Simple modifications to the NMR PGSE technique for investigations in the regime of short diffusion observation times

Abstract

Simple modifications to the spin-echo NMR technique with pulsed magnetic field gradients are proposed. These modifications make it possible to compensate the negative influence of gradient pulse fronts by applying an additional gradient pulse, to eliminate the acoustic effect of gradient pulses on a sample by decoupling the probe and gradient coils, and to simplify the generation of bipolar gradient pulses of the magnetic field using a scheme of two electronic circuits. KEY WORDS: nuclear magnetic resonance; magnetic field gradient; pulse sequences; diffusion Experience with pulsed magnetic field gradient spin echo (PGSE) NMR in investigations of translational molecular diffusion shows that experiments with short diffusion times require much more care than those with long diffusion times. As a rule, two approaches are used for short diffusion-time experiments. In the first case, the conventional two-pulse Hahn echo sequence (908-1808-echo) with two magnetic field gradient pulses is used. The gradient pulses are switched on during the defocusing (908-1808) and the refocusing (1808-echo) of the time intervals of the magnetization (1).In the second case, a sequence of bipolar magnetic field gradient pulses is used. These pulses are switched on during the time interval of the signal free induction decay (FID) (2). For liquids, the decay time of the FID, or the attenuation rate, is determined by the magnetic field heterogeneity of the NMR magnet.As a result, the magnetic field heterogeneity determines the available time interval for applying bipolar magnetic field gradient pulses.In conventional constructions of pulsed field gradient (PFG) systems, gradient coils (GCs) combined in series, are used to generate a gradient in the Zdirection. Most often, these are of the quadrupole type (3), and a group of rectangular coils is used to generate the gradient in the Y-direction (4, 5). The coils, as a rule, are constructively interlocked with an NMR probe and connected to the current source by a precision current amplifier (6). To produce the bipolar pulses, transistor-based current amplifiers are typically employed.With the increasing amplitude g and duration d of the gradient pulses (the interval between pulses is D), the range of measurable diffusion coefficients is extended, and the observation time (D 2 d/3) for molecular diffusion can be reduced. The increase in the PFG magnitude is achieved by increasing the number of GC turns and the pulse current. Correct measurements are carried out under the conditions of excellent matching of both gradient pulse areas (dg) 1 5 (dg) 2 .The following problems arise when carrying out experiments at short diffusion times and high magnetic field gradients:

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