The precision of nuclear magnetic resonance spectroscopy 1 (NMR) is limited by the signal-to-noise ratio, the measurement time T m and the linewidth ν = 1/(πT 2 ). Overcoming the T 2 limit is possible if the nuclear spins of a molecule emit continuous radio waves. Lasers 2,3 and masers 4-13 are self-organized systems which emit coherent radiation in the optical and microwave regime. Both are based on creating a population inversion of specific energy states. Here we show continuous oscillations of proton spins of organic molecules in the radiofrequency regime (raser 5 ). We achieve this by coupling a population inversion created through signal amplification by reversible exchange (SABRE) 14-16 to a high-quality-factor resonator. For the case of 15 N labelled molecules, we observe multi-mode raser activity, which reports di erent spin quantum states. The corresponding 1 H-15 N J-coupled NMR spectra exhibit unprecedented sub-millihertz resolution and can be explained assuming two-spin ordered quantum states. Our findings demonstrate a substantial improvement in the frequency resolution of NMR.Radio-wave masers (rasers) arise from the radiofrequency Zeeman splittings of nuclear spins such as 1 H, 3 He, 29 Al or 129 Xe. Rasers using 3 He and 129 Xe gas as the rasing medium employ spin exchange optical pumping 17 (SEOP) at T > 400 K to create sufficient population inversion 6-8 , whereas 29 Al solid 9 or 1 H Zeeman liquidstate masers 10 rely on dynamic nuclear polarization (DNP) techniques 18 or photochemical excitation 11 to invert populations. Solidstate maser action has been observed in pulsed mode at room temperature with pentacene 12 , and a continuous-mode solid-state maser based on nitrogen-vacancy centres in diamond has been proposed 13 .Here we report the observation of a liquid-state para-hydrogen pumped molecular raser that operates at 300 K with protons of organic molecules in solution and thereby avoids costly high magnetic fields, high vacuum, optical pumping or DNP techniques. We continuously supply para-hydrogen (p-H 2 ) gas into a solution containing the raser active molecules and an iridium-based SABRE catalyst [14][15][16] . This spin-order transfer catalyst creates population inversion, equivalent to a negative spin temperature, on target molecules without altering their molecular structure. Coupling of these hyperpolarized molecules to a high-Q resonator 19 produces a sustained raser signal comprised of frequencies that originate from the scalar couplings of nuclei within the molecule.The operating principles for the 3 He Zeeman maser 6-8 are a starting point to assess challenges associated with the design of a SABRE-pumped room-temperature proton raser working at low frequencies. Masing starts once the radiation-damping rate 1/τ rd , which quantifies the coupling between the resonator and the nuclear spins, satisfies the conditionHere the apparent transverse relaxation rate 1/T * 2 = 1/T 2 + 1/τ p is the sum of the transverse relaxation rate 1/T 2 and the pumping rate 1/τ p . The radiation-dampi...
The development of powerful sensors for the detection of weak electromagnetic fields is crucial for many spectroscopic applications, in particular, for nuclear magnetic resonance (NMR). Here we present a comprehensive, new theoretical model for boosting the signalto-noise ratio, validated by liquid-state 1 H, 129 Xe, and 6 Li NMR experiments at low frequencies (20-500 kHz), using an external resonator with a high quality factor combined with a low-quality-factor input coil. In addition to a very high signal-to-noise ratio, this external high quality-factor enhanced NMR exhibits striking features such as a large flexibility with respect to input coil parameters, and a square-root dependence on the sample volume, and signifies an important step towards compact NMR spectroscopy at low frequencies with small and large coils.Nuclear magnetic resonance spectroscopy preferably operates at high magnetic fields (10-24 T) to benefit from high sensitivity and high chemical shift dispersion. The signal is detected by nuclear induction in coils, which are part of a resonant circuit [1][2][3] . NMR operating at high frequencies (~500 MHz) is particularly in demand for microcoil-based spectroscopy with sample volumes smaller than 1 L (ref. 4). Microcoils, with their small inductance (L < 1 nH), cannot readily be tuned to lower resonance frequencies (<1 MHz) owing to the large capacity (C > 1 mF) necessary to fulfil the resonance condition L C . Sillerud et al. 5 were the first to address this problem by adding an external inductor to a microcoil for NMR detection at 40 MHz. Coffey et al. 6 reported a very weak frequency dependence of the signal-to-noise ratio (SNR) when comparing hyperpolarized NMR/MRI at 0.047 T and 4.7 T (keeping the polarization constant), with a possibly higher SNR of hyperpolarized low-field NMR/MRI. So far, NMR spectroscopy with microcoil detection and a large SNR have not be realized at very low Larmor frequencies -1000 kHz, where is the gyromagnetic ratio of the nuclear spin species and B0 is the strength of the static magnetic field. Instead, other unconventional schemes for NMR detection are being explored at low frequencies. Such lowfrequency NMR field sensors are superconducting quantum interference devices (SQUIDs, refs 7,8) and atomic magnetometers 9,10 with sensitivities better than 1 fT/Hz 1/2 . Moreover, a single nitrogen vacancy (NV) centre in diamond can detect 100 out of 10 4 proton spins, which are
Welcome to the guest zone: By combining hyperpolarized xenon and simple low-field NMR devices it is possible to obtain more control over hydrogels that show potential as drug delivery systems. An alternative way of polymer swelling-degree determination is demonstrated with real-time NMR analysis. An ideal region for solvent uptake can be defined in which the absorbed solvent molecules are completely confined in the nano-porous network of the hydrogel.
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