Chemical shift dependent phase errors

We saw with the gradient DQF-COSY experiment that the relatively long gradients ( 1 ms) allow chemical shift evolution that will produce large chemical-shift dependent phase errors in the final spectrum. In the sequence of Figure 11.40, the gradient placed in the second half of the t period will set a minimum value for t of twice the gradient time (and... 

Answer The phase error in the signals is frequency dependent across the spectrum. The error was actually generated by setting to zero the width of what should have been the % -pulse. There is now no refocussing of the chemical shifts before acquisition. In the same pulse sequence, one can generate the same error by having unequal delays before and after a good rc-pulse. 

For the desired pathway, the spatially dependent phase created by a gradient pulse is refocused by a subsequent gradient pulse. However, the underlying evolution of offsets (chemical shifts) and couplings is not refocused, and phase errors will accumulate due to the evolution of these terms. Since gradient pulses are typically of a few milliseconds duration, these phase errors are substantial. 

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