T2 with a spin-echo sequence

Nuclear spin relaxation is not a spontaneous process, it requires stimulation by a suitable fluctuating field to Induce the necessary spin transitions and there are four principal mechanisms that ate able to do this, the dipole-dipole, chemical shift anisotropy, spin rotation and quadiupolat mechanisms. Which of these is the dominant process can directly influence the appearance of an NMR spectrum and it is these factors we consider here. The emphasis is not so much on the explicit details of the underiying mechanisms, which can be found in physical NMR texts [7], but on the manner m which the spectra are affected by these mechanisms and how, as a result, different experimental conditions influence the observed spectrum. 

F ure 2A7. Spin-echo sequences for measuring relaxation times, (a) A basic spin-echo, (b) the Carr-Purcell sequence and (c) the Carr-Purcell-Meiboom iU (CPMG) sequence. 

Signal acquisition was started immediately after a 90° excitation pulse and a 180° pulse applied to refocus field inhomogeneity losses and produce the observed echo, (b) A train of spin-echoes reveals the true T2 relaxation of magnetisation (dashed line). 

The selective reduction of a solvent water resonance can also be achieved in a similar way if the transverse relaxation time of the water protons can be reduced (i.e. the resonance broadened) such that this becomes very much shorter than that of the solutes under investigation. This can be achieved by the addition of suitable paramagnetic relaxation agents (about which the water molecules form a hydration sphere) or by reagents that promote chemical exchange. Ammonium chloride and hydroxylamine have been used to great effect in this way [4,5], as illustrated for the proton spectrum of the reduced arginine 

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