Intensity in NMR spectra is acquired at regular time intervals during pulse sequences
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Peer-reviewed literature on nuclear magnetic resonance establishes that time-domain signals during pulse sequences are acquired by sampling at discrete, uniform time intervals.
Beginning with the introduction of Fourier Transform NMR by Ernst and Anderson in 1966, time domain measurement of the impulse response (the free induction decay, FID) consisted of sampling the signal at a series of discrete intervals. For compatibility with the discrete Fourier transform (DFT), the intervals are kept uniform, and the Nyquist theorem dictates the largest value of the interval sufficient to avoid aliasing. With the proposal by Jeener of parametric sampling along an indirect time dimension, extension to multidimensional experiments employed the same sampling techniques used in one dimension, similarly subject to the Nyquist condition and suitable for processing via the discrete Fourier transform. The challenges of obtaining high-resolution spectral estimates from short data records using the DFT were already well understood, however. Despite techniques such as linear prediction extrapolation, the achievable resolution in the indirect dimensions is limited by practical constraints on measuring time. The advent of non-Fourier methods of spectrum analysis capable of processing nonuniformly sampled data has led to an explosion in the development of novel sampling strategies that avoid the limits on resolution and measurement time imposed by uniform sampling. The first part of this review discusses the many approaches to data sampling in multidimensional NMR, the second part highlights commonly used methods for signal processing of such data, and the review concludes with a discussion of other approaches to speeding up data acquisition in NMR.
The arrival of very high field magnets and cryogenic circuitries, and the development of relaxation-optimized pulse sequences have added powerful tools for increasing sensitivity and resolution in NMR studies of biomacromolecules. The potential of these advances is not fully realized in practice, however, since current experimental protocols do not permit sufficient data sampling for optimal resolution in the indirect dimensions. Here we analyze quantitatively how increasing resolution in indirect dimensions affects the S/N ratio and compare this with currently used sampling routines. Optimal resolution would require sampling up to approximately 3 R (2)(-1), and the S/N reaches a maximum at approximately 1.2 R (2)(-1). Currently used data acquisition protocols rarely sample beyond 0.4 R (2)(-1), and extending evolution times would result in prohibitively long experiments. We show that a general solution to this problem is to use non-uniform sampling, where only a small subset of data points in the indirect sampling space are measured, and possibly different numbers of transients are collected for different evolution times. Coupled with modern methods of spectrum analysis, this strategy delivers substantially improved resolution and/or reduced measuring times compared to uniform sampling, without compromising sensitivity. Higher resolution in the indirect dimensions will facilitate the use of automated assignment programs.
Proton nuclear magnetic resonance (NMR) is useful for the analysis of biological samples such as serum. Free induction decays (FIDs) are NMR signals that follow a radio-frequency pulse applied at the resonance frequency. Short-time Fourier transform (STFT) is a basic method for time-frequency analyses. The purpose of this study was to ascertain whether the STFT of FIDs enables the sensitive detection of changes and differences in serum properties. FIDs were obtained from serum collected from young, healthy, male volunteers ≤ 40 years of age and seniors ≥ 65 years of age. Temporal changes in the instantaneous amplitudes for the time-domain analysis, fast Fourier transform for frequency-domain analysis, and STFT were applied to the FIDs. The STFT-based spectrogram represented the complex frequency components that changed dynamically over time, indicating that the spectrogram enabled the visualization of the features of an FID. Furthermore, the results of a partial least-squares discriminant analysis demonstrated that the STFT was superior to the other two methods for discriminating between serum from younger and older subjects. In conclusion, the STFT of FIDs obtained from proton NMR measurements was useful for evaluating similarities and dissimilarities in the FIDs obtained from serum samples.
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