The present invention relates to the field of nuclear magnetic resonance and, more particularly, to the field of detection of J- or scalar coupling.
Nuclear magnetic resonance (NMR) endures as one of the most powerful analytical tools for detecting chemical species and elucidating molecular structure. The fingerprints for identification and structure analysis are chemical shifts, nuclear Overhauser effects, and scalar couplings of the form JI1·I2. The latter yield useful information about molecular spin topology, bond and torsion angles, bond strength, and hybridization. NMR experiments are conventionally performed in high magnetic fields, requiring large, immobile, and expensive superconducting magnets. However, detection of NMR at low magnetic fields has recently attracted considerable attention in a variety of contexts, largely because it eliminates the need for superconducting magnets. Additional advantages of low and zero field NMR include extremely homogeneous fields (both spatially and temporally) for narrow lines and the appeal of measuring small contributions to the Hamiltonian in the absence of a much larger Zeeman interaction.
One-dimensional and two-dimensional spectroscopy (see, S. Appelt, H. Kühn, F. W. Häsing, B. Blümich, Chemical analysis by ultrahigh-resolution nuclear magnetic resonance in the Earth's magnetic field, Nat. Phys. 2 (2006) 105-109; and J. N. Robinson et al., Two-dimensional NMR spectroscopy in Earth's magnetic field, J. Magn. Res. 182 (2006) 343-347, respectively) have been demonstrated in the Earth's magnetic field using inductive detection. J-resolved spectra have been detected with superconducting quantum interference device (SQUID) magnetometers in ˜μT fields (see, R. McDermott et al., Liquid-state NMR and scalar couplings in microtesla magnetic fields, Science 295 (2002) 2247-2249). Atomic magnetometers have been used to perform one-dimensional spectroscopy (see, I. M. Savukov, M. V. Romalis, NMR detection with an atomic magnetometer, Phys. Rev. Lett. 94 (2005) 123001; I. M. Savukov, S. J. Seltzer, M. V. Romalis, Detection of NMR signals with a radio-frequency atomic magnetometer, J. Magn. Res. 185 (2007) 214-220; and M. P. Ledbetter et al., Zero-field remote detection of NMR with a microfabricated atomic magnetometer, Proc. Natl. Acad. Sci. (USA) 105 (2008) 2286-2290) and for remote detection of magnetic resonance imaging in low magnetic fields. Nuclear magnetic resonance in a zero-field environment has been detected indirectly using field cycling techniques (see, D. B. Zax, A. Bielecki, K. W. Zilm, A. Pines, Heteronuclear zero-field NMR, Chem. Phys. Lett. 106 (1984) 550-553; and D. B. Zax, A. Bielecki, K. W. Zilm, A. Pines, D. P. Weitekamp, Zero field NMR and NQR, J. Chem. Phys. 83 (1985) 4877-4905). However, this practice does not remove the requirement for a superconducting magnet.
Low Field Nuclear Magnetic Resonance
Nuclear magnetic resonance (NMR), conventionally detected in multi-tesla magnetic fields, is a powerful analytical tool for the determination of molecular identity, structure, and function. With the advent of prepolarization methods and alternative detection schemes using atomic magnetometers or superconducting quantum interference devices (SQUIDs), NMR in very low-(˜earth's field), and even zero-field, has recently attracted considerable attention. Despite the use of SQUIDs or atomic magnetometers, low-field NMR typically suffers from low sensitivity compared to conventional high-field NMR.
NMR1,2 in low or zero magnetic field has long been viewed as a curiosity due to the low nuclear spin polarization, poor sensitivity of inductive pickup coils at low frequencies, and the absence of site-specific chemical shifts.
Despite the use of atomic magnetometers or SQUIDS, low-field NMR using samples thermally prepolarized in a permanent magnet typically suffers from low signal-to-noise ratio compared to inductively-detected high-field NMR, in part because of the low polarization available from thermalization in a permanent magnet.
While parahydrogen induced polarization (PHIP) has been investigated in a variety of magnetic fields, ranging from the earth's field to high field, observation of the resulting NMR signals has always been performed in finite magnetic field.