Pulse field gradient method

A common pulse field gradient sequence due to Stejskal and Tanner (1965) is shown below. 

If there is no dc field gradient and identical width field gradient pulses of duration 6 are used, then the appropriate relationship for the echo amplitude M is 

When a dc gradient Gq exists in addition to the gradient pulses 

It should be noted that the duration over which the diffusion is being measured is well defined, which is one of the advantages of the pulse gradient method over the steady gradient method. This time is A-6/3 for the condition A 6 and becomes simply A when A 6. For an implementation of such a sequence for solids see, for example, the article by Gordon, et al. (1978). 

Potential problems with pulsed gradient methods 

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In this chapter we will focus on molecular ordering and confinement effects in pores. Diffusion experiments with the pulse-field gradient method ([162-165] and references therein) and characterization of the surface properties using NMR of noble gases such as 129Xe ([166-171] and references therein), or 83Kr [172], will be omitted due to excellent reviews that have appeared quite recently in these areas. 

Diffusional behavior of sorbed species is studied by NMR using one of three approaches the van Vleck method of moments, relaxation measurements, and the pulsed-field-gradient method. An example of the use of the method of moments is the work of Stevenson (194) on H resonances in zeolite H-Y (see Section III,K). Another is the study by Lechert and Wittem (284) of C6H6 and C6H3D3 adsorbed on zeolite Na-X. Analysis of second moments of H resonances allowed the intra- and intermolecular contributions to the spectra to be extracted. Similarly, second moments of H and 19F spectra of cyclohexane, benzene, fluorobenzene, and dioxane on Na-X provided information about orientation of molecules within zeolitic cavities (284-287). 

Since the pioneering work of Stejskal, the pulse field gradient method is currently used to characterize the diffusion process of small molecules or of macromolecules in dilute or semi-dilute solutions [18-20]. In this Chapter, the NMR approach is illustrated from the self-diffiision of ( dohexane molecules through polybutadiene. Variations of the Ds self-diffusion coefficient of cyclohexane in polybutadiene have been reported as a temperature function considering several concentrations [21]. 

The obtained parameter values are summarized in Table 3. The self-diffusion coefficient of the proton decreases with lowering temperature, suggesting that the translational motion of water molecules is restricted at the low temperatures. This is probably because of the water-water interaction will be enhanced with decreasing temperature. The self-diffusion coefficients obtained from QENS are less than those determined by the H-NMR pulsed-field gradient method. A probable reason for the difference in Z h is ascribed to different time scales observed by both methods ( 200 peV for LAM40). 

The next three sections deal with steady and pulsed field gradient methods for measuring translational diffusion in liquids and plastic solids. There are other methods for measuring slower diffusion which we will not really touch upon. See, for example, Ailion, referenced in IV.C.l., and Burnett and Harmon (1972). 

Pulsed field gradient methods may be used in combination with a spin-echo pulse sequence to measure average molecular displacements in a time In liquids, PFG methods can therefore be used to measure self-diffusion coefficients (i.e. the rate of diffusion due to Brownian motion in the absence of a concentration gradient). In porous media, there is the possibility of obtaining information about the pore geometry because the pore boundaries will influence molecular transport. PFG techniques can measure restricted diffusion and thus provide valuable information on pore sizes in the range 5-100 (im. 

The most recent, successful and widely applicable approach to suppressing zero-quantum contributions employs a pulsed field gradient method for the net dephasing of these coherences in a single scan with no requirement for experiment repetition [88]. Since PFGs alone cannot manipulate ZQCs, the gradients must be applied in a rather ingenious manner to be effective, tbe operation of wbicb may be understood as follows. 

Some of the most successful methods of nuclear magnetic resonance used in the study of disordered porous media have been reviewed and their applications to the measurement of pore size distribution, diffusion coefficients and permeability have been presented. This is the case of nuclear relaxation and pulsed field gradient method of a specific imbibed solvent in various fully or partially saturated organic or inorganic porous media. The cases of narrow and large distributions of pores have been... 

The pulsed field gradient method of self-diffusion measurement was originally developed by Stcjskal and Tanner " for the measurement of diffusion in liquids. The self-diffusivity is measured directly and no prior estimate of jump distance is needed so the results are, in principle, more reliable. The development and application of this technique to the study diffusion in zeolites and zeolitic adsorbents has been achieved largely through the researches of Pfeifer, Karger, and their co-workers at the Karl Marx University in Leipzig,... 

We start by summarizing the pulsed field gradient method and then describe two relevant topics the first deals with droplet size determination of emulsions and the second deals with the characterization of concentrated emulsions. The former is a well-developed area, which has been used for a long time not only by the present author, while the latter constitutes a novel field for the application of the method. We present some (very) preliminary data from such a system. 

The temperature dependent H, H, and chemical shifts of NH3 and ND3 have been calculated.The pressure and temperature dependence of self diffusion in fluid ammonia have been investigated using the NMR pulsed field gradient method. Substituent effects on the P chemical shifts of PX2Y, X, Y = F, OH, NH2, CH3, BH2, BeH, Li, H, have been analysed. The quadrupole spin relaxation of in NNO in collision with various molecules has been determined.chemical shifts and some /( P N) values have been reported for three classes of compounds containing P-N bonds. The relations of P chemical shift with degree of substitution and substituent have been determined for cyclotriphosphazenes. and NMR spectroscopy has... 

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