# Hyperpolarised \({}^{1}\)H-\({}^{13}\)C Benchtop NMR Spectroscopy
## 1 Introduction  
Benchtop NMR spectrometers have the potential to open up new applications for NMR spectroscopy outside of the traditional laboratory environment owing to the relative portability of these devices. Of particular interest are permanent magnet spectrometers with magnetic field strengths of 1-2 T that have the capability to record high-resolution NMR spectra [1]. Whilst several of the original NMR discoveries were made using permanent magnet NMR spectrometers [2; 3; 4; 5; 6; 7; 8; 9], their usage became limited once strong and stable superconducting electromagnets became readily available [10; 11; 12]. In the early 2000s, the prospect of cheap, cryogen-free, and compact NMR spectrometers prompted the resurgence of permanent magnet-based systems [13]. A major obstacle to these spectrometers was the need for highly homogenous magnetic fields as high-resolution spectra require field spatial variations of less than tens-of-parts per billion [14]. It was through specifically designed Halbach arrays of magnets [15], advancements in shimming electronics, and temperature stabilisation that these tens-of-ppb magnetic field homogeneities were achieved [16; 17; 18]. From this point, a range of high-resolution benchtop NMR spectrometers with different field strengths and heteronuclear detection capabilities have been developed and implemented across a plethora of applications [19], such as industrial quality control [20; 21; 22; 23; 24; 25; 26], \({}^{1}\)H and \({}^{13}\)C NMR undergraduate teaching [27; 28; 29; 30; 31; 32], and on-line reaction monitoring [33; 34; 35; 36; 37; 38; 39]. A significant limitation of these benchtop spectrometers is their low inherent sensitivity. All NMR experiments are considered to be insensitive due to the small Boltzmann population difference between the nuclear energy levels that form when NMR-active nuclei are placed within an external magnetic field. The energy level spacing is magnetic field strength dependent and so moving to lower magneticfield strengths further reduces sensitivity [40]. Indeed, overall NMR sensitivity scales with magnetic field strength approximately proportional to \({B_{0}}^{3/2}\). Additionally, lower magnetic field strengths reduce spectral chemical shift dispersion, which scales linearly with field, and can cause strong coupling issues, where coupling constants are similar in magnitude to chemical shift differences between coupled spins. As a result, broad and overlapping peaks are common in low-field NMR spectra, even for simple molecules. This is a particular challenge of \({}^{1}\)H benchtop NMR spectroscopy because of the relatively small chemical shift range of \({}^{1}\)H nuclei. To improve the general applicability of benchtop NMR spectrometers, novel methods are required to overcome the challenges of sensitivity and resolution. In this work, we explore the potential for natural abundance \({}^{13}\)C NMR spectra to be used to surmount the issue of reduced resolution. Due to isotopic dilution, the increased chemical shift range, when compared to \({}^{1}\)H NMR and the ability to simplify spectra through broadband \({}^{1}\)H decoupling and natural abundance \({}^{13}\)C NMR spectra, can be as readily interpreted at 43 MHz (1 T) as at 300 MHz (7 T). However, benchtop \({}^{13}\)C NMR spectroscopy poses a significant sensitivity challenge because the receptivity of natural abundance \({}^{13}\)C is 1.7 \(\times\) 10\({}^{-4}\) relative to \({}^{1}\)H.  
In principle, the low sensitivity of benchtop NMR can be overcome using hyperpolarisation. Hyperpolarisation methods generate a population difference that is orders of magnitude larger than at thermal equilibrium and so provide large NMR signal enhancements. Popular hyperpolarisation techniques include Dynamic Nuclear Polarisation (DNP) [41; 42; 43] and _Parahydrogen-induced_ polarisation (PHIP) [44; 45; 46]. One relatively inexpensive method, which has been successfully implemented at low-field, is a PHIP-based method called signal amplification by reversible exchange (SABRE) [47; 48; 49].