Chemical analysis by ultrahigh-resolution nuclear magnetic resonance in the Earth’s magnetic field

Abstract

Increasing requirements of sensitivity and of spectral dispersion have driven the development of NMR magnets to higher and more homogeneous magnetic fields, which are obtained by immobile and expensive superconducting magnets. With the best field homogeneities available ( B/B ∼ 10 −9 over 1 cm 3 , where B is the magnetic field), ultrahigh-resolution carbon ( 13 C) NMR spectra at 4.2 T with an instrumental broadening below 50 mHz have been realized 2 . For 1 H high-field NMR spectroscopy (1-20 T), it is difficult to measure linewidths with an instrumental broadening below 100 mHz.We define our ideal NMR spectrometer by two requirements: first, it should measure NMR spectra with high resolution and all relevant NMR parameters, such as the longitudinal (T 1 ) and transverse (T 2 ) relaxation times, the chemical shift and the dipolar and J-coupling, in a single scan; and second, the spectrometer should be robust, low cost and mobile. Low cost and mobile means that heavy electro or superconducting magnets as well as superconducting quantum interference devices (SQUIDs) should be avoided. Single-scan and high-resolution NMR implies

Optical detection of NMR J-spectra at zero magnetic field

“…A magnetically shielded, zero-field environment provides high absolute field homogeneity and temporal stability, allowing us to obtain 0.1-Hz linewidths without using spin echoes, and to determine scalar coupling parameters with a statistical uncertainty of 4 mHz. 3 The use of atomic magnetometers yields greatly improved sensitivity compared to inductive detection at low or zero fields because they sense magnetic field directly, rather than the time derivative of flux through a pickup coil. Furthermore, in contrast to SQUIDs, atomic magnetometers do not require cryogenics.…”

Musings on Hardware Advances and New Directions

“…Nobody predicted many of the recent advances in this fi eld, such as even chemical shift, NMR imaging or the Earth ' s fi eld Jcoupling spectroscopy experiments [14] . Who would have thought NMR would be performed in the grasslands of Siberia [15] or on the Ross Ice Shelf in Antarctica [16] ?…”

Detection of radio-frequency magnetic fields using nonlinear magneto-optical rotation

“…[7] where the signal occurs at a fixed resonance frequency or in NMR spectroscopy where high spectral resolution is required to observe small splittings of NMR lines, due to, for example, scalar spin-spin (J) coupling between nuclei of the form JI 1 · I 2 . Such couplings can yield valuable information on molecular structure [9,10] and can be difficult to access in high field environments where the absolute field homogeneity and differences in diamagnetic susceptibility limit the spectral resolution. Hence, recent attention has been given to performing such measurements in a low field environment using broadband, low transition-temperature superconducting quantum interference devices (SQUIDs) [9] or inductive detection [10].…”

“…Such couplings can yield valuable information on molecular structure [9,10] and can be difficult to access in high field environments where the absolute field homogeneity and differences in diamagnetic susceptibility limit the spectral resolution. Hence, recent attention has been given to performing such measurements in a low field environment using broadband, low transition-temperature superconducting quantum interference devices (SQUIDs) [9] or inductive detection [10]. As inductive detection becomes less efficient at low frequencies, the technique described in this letter offers the possibility of significant gains in signal-to-noise ratio without requiring cryogenics.…”