Shaped pulse pure phase

This simple phase-incrementation idea, not particularly emphasized by the authors at the time, has more recently had a considerable impact on NMR methodology. First, it was made the basis of one of the standard methods for obtaining pure-phase two-dimensional spectra, replacing the undesirable phase-twist line shape with a pure absorption-mode signal. Secondly, it has provided a neat way to generate an extensive array of simultaneous soft radiofrequency pulses covering an... 

Such excitations have been called shaped pulses, and a considerable effort has been expended in an attempt to optimize their shapes. A simple Gaussian shape is a considerable improvement over a rectangular pulse, but is not entirely effective in achieving an optimal peak shape. The use of more elaborate mathematical functions improves the shape of the signal, although with increasing loss of intensity. The BURP Band selective. Uniform Response, Pure phase) family utilizes an exponentially dependent sinusoidal series of Gaussians with considerable success in a variety of situations (EBURP for REBURP for 180 ). 

Another simple and general solution to obtain heteronuclear correlation spectra that yield truly pure absorption fine shapes and IP multiplet structures for all available cross-peaks with respect to both J(CH) and aU passiveJ(HH) coupling constants along the detected dimension is the so-called Pure In-Phase HSQC (PIP-HSQC) experiment [111]. The key point is an appended adiabatic zero-quantum filter (ZQF) [112] applied just before a refocusing gradient perfect-echo element and the acquisition. Thus, after the 90°j,( H) pulse (point b in Fig. 8C) the above four components derived from Eq. (2) are converted to... 

Pure 2D absorption line shapes are readily obtained in heteronuclear 2D /-resolved spectra. The incorrect setting of 90° and 180° pulses can, however, cause ghost peaks that can be removed by a phase cycling procedure, appropriately named Exorcycle (Rutar, 1984b). A... 

First, either a Lorentzian or Gaussian filter is applied to the FID to reduce the amount of noise. The choice of lineshape will depend on the shape of the frequency domain spectrum, the lineshape is related to how the fluorine spins interact with their environment. The filter linewidth is generally similar to or slightly less than the T2 value (T2 can be estimated from the spectral linewidth). After application of the time domain filter, a fast Fourier transform (FFT) is performed. The resultant frequency domain spectrum will then need to undergo phase adjustment to obtain a pure absorption spectrum. The amount of receiver dead time (time lost between the end of the excitation pulse and the first useful detection time point) will determine the presence and extent of baseline artifact present as well as how difficult phase adjustment will be to accomplish. 

In solution-state NMR, many important experiments incorporate the creation and evolution of MQ coherence (MQC).5,6,84-86 Since MQC cannot be directly detected, experiments that follow the evolution of a MQC are inherently at least two-dimensional. This is the case with H- H DQ MAS spectroscopy. Figure 7 shows a corresponding pulse sequence and coherence transfer pathway diagram first, a DQC is excited, which subsequently evolves during an incremented time period q the DQC is then converted into observable single-quantum (SQ) coherence (SQC), which is detected in the acquisition period, q. To select the desired coherence transfer pathways, e.g., only DQC during q, a phase cycling scheme is employed.79,80 Pure absorption-mode two-dimensional line shapes are ensured by the selection of symmetric pathways such that the time-domain... 

The harmonic pattern of a full wave diode bridge is well known and well documented in literature. In the diagram of Figure 2, six diodes are used for full wave rectification of a three phase AC supply, feeding the intermediate DC circuit of a VFD. The AC current will exhibit a 6 pulse pattern, resulting from each half of the AC sine wave conducting via a particular diode. This AC current does not have a pure sinusoidal shape anymore it now includes the combination of a fundamental component (50 or 60 Hz) to which additional frequencies are superimposed. 

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