Probing four orders of magnitude of the diffusion time in porous silica glass with unconventional NMR techniques German Farrher a , Ioan Ardelean b, * , Rainer Kimmich a a Sektion Kernresonanzspektroskopie, Universita ¨ t Ulm, 89069 Ulm, Germany b Technical University from Cluj-Napoca, Physics Department, 400020 Cluj-Napoca, Romania Received 17 February 2006; revised 21 June 2006 Available online 21 July 2006 Abstract The combined use of two unconventional NMR diffusometry techniques permits measurements of the self-diffusion coefficient of flu- ids confined in porous media in the time range from 100 microseconds to seconds. The fringe field stimulated echo technique (FFStE) exploits the strong steady gradient in the fringe field of a superconducting magnet. Using a standard 9.4 T (400 MHz) wide-bore magnet, for example, the gradient is 22 T/m at 375 MHz proton resonance and reaches 60 T/m at 200 MHz. Extremely short diffusion times can be probed on this basis. The magnetization grid rotating frame imaging technique (MAGROFI) is based on gradients of the radio fre- quency (RF) field. The RF gradients not necessarily need be constant since the data are acquired with spatial resolution along the RF gradient direction. MAGROFI is also well suited for unilateral NMR applications where all fields are intrinsically inhomogeneous. The RF gradients reached depend largely on the RF coil diameter and geometry. Using a conic shape, a value of at least 0.3 T/m can be reached which is suitable for long-time diffusion measurements. Both techniques do not require any special hardware and can be imple- mented on standard high RF power NMR spectrometers. As an application, the influence of the tortuosity increasing with the diffusion time is examined in a saturated porous silica glass. Ó 2006 Elsevier Inc. All rights reserved. Keywords: NMR; Diffusion; Porous media; Radiofrequency gradients; Fringe field gradients; Pulse field gradients 1. Introduction The most popular principle of NMR diffusion measure- ments is based on the attenuation of spin echoes due to incomplete refocusing of coherences as a consequence of incoherent molecular displacements during the pulse sequence. Echo attenuation on these grounds arises in the presence of pulsed or steady gradients of the main magnetic flux density. Any sort of gradient-based echo can be used. Typical examples are the Hahn and the stimulated echo [1– 3]. Here we consider the stimulated echo arising after three RF pulses in the presence of the steady fringe field gradient of a superconducting magnet. The method will be called fringe field stimulated echo (FFStE) technique. On the other hand, there are methods employing gradients of the amplitude of the radio frequency flux density, i.e. RF field gradients [4]. Such an alternative protocol for diffusion measurements was successfully demonstrated with rotating- frame echo phenomena [3–7]. Furthermore, B 1 and B 0 gradi- ents can be applied in mixed form. If suitably matched, such mixed combinations of gradients lead to ‘‘nutation echoes’’ [8,9] the diffusive attenuation of which can also be used for molecular displacement studies [10]. The localized character of nutation echoes [9] in principle permits remote measure- ments of diffusion coefficients. Moreover, it may be possible to accomplish diffusion measurements with chemical shift resolution in inhomogeneous static magnetic fields [11], pro- vided that the directions of B 1 and B 0 gradients coincide. With the techniques mentioned so far, a non-equilibri- um magnetization distribution is first prepared in the form of a ‘‘helix’’ or—with respect to a certain component—as a 1090-7807/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2006.06.032 * Corresponding author. Fax: +40 264 401 536. E-mail address: ioan.ardelean@phys.utcluj.ro (I. Ardelean). www.elsevier.com/locate/jmr Journal of Magnetic Resonance 182 (2006) 215–220