Pulse-field-gradient spin-echo technique

FIGURE 9.3 Pulse sequence for measurement of diffusion coefficient D by the pulsed field gradient spin echo technique. The gradient G is applied for a period 8 both before and after the 180° pulse, with separation A. 

We finish this section by comparing our results with NMR and incoherent neutron scattering experiments on water dynamics. Self-diffusion constants on the millisecond time scale have been measured by NMR with the pulsed field gradient spin echo (PFGSE) method. Applying this technique to oriented egg phosphatidylcholine bilayers, Wassail [68] demonstrated that the water motion was highly anisotropic, with diffusion in the plane of the bilayers hundreds of times greater than out of the plane. The anisotropy of... 

Umecky, T, Kanakubo, M., and Ikushima, Y., Self-diffusion coefficients of l-butyl-3-methylimidazolium hexafluorophosphate with pulsed-field gradient spin-echo NMR technique. Fluid Phase Equilib., 228-229, 329, 2005. 

The double pulsed field gradient spin echo (DPFGSE) pulse sequence has been used to improve the measurement of proton-proton nuclear Overhauser effect (NOE) [28]. The DPFGSE NOE does not rely on difference spectroscopy and very small NOEs can be measured. This technique has been used to determine the structure of organosilicon compounds [28]. 

Molecular self-diffusion coefficients are experimentally easily accessible and can be measured accurately with various techniques. However, the Fourier transform pulsed field gradient spin-echo NMR (FT PGSE NMR) approach has during the last decade proved to be superior to other approaches for several reasons. In this technique the displacement of nuclear spins in a controlled magnetic field gradient is monitored, and the contributions of different components are resolved by Fourier transformation of the NMR signal (spin echo). 

Recently, data regarding the lignin study both in the solid state and solution with the modem and performance techniques, namely the small-angle X-ray scattering (SAXS) and ultra-smaU-angle X-ray scattering (USAXS) [166], pulsed field gradient spin-echo NMR [167, 168], nano zeta sizer techniques [169] have been reported. 

As a qualitative means to investigate the ionic nature of acid-base systems, various techniques have been employed such as one- or two-dimensional nuclear magnetic resonance (NMR) spectroscopy [31], infrared (IR) spectroscopy [32], the Walden plot [33], the ratio of molar conductivities AjmpMNMR (determined by electrochemical impedance spectroscopy and pulse-field-gradient spin-echo NMR, respectively) [34], Some of these will be discussed below. 

Pulsed-Field Gradient Spin-Echo NMR Diffusion NMR experiments resolve different compounds in a mixture based on their diffusion coefficients, depending on physical parameters such as size and shape of the molecules, temperature, and viscosity. The diffusion NMR technique is often referred to as diffusion-ordered spectroscopy (DOSY) or pulsed-field gradient spin-echo (PGSE) NMR. A series of NMR diffusion spectra are acquired as a function of the gradient strength G (Fig. 2.19) [56], and the slope of the peak decay is used to obtain the diffusion coefficient D. Furthermore, the hydrodynamic radius can be obtained from the Stokes-Einstein equation (Eq. 2.3). 

Diffusion Coefficients of l-Butyl-3-methylimidazolium Hexafluorophosphate with Pulsed-Field Gradient Spin-Echo NMR Technique. 

Figure 15.1. Pulse sequences for the standard pulsed field gradient spin-echo (a) and pulsed field gradient stimulated spin-echo (b) techniques. Note that in the latter case, the length (5) and strength (C) of the gradient pulse is omitted for clarity but have the same meaning as in part (a)...

Pulsed NMR technique, 250-2 self-difiusion coefficient, 252-7 Pulsed-field-gradient spin echo (POSE) technique, 254-7 measurement examples, 257 pulse sequence, 255... 

There are a number of NMR methods available for evaluation of self-diffusion coefficients, all of which use the same basic measurement principle [60]. Namely, they are all based on the application of the spin-echo technique under conditions of either a static or a pulsed magnetic field gradient. Essentially, a spin-echo pulse sequence is applied to a nucleus in the ion of interest while at the same time a constant or pulsed field gradient is applied to the nucleus. The spin echo of this nucleus is then measured and its attenuation due to the diffusion of the nucleus in the field gradient is used to determine its self-diffusion coefficient. The self-diffusion coefficient data for a variety of ionic liquids are given in Table 3.6-6. 

While the nuclear magnetic resonance (NMR) technique has widely been used to study diffusion processes of normal liquids, solids, or colloidal systems, there are only a few applications to molten salts. The spin echo self-diffusion method with pulsed field gradients was applied to molten salts by Herdlicka et al. "" There is no need to set up or maintain a concentration gradient. 

When pulsed magnetic field gradients are applied to study diffusive processes, the MR technique is often referred to as pulsed field gradient or pulsed gradient spin echo (PGSE) MR. Application of PGSE MR techniques to quantify molecular diffusion was pioneered by Stejskal and Tanner 17,18), and the techniques typically probe molecular displacements of 10 -10 m over time scales of the order 10 M s. 

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