PPT Slide
Part of the ensemble/ population of labile protons will exchange depending of the delay length t. This will reduce the intensity of the signal during the acquisition as follows:
This only adds to the natural decay of the signal during acquisition due to relaxation ( e -t/T2 ):
which shortens the observed transversal relaxation rate since : 1/T2eff = 1/T2 +kex
The shortened relaxation time leads to a broader signal as: line width= 1/2T2
This effect of chemical exchange is termed “exchange broadening”.
Exchange before FID-acquisition during the pulse sequence:
Part of the ensemble/ population of labile protons will exchange depending of the delay length D between water supression and begin of acquisition. This will reduce the intensity of the signal observed depending on the exchange kinetics: Note: D (usually ms, occasionally ms range)
Exchange before the “read-out” pulse, during the preparation/relaxation period.
The intensity of the resonance will be reduced as a function of the exchange rate, and the relaxation properties of the NH-proton. This phenomen is termed “presaturation” transfer, and is the only significant disadvantage of presaturation!
The signal reduction is significant larger: long delay = 2-3 sec = we reach equilibrium for water supression , but also via exchange for the labile proton intensity.
Residual fractional intensity: I/Io = 1/ (1+ kexT1)