(5c) Quantitative, Multinuclear Magnetic Resonance Microscopy Methods for the Study of Multiphase Systems

Magnetic resonance microscopy
(MRM)

MRM is a non-invasive, non-destructive
technology that can be used to probe optically opaque systems.  By judicious design
of magnetic resonance (MR) methodologies, this mode of visualization can be
made sensitive to a number of phenomena including chemical composition and both
coherent (velocity, acceleration) and incoherent molecular motion (diffusion,
dispersion).  While it is better known in the context of medical diagnosis, MR
imaging has found application in fields ranging from Physics to Engineering and
Geology ? with applications as varied as microfluidics, catalysis, materials
characterization, multiphase flows, fluidization, oil recovery, and drug
delivery. 

Multinuclear MR and multiphase
systems

The extent to which
chemically-specific information can be resolved in a magnetic resonance
experiment depends on both the physical attributes of the system being studied
and the nature of the particular NMR technique employed. Frequently, in a
magnetic resonance imaging experiment, the ¹H nucleus is observed,
because it has the highest detection sensitivity of all the NMR-active nuclei.
However, in the study of heterogeneous porous media, observation of the ¹H
nucleus has some distinct disadvantages. Although NMR signal is inherently both
chemical and nucleus-specific, the frequency characteristics that allow
individual resonances to be identified can easily become obscured by a number
of effects. Here, a multinuclear approach to chemical mapping has been adopted
as a means of improving the versatility of the MRM toolkit for the study of
heterogeneous systems.  Advantages of the multinuclear approach will be
highlighted; these include: (i) improved frequency resolution via the inherent
superior chemical shift dispersion of X-nuclei, (ii) reduced vulnerability to
magnetic susceptibility-induced field gradients at interfaces, (iii)
longer signal lifetimes, permitting observation of transport phenomena on
longer time-scales (and hence, length-scales), and (iv) inherent or
highly efficient solvent suppression in aqueous systems.  The studies reported
here focus primarily on the detection of ¹³C at natural abundance.
However, the techniques presented are applicable to other spin-½ nuclei (e.g. ³¹P,
¹⁵N).

Expanding the MRM toolkit to
access new information about complex systems

Tailored MRM methods have been
implemented to achieve the following.

Heteronuclear imaging methods for
the study of ¹³C at natural abundance, more specifically: In situ,
quantitative mapping of conversion and selectivity in heterogeneous
catalytic reactors^1-3.

Hybrid polarization transfer and pulsed-field
gradient (PFG) methods for the study of species-specific diffusion and
dispersion in multicomponent systems, including: Flow in packed beds⁴, mass transport in biofilm systems⁵, and characterization of emulsion structure⁶ with superior phase resolution and extended observation time-scales.

Characterization of anomalous
diffusion as probed by MR, focusing on: The relevance of observation time-scale⁵, reconciliation of proposed physical models with the observed displacement dynamics, and a new approach to characterizing structure and improving image contrast by application of fractional calculus⁷.

Quantitative visualization of mixing
processes in a microfluidic device, including: Concentration mapping in
optically opaque microchannels of arbitrary cross-section and mapping of
evolving flow fields^{8, 9}.

References:

1. B. S. Akpa, M. D. Mantle, et al. In situ ¹³C DEPT-MRI as a tool to spatially
resolve chemical conversion and selectivity of a heterogeneous catalytic reaction
occurring in a fixed bed. Chemical Communications. 21, 2741 (2005)

2. L. F. Gladden,
B. S. Akpa, et al. (2005). In situ reaction imaging in fixed-bed
reactors using MRI. NMR Imaging in Chemical Engineering. S. Stapf and H.
Song-I. Weinheim, Wiley-VCH: 590-606.

3. A. J.
Sederman, M. D. Mantle, et al. In situ MRI study of 1-octene
isomerisation and hydrogenation within a trickle-bed reactor. Catalysis
Letters. 103, 1-8 (2005)

4. B. S. Akpa, D.
J. Holland, et al. Enhanced C-13 PFG NMR for the study of hydrodynamic
dispersion in porous media. Journal of Magnetic Resonance. 186, 160-165 (2007)

5. D. A. Graf von
der Schulenburg, B. S. Akpa, et al. Non-invasive mass transfer
measurements in complex biofilm-coated structures. Biotechnology and
Bioengineering. Accepted Article, (2008)

6. B. S. Akpa, M.
L. Johns, et al. (2008). Heteronuclear PFG-NMR methods for the sizing of
emulsion droplets using natural abundance ¹³C. 49th Experimental NMR
Conference, Asilomar, CA.

7. B. S. Akpa, O.
Abdullah, et al. (2008). Anomalous diffusion expressed through
fractional order differential operators in the Bloch-Torrey equation. Magnetic
Resonance in Porous Media, Cambridge, MA.

8. B. S. Akpa, S.
M. Matthews, et al. Study of miscible and immiscible flow in a
microfluidic device using magnetic resonance imaging. Analytical Chemistry. 79,
6128-6134 (2007)

9. S. P.
Sullivan, B. S. Akpa, et al. Simulation of miscible diffusive mixing in
microchannels. Sensors and Actuators B. 123, 1142-1152 (2007)