Chemical shift concertina

Ellett and Waugh (1969) proposed the chemical shift concertina which is a method for scaling down the chemical shifts by arbitrary amounts at a fixed magnetic field while leaving spin-spin splittings unchanged in liquid samples. This may be a very useful technique in some circumstances. 

The correct explanation for how the chemical shift concertina works is complex but the following discussion provides a plausibility for the experiment. Suppose there is a train of equally spaced n/2 pulses whose phase alternates between 0° and 180°. 

Pulse sequences can be used to manipulate information in other ways, as well. The chemical shift concertina uses it to manipulate the relative importance of two terms in the Hamiltonian, the chemical shift term and the spin-spin coupling term. Echo sequences can be viewed as causing the system to evolve back in time to enable us to view information that existed earlier but was obscured by instrumental problems. The same is true of the ZTR experiment to observe the FID at zero time. The cross polarization experiments enable us to exploit the thermodynamic properties of a spin system to enhance the spectrum S/N of a second spin system. 

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