Abstract
Resolving NMR signals which are separated in frequency on the order of their line widths requires obtaining the time domain free induction decay for a maximum time t~max~ = Ο__T__~2~, where T~2~ is the transverse relaxation time of the given signals. Unfortunately, samples acquired beyond βΌ1.26__T__~2~ contribute more noise than signal to the data; and samples in the range of about (0.75β1.26)Γ T~2~ have a negligible effect on the signalβtoβnoise ratio (SNR). Therefore, one must sacrifice SNR to reach evolution times of Ο__T__~2~. One can preserve resolution in a shorter total experimental time by selecting a reduced set of samples from the Nyquist grid according to an exponential probability density which is on the order of the T~2~ of the signals. This practice is widely termed nonuniform sampling (NUS). We derive analytic theory for the enhancement of the intrinsic SNR of NUS time domain data compared with uniformly sampled data when the total experimental times are equivalent. This theory is general for any t~max~ and exponential weighting and is further carefully validated with simulations. Enhancements of SNR in the time domain on the order of twofold are routinely available when t~max~ βΌ Ο__T__~2~ and are reflected in the subsequent maximum entropy reconstructed spectra. SNR enhancement by NUS is demonstrated to be helpful in enabling the acquisition of HMQC spectra of dilute bile salts in which high resolution in the indirect carbon dimension is required. Copyright Β© 2011 John Wiley & Sons, Ltd.