Abstract
The development of hyperpolarization technologies enabled several yet exotic NMR applications at low and ultra‐low fields (ULF), where without hyperpolarization even the detection of a signal from analytes is a challenge. Herein, we present a method for the simultaneous excitation and observation of homo‐ and heteronuclear multiple quantum coherences (from zero up to the third‐order), which give an additional degree of freedom for ULF NMR experiments, where the chemical shift variation is negligible. The approach is based on heteronuclear correlated spectroscopy (COSY); its combination with a phase‐cycling scheme allows the selective observation of multiple quantum coherences of different orders. The nonequilibrium spin state and multiple spin orders are generated by signal amplification by reversible exchange (SABRE) and detected at ULF with a superconducting quantum interference device (SQUID)‐based NMR system.
Highlights
Encounters difficulties due to magnetic field inhomogeneity caused by pH2 bubbling;[38] this is not a problem for ultra-low field (ULF) NMR and even allows continuous signal amplification by reversible exchange (SABRE),[39] radiowave amplification by stimulated emission of radiation (RASER)[13] and QUASi-Resonance SABRE (QUASR).[40]
We report the opposite effect: we discovered that in SABRE experiments at low magnetic fields various multiple spin orders are hyperpolarized. It was revealed by simultaneous observation of hyperpolarized homo- (1H) and heteronuclear (1H-19F) zero-order and multiple quantum coherences (QCs), up to the third order
H ULF SABRE spectrum is evident (Figure 1b) and in the absence of chemical shift resolution the sign of polarization can serve as a contrast; at 91 μT the chemical shift difference of ppm corresponds to only 4 mHz frequency variation
Summary
Much effort is being undertaken to bring SABRE to “life sciences”, but despite considerable efforts, a clean, highly polarized, highly concentrated biologically relevant contrast agent was not produced yet.[41,42,43,44,45,46] When it comes to biomedical applications, usually, it is the goal to populate one dedicated spin state and, as a result, boost the MRI signal of the targeted nuclei. We report the opposite effect: we discovered that in SABRE experiments at low magnetic fields various multiple spin orders are hyperpolarized It was revealed by simultaneous observation of hyperpolarized homo- (1H) and heteronuclear (1H-19F) zero-order and multiple quantum coherences (QCs), up to the third order. This confirms that redistribution of pH2 spin alignment in SABRE results in the substrate’s magnetization and in the population of multiple spin orders, including homo- and heteronuclear zz-orders and singlet spin states.[27,35,36,47,48,49] This observation illustrates that the transfer of pH2 spin order to a substrate at the low magnetic field can be greatly improved by using more targeted polarization transfer techniques discussed elsewhere.[40,50]
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