2D Diffusion-Diffusion Correlation NMR Spectroscopy in Study of Diffusion Anisotropy in Wood

This chapter describes how to use 2-dimensional pulsed field gradient (PFG) NMR spectroscopy to explore the anisotropic diffusion of water in wood. Simulations based on the theory of 2D diffusion-diffusion correlations with the application of inverse Laplace transform (ILT) have been carried out for orthogonal and collinear pairs of gradients with parameters of 2D experiment on wood. The algorithm uses analytical solution for the echo attenuation in the 2D PFG experiment with collinear/orthogonal applications of two gradient pulses pairs. Numerical expressions calculated for the gradients and the model diffusion coefficients in both orthogonal directions were treated by 2D ILT on various artificial and experimental data sets. The algorithm is presented in a NMR context for wetting wood when the gradient directions are switched. The results showed how 2D (D1, D2) ILT maps reflected different ratios D1/D2 in two orthogonal directions. When two pairs of gradients were applied orthogonally in 2D experiment on wood, the features of diffusion anisotropy appeared as off-diagonal “wings” in 2D diffusion map whereas in the experiment with two collinear pairs of gradients, anisotropic diffusion of water in wood cells has been observed as two diagonal peaks in 2D map.

Keywords: 2D diffusion-diffusion correlation NMR spectroscopy (DDCOSY), Anisotropic diffusion, Apparent diffusion coefficient Dapp, Free induction decay (FID), Inverse Laplace transform (ILT), NMR, Pulsed Field Gradient (PFG) NMR, Spin-echo (SE), Spin-lattice (longitudinal) relaxation time T1, Spin-spin (transverse) relaxation time T2, Stimulated echo (STE), Water, Wood.