Pulse Fourier-Transform acquisition

Greatly enhanced sensitivity with very short measuring time is the major advantage of PFT (pulse Fourier transform) experiments. In the CW (continuous wave) experiment, the radiofrequency sweep excites nuclei of different Larmor frequencies, one by one. For example, 500 s may be required for excitation over a 1-KHz range, while in a PFT experiment a single pulse can simultaneously excite the nuclei over 1-KHz range in only 250 jits. The PFT experiment therefore requires much less time than the CW NMR experiment, due to the short time required for acquisition of FID signals. Short-lived unstable molecules can only be studied by PFT NMR. 

Radiofrequency spectroscopy (NMR) was introduced in 1946 [158,159]. The development of the NMR method over the last 30 years has been characterised by evolution in magnet design and cryotechnology, the introduction of computer-based operating systems and pulsed Fourier transform methods, which permit the performance of new types of experiment that control production, acquisition and processing of the experimental data. New pulse sequences, double-resonance techniques and gradient spectroscopy allow different experiments and have opened up the area of multidimensional NMR and NMRI. 

The 500-MHz, H-n.m.r. spectra were recorded with a Bruker WM-500 spectrometer operating in the pulsed, Fourier-transform mode and equipped with a Bruker Aspect2000 computer having an 80k memory-capacity. The D resonance of D20 was used as the field-frequency lock-signal. The spectra were obtained by using a 90° pulse-width, and accumulated into 16k addresses with an acquisition time of... 

NMR spectra were recorded for an 8.06 wt % D2O solution at 62.89 MHz and ambient temperature on a Bruker WM 250 spectrometer operating at 5.87 T in the pulsed Fourier transform mode with inverse-gated decoupling. The pulse width and acquisition time were 30.5 ms and 0.442 s, respectively. Each spectrum contained 16,000 data points over a frequency of 18,518 Hz with about 2000 acquisitions. 

Just as important as these developments in magnet design has been the introduction of pulsed Fourier transform methods, for these permit the performance of new types of experiment by the computerised systems that control the production, acquisition and processing of the experimental data. New pulse sequences increasingly made available by instrument manufacturers within their software suites permit the routine performance of these new experiments an early example is the distortionless enhancement polarisation transfer, or DEPT, experiment to identify the number of protons attached to a carbon by controlling the final... 

The detection period corresponds to the data-acquisition time in a pulse Fourier-transform experiment, with the exception that the data being digitized are a function of two variables, f, and (2-Figure 2 depicts a twice-Fourier-transformed, two-dimensional data set... 

Two-dimensional NMR spectroscopy may be defined as a spectral method in which the data are collected in two different time domains acquisition of the FID tz), and a successively incremented delay (tj). The resulting FID (data matrix) is accordingly subjected to two successive sets of Fourier transformations to furnish a two-dimensional NMR spectrum in the two frequency axes. The time sequence of a typical 2D NMR experiment is given in Fig. 3.1. The major difference between one- and two-dimensional NMR methods is therefore the insertion of an evolution time, t, that is systematically incremented within a sequence of pulse cycles. Many experiments are generally performed with variable /], which is incremented by a constant Atj. The resulting signals (FIDs) from this experiment depend... 

Scalar coupled experiments COSY and TOCSY The correlated spectroscopy (COSY) experiment is one of the most simple 2D-NMR pulse sequences in terms of the number of RF pulses it requires [32]. The basic sequence consists of a 90-C-90-acquire. The sequence starts with an excitation pulse followed by an evolution period and then an additional 90° pulse prior to acquisition. Once the time domain data are Fourier transformed, the data appear as a diagonal in... 

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