Zero to ultralow magnetic field NMR of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mn>1</mml:mn><mml:msup><mml:mrow><mml:mtext>−</mml:mtext></mml:mrow><mml:mn>13</mml:mn></mml:msup><mml:mi mathvariant="normal">C</mml:mi></mml:mrow><mml:mo>]</mml:mo><mml:mi>pyruvate</mml:mi></mml:mrow></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mn>2</mml:mn><mml:msup><mml:mrow><mml:mtext>−</mml:mtext></mml:mrow><mml:mn>13</mml:mn></mml:msup><mml:mi mathvariant="normal">C</mml:mi></mml:mrow><mml:mo>]</mml:mo><mml:mi>pyruvate</mml:mi></mml:mrow></mml:math> enabled by SQUID sensors and hyperpolarization

Abstract

Accurately characterizing magnetic resonance of molecules at zero to ultralow magnetic field (nTs-µTs) is challenging, due to vanishingly small sensitivity, which depends on the thermal equilibrium polarization of the nuclear spins and instrumentation. We overcome the former limitation with the parahydrogen-based hyperpolarization method SABRE-SHEATH (signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei). This method allows for the continuous transfer of spin order from parahydrogen to a substrate via chemical exchange, reaching polarization levels of some percent (level equivalent to <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mmultiscripts><a:mi mathvariant="normal">C</a:mi><a:mprescripts/><a:none/><a:mn>13</a:mn></a:mmultiscripts></a:math> polarization at 20 kT). We address the latter with our application of a superconducting quantum interference device (SQUID)-based detector setup that allows for broadband detection (dc-MHz) with exquisite sensitivity over its entire range. Here, we present the results of our comprehensive characterization of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mo>[</c:mo><c:mn>1</c:mn><c:msup><c:mrow><c:mtext>−</c:mtext></c:mrow><c:mn>13</c:mn></c:msup><c:mi mathvariant="normal">C</c:mi><c:mo>]</c:mo><c:mi>pyruvate</c:mi></c:mrow></c:math> and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mo>[</e:mo><e:mrow><e:mn>2</e:mn><e:msup><e:mrow><e:mtext>−</e:mtext></e:mrow><e:mn>13</e:mn></e:msup><e:mi mathvariant="normal">C</e:mi></e:mrow><e:mo>]</e:mo><e:mi>pyruvate</e:mi></e:mrow></e:math>, hyperpolarized via SABRE-SHEATH, from zero field to 100 µT. To this end, we show low-noise, high-resolution spectra for both molecules, detecting how the NMR spectrum changes from the -coupling dominated zero-field spectrum to the strongly coupled spectrum, and then finally to the conventional high-field, otherwise known Zeeman-dominated spectrum. We also analytically derive the evolution of product operators in arbitrary magnetic fields, which aid in the understanding of the differences between spin evolution and spin-coupling regimes. We predict and confirm that the absence of spin precession at zero field can result in observable oscillation of magnetization along one axis with a frequency of the -coupling constant, no observable spin evolution, or observing spin evolution that corresponds to “forbidden” transitions at high field. The zero-field spectra with their near-dc signals reveal different relaxation rates for the different spin states of hyperpolarized <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mmultiscripts><g:mi mathvariant="normal">C</g:mi><g:mprescripts/><g:none/><g:mn>13</g:mn></g:mmultiscripts></g:math> pyruvates, demonstrating the utility of SQUID detectors and hyperpolarization for the characterization of magnetic resonance phenomena. Published by the American Physical Society 2024

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