In phase selective excitation of overlapping multiplets by gradient-enhanced chemical shift selective filters

We have developed gradient-enhanced chemical shift selective filters (ge-CSSF) for inphase excitation of overlapping multiplets 1H. This method relies on the constructive addition of on resonance signal while off resonance magnetization is eliminated by destructive interference due to variable chemical shift evolution. This is achieved by co-addition of several FIDs acquired with a gradually incremented chemical shift evolution period. Two variable-time and one constant-time ge-CSSFs are proposed that can be combined with TOCSY, NOESY, and ROESY mixing schemes yielding highly selective 1D experiments. Analytical and numerical expressions are derived to calculate the excitation profiles of the ge-CSSFs and to examine the effects of spin-spin relaxation, the length of the CSSF increment, and selective inversion pulses. We demonstrate, both theoretically and experimentally, that CSSFs yield fast signal separation for compounds with a range of spin-spin relaxation times and chemical shift differences as small as 1-2 Hz. The use of pulsed field gradients ensures that very clean spectra are obtained. The main application of these techniques lies in analysis of mixtures where severe spectral overlap prevents the use of simple 1D selective methods.