HYPERPOLARISED BENCHTOP NMR SPECTROSCOPY FOR ANALYTICAL APPLICATIONS

Benchtop NMR spectrometers provide a portable and more affordable alternative to high-field NMR, enabling new analytical applications outside of the traditional laboratory environment. However, benchtop NMR suffers from reduced sensitivity and increased peak overlap relative to high-field NMR due to its moderate magnetic field strengths (1 – 2 T). Hyperpolarisation methods can overcome these limitations by breaking the link between sensitivity and detection field strength. Parahydrogen (p-H₂) induced polarisation (PHIP) [1] uses the singlet nuclear spin isomer of molecular hydrogen (p-H₂) as the source of hyperpolarisation and can deliver signal enhancements with build-up times comparable to typical relaxation delays (e.g. 10 s) in standard NMR pulse sequences. The Signal Amplification by Reversible Exchange (SABRE) [2] method is a variant of PHIP that catalytically transfers the spin order of p-H₂ to target analyte(s) in solution using a reversible exchange reaction. SABRE requires relatively simple instrumentation and has been integrated with benchtop NMR detection to achieve signal enhancements of up to tens of thousands.[3] However, several challenges need to be overcome for SABRE-enhanced benchtop NMR to be used for analytical applications.

One challenge is quantification. In any hyperpolarisation experiment, the signal observed is dependent on the efficiency of the polarisation transfer method, which can vary significantly between different analytes and samples. In SABRE, the signal is dependent on the catalytic generation of hyperpolarisation and the need to transfer the sample between a low-field regime (< 10 mT) for polarisation build-up and the benchtop NMR spectrometer for detection. It has been demonstrated previously, with high field detection, that the incorporation of a suitable co-substrate into the SABRE catalysis step can enable quantification of ¹H SABRE signals for analytes down to the nanomolar regime.[4] Here we show that a similar approach can be used to deliver quantification of ¹H and ¹⁹F SABRE-enhanced benchtop NMR signals, extending the limits of detection for a single scan ¹⁹F SABRE-enhanced benchtop (1 T) NMR spectrum to below 1 μM.[5]

A second challenge to automate the hyperpolarisation process such that it can be integrated into any multi-step NMR pulse sequence (such as 2D spectroscopy). For this to be effective, instrumentation is required for both mixing p-H₂ gas into solution and shuttling the sample between the polarisation transfer and signal detection regimes. Here we present a compact, computer-controlled instrumentation solution interfaced to a commercial benchtop NMR spectrometer. We demonstrate the efficacy of our approach using a range of NMR experiments for mixture analysis (e.g. COSY, TOCSY, HETCOR, pure shift and GEMSTONE). In addition, we present progress towards fully in situ SABRE hyperpolarisation, where RF irradiation is used to achieve ¹H and ¹³C hyperpolarisation within the detection field of the benchtop NMR spectrometer. The relative advantages of in situ and ex situ SABRE will be discussed in relation to hyperpolarisation efficiency, specificity and reproducibility.

REFERENCES

1. Bowers and Weitekamp, J. Amer. Chem. Soc., 1987, 109, 5541.
2. Adams et. al., Science, 2009, 323,1708.
3. Richardson et. al., Analyst, 2018, 143, 3442.
4. Eshuis et al., J. Chem. Soc., 2014, 136, 2695.   
5. Silva Terra and Halse, Chemistry-Methods, 2025, in press.