Record a normal ¹H NMR spectrum of the sample and center the offset at the water resonance; set and control the temperature at 298 K. In this experiment one can vary either the gradient strength or the gradient length δ. For observation of restricted diffusion Δ would be varied, since Δ is the time during which the diffusion process occurs. In this example we vary the gradient strength. You have to set:

td	1 k
sw	1000 Hz
o1	on resonance of water signal
p1	90° ¹H transmitter pulse
p2	180° ¹H transmitter pulse
d1	5 s
d2	1 ms
d3	10 ms (dependent on gradient ring-down time)
d4	d3 minus pre-acquisition delay
g1, g2	rectangular-shaped field gradients, 4 ms duration and variable strength from 0 to 0.2 T/m in 10 steps, with gradient loop counters, ring-down delays (100 μs), lock blanking and gradient coil blanking switches according to the actual instrumentation used. Gradient strength ratio: 1 : 1.
ns	1

5. Processing

Process all 10 spectra identically with exponential multiplication using lb = 5 Hz and a base-line correction in order to obtain good integrals. Integrate the water signal in all spectra and refer all integrals to the integral value of the starting spectrum with gradient strength of 0. Compile a table of integral ratios I_g/I₀ vs. gradient strength G used, where the gradient strength is determined as described in Experiment 11.1. Equation (1) relates the integral ratio to the diffusion constant D.

ln (I_g/I₀) = − [γ² δ² G² (Δ − δ/3)] D(1)

Thus, a plot of ln(I_g/I₀) versus G² yields the diffusion constant from the slope, when the values γ = 2.675·10⁸ T⁻¹s⁻¹, δ = 0.004 s, and Δ = 0.0151 s are inserted.

6. Result

The table and figure give the results obtained on an AMX-500 spectrometer equipped with a BGU (10 A) gradient unit and an inverse multinuclear z-gradient probe-head, giving a value of D = 2·10⁻⁹ m²/s, which is reasonably close to the accepted literature value of 2.30·10⁻⁹ m²/s [4, 5].

G [T/m]	0	0.022	0.045	0.065	0.089	0.11	0.13	0.15	0.18	0.19	0.22
I_g/I₀	1	0.96	0.92	0.88	0.82	0.73	0.62	0.52	0.40	0.30	0.22