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Low Field Nuclear Magnetic Resonance

6.1. Low Field Nuclear Magnetic Resonance. High Field (Resolution) NMR: 7.5 T < B < 37 T. Study of chemical structures, reactions (only solution). Study of physical structures (solid, liquid, gel, solution, suspension, emulsion). Low Field (Resolution) NMR: 0.37 T < B < 2.43 T. Hydrogen.

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Low Field Nuclear Magnetic Resonance

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  1. 6.1 Low Field Nuclear Magnetic Resonance High Field (Resolution) NMR: 7.5 T < B < 37 T Study of chemical structures, reactions (only solution) Study of physical structures (solid, liquid, gel, solution, suspension, emulsion) Low Field (Resolution) NMR: 0.37 T < B < 2.43 T Hydrogen B0 = 0 mi(permanent magnetic momentum) H H H H H H H H H H H H

  2. B0≠ 0 n0 = g B0 n0=Larmor frequency (Hz) H H H H H H H H H H H H H H H H H H H H H H H g = hydrogen giromagnetic ratio (2.67*108 rad/Ts)

  3. Z B0 H H B1 X t = t0 - - - > B1>> B0 Y B0≠ 0

  4. H Relaxation (T1) Mz/M0 X Mxy/M0 Relaxation (T2) t = t1 > t0 - - - > B1= 0 Z B0 Y

  5. Discontinuous spectrum A1 A3 A2 A4 Continuous spectrum A3 B A4 C D a(T2) A1 A2 Determination of T2i

  6. PORES CONFINED WATER WATER MOLECULE = x x HOMOGENEOUS HYDROGEL EXTERNAL WATER “Bound” water molecules: SHORT RELAXATION TIME (T2) T2≤ 300 ms “Free” water molecules: LONG RELAXATION TIME (T2) T2~ 2200 ms (25°C) It can be demonstrated1 that T2 f(x) Effect of environment on T2

  7. 1) Diffusion between bulk and surface much faster than relaxation 2) If fb≈ 1 and fs≈ 0 (x > 10 nm) 3) T2<< T2b (≈ 2200 ms T = 25°C, B = 0.47 tesla) TWO FRACTION – FAST EXCHANGE MODEL2 x T2b T2s Water molecule

  8. T2b = 2200 ms

  9. k determination Mesh size distribution

  10. B0 B0 B1(p/2) B1(p) Signal intensity (A0) t(ms) B1(p) B1(p/2) t t B0 B1(p/2) B1(p) Signal intensity (A) gradient gradient t(ms) d t d D determination: gradient test

  11. B0 n0 = g B(x) B0 n0 = g B(x) phase shift still present phase shift zeroed B1(X) B1(X) B1(p) B1(p) H DIFFUSION n0 = g B(x) NO H DIFFUSION n0 = g B(x) phase shift phase shift It can be demonstrated that the following relation holds2: Ln(A/A0) = -g2Dd2 (D - d/3) G2 • A = signal intensity with gradients • A0 = signal intensity without gradients • D = water molecules self diffusion coefficient • = hydrogen giromagnetic ratio d = gradient duration D = intergradient separation G = gradient intensity (T/m) D can be determined as a function of the diffusion time td = D-d/3

  12. It can be demonstrated that the for small td, the following relation holds3: td = diffusion time (= D-d/3) D(td) = water self diffusion coefficient inside the hydrogel at td D0 = water self diffusion coefficient

  13. Latour4 proposed the following expression holding for every td • = characteristic time a = network tortuosity

  14. Water diffusion coefficient (DH2O) dependence on temperature (T)5

  15. REFERENCES • Brownstein K.R., et al. Physical Review A, 197919, 2446. • Brownstein K.R., et al., J. Magnetic Resonance, 1977, 26, 17 • Mitra P.P., et al. Physical review B, 1993, 47(14), 8565. • Latour L.L. et al., J. Magnetic Resonance A, 1993, 101, 342. • Holz M. et al., Phys. Chem. Chem. Phys., 2000, 2, 4740.

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