Versatile pulse sequence

Versatile Pulse Sequence One of the great strengths of FTMS is the flexibility to selectively accelerate, activate, and eject ions in any combination and any sequence without hardware modifications. This versatility makes FTMS the method of choice for MS/MS and hence for establishing pathways and rate constants for gas-phase ion-molecule reactions, and to correlate this data with structural information. Recently up to (MS)5 has been demonstrated (18). 

Recently, a robust, sensitive, and versatile HMBC experiment for rapid structure elucidation has been proposed. The suggested IMPACT-HMBC experiment eliminates the weaknesses of the basic HMBC experiment and the overall performance of the pulse sequence is improved significantly. In addition, it can be recorded with short recovery times, which is useful in routine analysis by NMR when the experimental time is limited. 

With an understanding of the response of the nuclear magnetization to an rf pulse, we can further manipulate the spin system by applying a sequence of pulses. The magnetization is rotated by the first pulse, allowed to precess for some period, and rotated further by subsequent pulses. Chapters 9 and 10 are devoted largely to the application of a variety of pulse sequences and the resultant spin gymnastics that the nuclear magnetizations experience. Here we consider only two simple pulse sequences to illustrate the versatility and to lay the foundation for later discussion. 

Homonuclear or heteronuclear Hartmann-Hahn mixing periods are versatile experimental building blocks that form the basis of a large number of combination experiments (see Section XIII). In practice, the actual multiple-pulse sequence that creates Hartmann-Hahn mixing conditions can usually be treated as a black box with characteristic properties. In this section, design principles and practical approaches for the development of Hartmann-Hahn mixing sequences are discussed. 

NMR spectroscopy represents a valuable and versatile tool for the characterization of dispersed nanoparticles. In contrast to alternative analytical techniques, it combines a distinctly non-invasive character with the ability to analyse for chemical composition as well as for local mobility of individual system components. Its main disadvantages - poor sensitivity and time consuming acquisition of experimental data - can be overcome by a suitable choice of the pulse sequence and the experimental conditions. The advantages of the NMR approach are especially promising for the study of nanoparticle dispersions used as drug carriers, where many important system characteristics such as release properties, surface exchange processes or decomposition pathways are readily available by relatively simple pulse experiments. 

Pulses, delays and gradients are the basic elements of a pulse sequence. They are often used repeatedly and may be changed in a very versatile way. Their exact definition and time related execution follows a specific set of rules which the pulse program interpreter uses to translate the line ordered execution commands into a computer readable list of commands. 

Some modern (new in 1981) commercial NMR spectrometers are extremely versatile with computer controlled pulse sequence generators which are easily programmable even for complex sequences. Thus, even for a homemade machine, you should seriously consider purchasing the operating system software with a computer and a pulse sequence generator. 

Fig. 10. Three types of DAS pulse sequence, phase cycling and rotor axis orientations, (a) A basic DAS experiment where all pulses are selective 90° pulses calibrated for the particular axis orientations (used from Mueller era/. with permission) (b) A more versatile pure-phase experiment, allowing axis flip to any angle 0 during acquisition. The Z filter mixes the two coherence orders during the flip. The time r for the Z filter is set equal to the storage time needed for the axis flip (from Mueller et al. with permission) (c) A combined shifted-echo and hypercomplex DAS experiment with two phase cycling schemes corresponding to the acquisition of real (upper) and imaginary (bottom) part of r, evolution, respectively (from Grandinetti et al. with permission).

For applied NQR spectroscopy, pulsed techniques are used in conjuction with a variety of pulse sequences and FT data processing. This approach provides for maximum sensitivity and versatility. 

This limitation is not there for pulsed ENDOR methods, which can be used at all microwave frequencies. In ENDOR, the sample is irradiated with a combination of microwaves and radio waves. Continuous-wave ENDOR was already introduced in 1956 by Feher [1] and for a long time remained an important tool to determine the hyperfine and nuclear quadrupole interactions. However, nowadays this technique is largely replaced by the pulsed counterparts, which are more versatile. Two of the most commonly used ENDOR pulse sequences are Davies ENDOR [18] and Mims ENDOR [ 19]. In these techniques a combination of microwave pulses and a n radio frequency (RF) pulse with variable RF is used. A first set of microwave pulses creates electron polarization. When the RF matches one of the nuclear transitions, the populations of the different energy levels will be affected. This will change the electron polarization that is read out by a last sequence of microwave pulses, usually via electron spin echo detection as a function of the radio frequency. In this way, the nuclear frequencies can be directly detected. 

Beyond the standard quadrupole echo experiment, multipulse sequences provide an alternative and versatile approach to measure transverse relaxation. The relaxation time constant T is obtained from a series of experiments with different pulse spacings t. Extending this sequence by n further 90 ... 

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