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Double Resonance Techniques: Overview

Double Resonance Techniques: Overview

Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
Spin decoupling is usually achieved by...

Atomic Nuclei: Types of Nuclear Relaxation

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Nuclear relaxation restores the equilibrium population imbalance and can occur via spin–lattice or spin–spin mechanisms, which are first-order exponential decay processes.
In spin–lattice or longitudinal relaxation, the excited spins exchange energy with the surrounding lattice as they return to the lower energy level. Among several mechanisms that contribute to spin–lattice relaxation, magnetic dipolar interactions are significant. Here, the excited nucleus transfers...

NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences

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A pulse is a short burst of radio waves distributed over a range of frequencies that simultaneously excites all the nuclei in the sample. Upon passing a radio frequency pulse along the x-axis, the nuclei absorb energy corresponding to their Larmor frequencies and achieve resonance. This shifts the net magnetization vector from the z-axis toward the transverse plane. This angle of rotation of the magnetization vector, or the flip angle, is proportional to the duration and intensity of the pulse.

NMR Spectrometers: Resolution and Error Correction

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When magnetic nuclei in a sample achieve resonance and undergo relaxation, the signal detected in NMR is an approximately exponential free induction decay. Fourier transform of an exponential decay yields a Lorentzian peak in the frequency domain. Lorentzian peaks in an NMR spectrum are defined by their amplitude, full width at half maximum, and position, where the peak width is governed by the spin-spin relaxation time alone. In real experiments, however, the applied magnetic field is rendered...

Atomic Nuclei: Nuclear Relaxation Processes

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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis, the precessing magnetic moments are randomly oriented around the z-axis.

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Rejection of Fluorescence Background in Resonance and Spontaneous Raman Microspectroscopy

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Relaxational signal attenuation during soft refocusing pulses.

Runchao Li¹, Laura Castañar¹, Mathias Nilsson¹

¹Department of Chemistry, University of Manchester, Manchester M13 9PL, UK.

Journal of Magnetic Resonance (San Diego, Calif. : 1997)

|December 5, 2022

Summary

Quantitative NMR accuracy can be improved by understanding spin relaxation during soft pulses. This study shows relaxation during refocusing pulses follows biexponential decay, enabling error correction in NMR experiments.

Keywords:

Relaxation Shaped pulses Spin echoes

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Area of Science:

Nuclear Magnetic Resonance (NMR) Spectroscopy
Quantitative Analysis
Magnetic Resonance Imaging (MRI)

Background:

Quantitative NMR (qNMR) is crucial for accurate molecular quantification.
Systematic errors in qNMR, particularly from non-hard pulses, are not fully understood.
Spin relaxation during pulse sequences is a significant source of error in soft pulse experiments.

Purpose of the Study:

To investigate and model spin relaxation occurring during shaped soft 180° refocusing pulses in on-resonance NMR experiments.
To determine the mathematical representation of relaxation losses during these pulses.
To provide a basis for correcting systematic errors in quantitative NMR.

Main Methods:

Theoretical modeling of spin relaxation during shaped radiofrequency pulses.
Analysis of relaxation contributions from both spin-spin (T2) and spin-lattice (T1) relaxation.
Characterization of relaxation rates using biexponential decay models.

Main Results:

Spin relaxation on resonance during shaped soft 180° pulses is accurately described by biexponential decay.
The decay rates involve spin-spin relaxation (R2) and a combination of spin-lattice (R1) and spin-spin (R2) relaxation, dependent on pulse shape.
Identified rate constants are R2 and a shape-dependent linear combination of R1 and R2.

Conclusions:

Relaxation during on-resonance selective refocusing pulses can be quantitatively modeled.
The findings enable the correction of relaxational losses in qNMR experiments employing soft pulses.
Improved accuracy in quantitative NMR is achievable through understanding and correcting pulse-induced relaxation effects.