An optimised detector for in-situ high-resolution NMR in microfluidic devices

Integration of high-resolution nuclear magnetic resonance (NMR) spectroscopy with microfluidic lab-on-a-chip devices is challenging due to limited sensitivity and line broadening caused by magnetic susceptibility inhomogeneities. We present a novel double-stripline NMR probe head that accommodates planar microfluidic devices, and obtains the NMR spectrum from a rectangular sample chamber on the chip with a volume of 2μl. Finite element analysis was used to jointly optimise the detector and sample volume geometry for sensitivity and RF homogeneity. A prototype of the optimised design has been built, and its properties have been characterised experimentally. The performance in terms of sensitivity and RF homogeneity closely agrees with the numerical predictions. The system reaches a mass limit of detection of 1.57nmols, comparing very favourably with other micro-NMR systems. The spectral resolution of this chip/probe system is better than 1.75Hz at a magnetic field of 7T, with excellent line shape.