Decoupling Hardware

How does a spectrometer deliver this RF irradiation to the probe Compared to normal excitation pulses, which are very high-power and short ( 10 p,s) duration, decoupling requires low-power irradiation for the entire acquisition time (1-2 s). This is usually accomplished by having two separate sources of RF power, a broadband transmitter that can be 

For routine acquisition (Fig. 4.8), the transmitter is set to the frequency (300 MHz) and pulses from the transmitter are directed to the outer coil of the probe, which is tuned to the 1H frequency. The decoupler is not used. After the exciting pulse, the H FID is detected on the outer coil of the probe and directed to the receiver, which uses a continuous signal from 

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The spectrometer supports phase cycling, asynchronous sequence implementation, and parameter-array experiments. Thus, most standard solid-state NMR experiments are feasible, including CPMAS, multiple-pulse H decoupling such as TPPM, 2D experiments, multiple-quantum NMR, and so on. In addition, the focus of development is on its extension of, or modification to, the hardware and/or the software, in the spirit of enabling the users to put their own new ideas into practice. In this paper, several examples of such have been described. They include the compact NMR and MRI systems, active compensation of RF pulse transients, implementation of a network analyzer, dynamic receiver-gain increment,31 and so on. 

A key feature of the DSMC technique in comparison to continuum methods is its relatively high computational expense. To allow the decoupling between molecular motion and intermolecular collisions to occur in a physically accurate way, the time-step used in the DSMC technique must be smaller than the mean time between collisions. Similarly, the size of the cells employed in the DSMC computational grid must be of the order of the local mean free path everywhere in the flow domain. These physical restrictions on the size of the numerical parameters results in the time steps and cell sizes employed in DSMC calculations being usually significantly smaller than those employed in continuum computations. For this reason, significant work has been performed in the optimization of the DSMC technique for different types of computer hardware. Examples of specific implementations are described in Refs. 23-26. 

The multicomputer model decouples node-local activities from communication. This simplification decouples the performance critical aspects of parallel hardware calculations and communications. In practice it treats a multi-... 

BIRD-HMQC. The most difficult aspect of implementing the HMQC experiment is the suppression of signals from protons attached to C (the center-band or single quantum coherences) in favor of the protons attached to C (the satellites or double quantum coherences). The use of pulse field gradients (PFG, Section 6-6) is the most effective technique, but relatively few spectrometers are equipped with the hardware required for their generation. Fortunately, there is an effective alternative for the suppression of center bands by means of the BIRD Bilinear Rotation Decoupling) sequence, which is outlined by the vector... 

Hardware requirements are slightly different from those for traditional NMR measurements NMR probe heads must be absolutely free from fluorine, but such probes are now commercially available. It is usually necessary to have two high-frequency channels on the NMR spectrometer to allow fluorine measurements with proton decoupling, and good RF-filters must be used to separate the 19F and ll channels. 

The APT sequence was the first experiment to decode the sign of the signal amplitude as a function of I S multiplicity. Because of the hardware restrictions in early NMR spectrometers, particularly applying simultaneous pulses on both the acquisition and second channel, emphasis was made on making the pulse sequence as simple as possible. The sequence only requires a 90° pulse and 180° pulse on the acquisition channel, which may be easily determined, and a simple decoupler switch-on/off on the second channel. Nevertheless the experiment is still included in modern pulse program libraries the experiment seems to be very robust and in contract to DEPT or INEPT type experiments quaternary carbon atoms can also be detected by the same experiment. 

The spin-lattice relaxation time for in solids is very long (several minutes). Since the nuclei have to relax before another excitation pulse can be sent, this requires hours of instrument time in order to collect a spectrum of reasonable intensity. A pulse technique called cross-polarization can be used to reduce this effect by having the protons interact with the carbon nuclei, causing them to relax more rapidly. FTNMR systems for sohd samples include the hardware and software to produce narrow line spectra from solid samples in a reasonable amount of time using high-power dipolar decoupling, MAS, and cross-polarization. 

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