In this experiment, performed at the University of California at Berkeley, we demonstrate remote detection of nuclear magnetic resonance (NMR) with a microchip-like sensor consisting of a microfluidic channel and a microfabricated vapor cell (the heart of an atomic magnetometer). Detection occurs at zero magnetic field, which allows operation of the magnetometer in the spin-exchange relaxation-free (SERF) regime and increases the proximity of sensor and sampleby elimi-nating the need for a solenoid to create a leading field.
The experimental setup is shown in Figure 1. Tap water flows though 800 μm inner diameter teflon tubes from a reservoir in a prepolarizing field to an encoding region, and finally through the microchip housed in a 4-layer set of magnetic shields. In the first experiment, we achieved NMR linewidth of 31 Hz, limited, we believe, by the residence period in the encoding region. In a fully optimized system, we estimate that for 1 second of integration, 7 x 1013 protons in a volume of 1 mm3, prepolarized in a 10 kG field, can be detected with a signal- to-noise ratio of about 3.
In a second experiment, we used a slightly improved version of this system to demonstrate direct detection of J-spectra due to both heteronuclear and homonuclear J-coupling in a zero-field environment where the Zeeman interaction is completely absent. Scalar couplings of the form JI1∙I2 between nuclei impart valuable information about molecular structure to nuclear magnetic-resonance spectra. We show that characteristic functional groups exhibit distinct spectra with straightforward interpretation for chemical identification. Detection is performed with a microfabricated optical atomic magnetometer, providing high sensitivity to samples of microliter volumes. We obtain 0.1 Hz linewidths and measure scalar-coupling parameters with 4 mHz statistical uncertainty. We anticipate that the technique described here will provide a new modality for high-precision “J spectroscopy” using small samples on microchip devices for multiplexed screening, assaying, and sample identification in chemistry and biomedicine.
Contact: Micah Ledbetter, UC Berkeley
References:
M.P. Ledbetter, C.W. Crawford, A. Pines, D.E. Wemmer, S. Knappe. J. Kitching, and D. Budker, Optical detection of scalar coupling at zero magnetic field, J. Magn. Reson., accepted.
Acknowledgements:
This work was carried out in collaboration with the groups of Dima Budker and Alex Pines at UC Berkeley.
Fig. 1. Experimental setup. Water flows from a reservoir inside a 7-kG permanent magnet through the encoding region where there is a Helmholtz coil used to apply AF ( ≈ 1 kHz) pulses. The water subsequently flows into a channel with dimensions ≈ 1×2×3 mm3 adjacent to a microfabricated atomic magnetometer vapor cell containing Cs and 5000 torr of N2. Insets hows a photograph of the chip before the ITO heaters were installed.
Experimental and simulated zero-field NMR spectra for ethanol 1, 12CH3-13CH2- OH. To the extent that signal is above the noise level, experiment and simulation are in agreement. The positions of the multiplets are determined by the heteronuclear J-coupling and the splittings within the multiplets are due to homonuclear J-coupling and higher-order effects of heteronuclear J-coupling.