Reducing relaxation losses

From NMR Wiki

1 Introduction
2 Factors causing enhanced relaxation
- 2.1 Motion dependent and external factors
- 2.2 Factors that involve the magnetic field variation amplitude
3 Methods to reduce the enhanced relaxation
4 Specific to solid state
5 References

Introduction

Relaxation in the magnetic resonance is a complex subject. On one side relaxation allows the experiments to be repeated at faster rates, but on the other side, it makes the signal or higher-order coherences to disappear faster. Relaxation of the coherence evolving at frequency $\nu = \frac{\omega}{2 \pi}$ is caused by the fluctuations of the local magnetic field occuring on the time scale of $\frac{1}{\nu}$ .

Reducing the relaxation losses is a very important subject, since relaxation limits the size of the molecules that can be analyzed, increases necessary time for fid accumulation and makes measurements of the chemical shift less precise.

Factors causing enhanced relaxation

Anything that slow down the motion of the macromolecule we are watching on, and also the factor that raise the square of the amplitude of the local magnetic field variation. It has to be noted that relaxation could cause other effect than signal loose, like dynamic frequency shift, in which case the peak are deformed, or shifted.

Motion dependent and external factors

molecular tumbling (random rotations of molecules in fluid phases)
presence of radical species in the solution

And the following will also account for relaxation by the experimentalist :

diffusive or convective motions of molecules in an inhomogeneous field
unstable magnetic field
inhomogeneous radio frequencies pulses

Factors that involve the magnetic field variation amplitude

Methods to reduce the enhanced relaxation

There are different ways to reduce the relaxation, we could modulate both kind of factors cited above:

having homogeneous magnetic field ( Shimming )
limiting convection in sample
deoxygenate the solution (a common free radical species present in samples)
increase temperature (accelerate molecular tumbling)
use reverse-micelles in less-viscous solvent - such as liquid alkane (also accelerates molecular tumbling)
use lower magnetic field instrument
record spectra of lower gyromagnetic ratio element, that is called protonless NMR
labeling the protein with deuterium (reduce spin diffusion)
use experiments that cancel the dipolar relaxation with the csa relaxation ( 15N Trosy ) or dipolar relaxation with dipolar relaxation ( 13C Trosy )
use experiments in which the preparation of the spin system take less time : less complexe experiment
use ways of magnetic polarization transfer other than the conventional INEPT : CRINEPT is an example [1]