Nuclear spin dynamics in a microfluidic chip

Abstract

The developments in Nuclear Magnetic Resonance experiments have evolved from static samples towards more realistic conditions in which spin dynamics are simultaneously coupled to diffusion, flow and chemical kinetics. In many such systems, including microfluidic NMR, hyperpolarised NMR, metabolic MRI, and catalytic MRI, the underlying chemistry is intrinsically second-order. However, in many simulation frameworks, these processes are often approximated as first-order because quantum mechanical equations of motion are strictly linear in density operators, but equations describing chemical kinetics and hydrodynamics may be nonlinear in concentrations.This thesis introduces a numerically stable, time-domain description of nuclear spin dynamics in the simultaneous presence of isotropic diffusion, flow, and second-order chemical reactions, enabling the treatment of nonlinear kinetics within a linear Liouville-space framework. As an illustration, the method is applied to the Diels–Alder cycloaddition of acrylonitrile and cyclopentadiene under continuous flow in a microfluidic NMR probe, modelled using a finite-volume discretisation comprising thousands of Voronoi cells and a spatially localised stripline radio-frequency coil. In addition, the feasibility of optimal control techniques, namely Gradient Ascent Pulse Engineering (GRAPE), is tested in isolation to realise the polarisation-transfer step of a para-hydrogen-induced polarisation (PHIP-on-a-chip) experiment, under static sample conditions. Preliminary tests indicate that the optimised pulses only match the performance of conventional block-pulse sequences when explicit robustness to B0, B1 inhomogeneities, the scalar J-coupling constant, and phenomenological relaxation effects is enforced during optimisation.

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Identifiers

ORCID for Anupama Acharya: orcid.org/0009-0005-6404-2183

ORCID for Ilya Kuprov: orcid.org/0000-0003-0430-2682

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