 61-62: p. 35-8.

We demonstrate a dramatic increase in the signal-to-noise ratio (SNR) of a nuclear quadrupole resonance (NQR) signal by using a polarization enhancement technique. By first applying a static magnetic field to pre-polarize one spin subsystem of a material, and then allowing that net polarization to be transferred to the quadrupole subsystem, we increased the SNR of a sample of ammonium nitrate by one-order of magnitude.

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