Microcoils for Broadband Multinuclei Detection

To date, NMR microcoils are mostly used for enhancing the spin sensitivity in homonuclear 1D NMR experiments because the design and manufacturing of standard microcoil probes for multinuclear detection require complex and sophisticated electronic circuitry. In this chapter, an alternative approach toward microcoil NMR, which uses a simplified front-end consisting of a planar spiral microcoil-on-a-chip terminating a coaxial cable with no tuning and matching circuitry, is discussed, both experimentally and theoretically. Due to the simple nature of the front-end without tuning or matching elements, the proposed solution can operate as a high-resolution “all-in-one” NMR system, with broadband character. Moreover, this relatively simple setup is capable of executing 1D broadband as well as complex heteronuclear 2D pulse sequences, on practically any combination of nuclides and with excellent mass sensitivity. The exciting broadband properties of microcoils require a radical shift in the conceptual thinking of RF circuitry for NMR applications and probe design and, moreover, the broadband coil concept provides a low-cost alternative to commercial NMR probe systems, enabling mono- and multidimensional experiments using a single microcoil. It is therefore somewhat surprising that broadband circuit probes are not in the normal arsenal of commercially available probes, which in turn raises the question how widely applicable and robust the concept is. To answer this question, in this chapter, we will also discuss how non-tuned circuits demand a different view on the classical electronics in NMR probes and open up the window to explore (micro) technologies to make integrated small NMR systems. We will further motivate why for these systems it becomes particularly attractive to co-design the spectrometer electronics together with the broadband coils to enhance system performance and robustness. Finally, in the conclusions and outlook section, we will outline how the paradigm-shifting idea of a non-resonant system opens up new horizons for NMR spectrometers, such as the reality of magnetic field-independent NMR probes.