Comparisons of quadrature and single-phase fourier transform NMR

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Abstract

Reflections of lines in time-averaged Fourier transform NMR spectra obtained by imperfect quadrature detection can be removed either by 90° rf phase shifting of the excitation pulse together with simple data routing, or by phase and amplitude manipulations of the two imperfect free induction decays after completion of data accumulation. The sensitivity of the resulting reflection-free spectra is comparable to that of spectra obtained by single-phase detection using a rf crystal filter to remove aliased noise. However, since the quadrature experiment involves data sampling rates half as fast as the comparable single-phase experiment, distortions associated with the finiterecovery time of the spectrometer areless severe. Even for fast-recovery spectrometers, asymmetric spectral distortions in Fourier transform experiments involving moderately broad lines can occur. In quadrature detection schemes, these distortions are less severe since they are not folded through the entire spectral width as occurs in single-phase experiments.

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