Self-diffusion measurements using a radiofrequency field gradient

Abstract

Self-diffusion coefficient measurements have been carried out by means of a novel and efficient NMR technique, which employs the gradient of an RF field produced by a single-turn coil. The sequence involves also pulses of a homogeneous RF field and has the form δ(g) x−( Δ 2 )−180°(h) y−( Δ 2 )−δ(g) x−90°(h) x -Acq., where h refers to the homogeneous RF field whereas g denotes the application of the RF field gradient. The theory of the classical PGSE ( pulsed field gradient spin echo) technique (which makes use of a static field gradient) is reconsidered according to a simplified approach. This shows that similar equations prevail for the sequence using RF field gradient pulses. A probe has been designed in order to obtain a linear RF gradient across a 10 mm o.d. sample. A conventional saddle-shaped coil orthogonal to the single-turn coil produces the homogeneous RF field. It is doubly tuned at proton and deuterium frequencies and therefore allows stabilization of the spectrometer in the course of the experiment. The use of an RF field makes it possible to determine directly the magnitude of the gradient by a capillary whose position is varied. The method has been checked on water, benzene, and acetone samples at room temperature, with self-consistent results, in agreement with literature data. The proposed method permits measurement of self-diffusion coefficients with good accuracy (error less than 5%) in a much simpler way than with pulsed B 0 gradients.