Untuned broadband spiral micro-coils achieve sensitive multi-nuclear NMR TX/RX from microfluidic samples

The low frequency plateau in the frequency response of an untuned micro-resonator permits broadband radio-frequency reception, albeit at the expense of optimal signal-to-noise ratio for a particular nucleus. In this contribution we determine a useful figure-of-merit for broadband microcoils, and thereby explore the parametric design space towards acceptable simultaneous excitation and reception of a microfluidic sample over a wide frequency band ranging from 13C to 1H, i.e., from 125-500 MHz in an 11.74 T magnet. The detector achieves 37 % of the performance of a comparably sized, tuned & matched resonator, and a linewidth of 16.6 ppb using standard magnet shims. The use of broadband detectors circumvents numerous difficulties introduce by multi-tuned RF detector circuits, including sample loading effects in matching, channel isolation, and field distortion.