Abstract
One of the greatest challenges of NMR to overcome is its low sensitivity, which is directly proportional to nuclear spin polarization. Hyperpolarization techniques address this by being able to increase the nuclear polarization of a molecule of interest from the range of ppm or lower to the percentage regime. One such technique of interest is signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH). A key feature of this technique is that the hyperpolarization develops spontaneously in a B₀ holding field that is only on the order of 100s nT, in contrast to the mT fields necessary for other SABRE techniques, and the T fields, common to other hyperpolarization techniques. Typically, in order to perform detection in NMR, the nuclear magnetisation of the sample is rotated perpendicular to the B₀ field, either via an rf pulse, or by non-adiabatic field switching. This is usually necessary, due to the orders of magnitude smaller magnetic flux density attributable to the sample vs. that of the B₀ holding field. This, combined with the lack of detector sensitivity to such small variations in flux density and drifts in B₀, due to vibrational noise, which are often larger than the magnetic flux density from the sample, makes direct detection of sample magnetisation usually infeasible. Here, we operate a highly sensitive, single order, SQUID-based gradiometer in the ultra-low, low-noise SHEATH field. Through this setup, we are able to directly detect the pT DC changes in samples hyperpolarized with SABRE- SHEATH, without any field switching or rf pulses. We applied this methodology to a sample of [1-¹³C]pyruvate, Ir- IMes catalyst and DMSO in methanol. The sample was hyperpolarized, using SABRE-SHEATH by bubbling 99% parahydrogen gas through the sample at a rate of 2 sL/h in a B₀ field on the order of 100s nT. Direct detection was performed by measuring the DC magnetic flux density of the sample, while parahydrogen was periodically supplied into the sample. An increase in longitudinal, magnetic flux density on the order of pT was observed, as a result of SABRE-SHEATH, and the corresponding decrease was detected, during the signal decay. Additionally, using the averaged data, it was possible to fit a first order exponential function to determine the buildup and decay time constants that were on the order of 10s of seconds for SABRE-SHEATH. This demonstrates how this direct detection technique can be used to make measurements of these time constants faster and more simply than with traditional methods in the future.
