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We optimised the magnetic field homogeneity of two canonical designs formobile microfluidic NMR applications: two parallel magnets with an air gap anda modified Halbach array. Along with the influence of the sample length,general design guidelines will be presented. For a fair comparison thesensitive length of the sample has been chosen to be the same as the gap sizebetween the magnets to ensure enough space for the transmitting and receivingunit, as well as basic electric shimming components. Keeping the compactness ofthe final device in mind, a box with an edge length 5 times the gap size hasbeen defined, in which the complete magnet configuration should fit. With thechosen boundary conditions, the simple parallel cuboid configuration reachesthe best homogeneity without active shimming (0.5$\mathrm{B_{s}}$, 41 ppm),while the Pseudo-Halbach configuration has the highest field strength(0.9$\mathrm{B_{s}}$, 994 ppm), assuming perfect magnets. However, permanentmagnet configurations suffer from imperfections, such as magnetisation,fabrication and positioning errors, which results in worse magnetic fieldhomogeneities than expected from simulations using a fixed optimised parameterset. We present a sensitivity analysis for a magnetic cube and the results ofstudies of the variations in the magnetisation and angle of magnetisation ofmagnets purchased from different suppliers, composed of different materials andcoatings, and of different sizes. We performed a detailed Monte Carlosimulation on the effect of the measured distribution of magnetic properties onthe mentioned configurations. The cuboid design shows a mean homogeneity of 430ppm (std dev. 350 ppm), the Pseudo-Halbach has a mean homogeneity of 1086 ppm(std dev. 8 ppm).

Influence of the distribution of the properties of permanent magnets on the field homogeneity of magnet assemblies for mobile NMR