Abstract
Benchtop NMR spectrometers with sub-ppm spectral resolution have opened up new opportunities for performing NMR outside of the standard laboratory environment. However, the relatively weak magnetic fields of these devices (1–2 T) results in low sensitivity and significant peak overlap in 1H NMR spectra. Here, we use hyperpolarised 13C{1H} NMR to overcome these challenges. Specifically, we demonstrate the use of the signal amplification by reversible exchange (SABRE) parahydrogen-based hyperpolarisation technique to enhance the sensitivity of natural abundance 1D and 2D 13C{1H} benchtop NMR spectra. We compare two detection methods for SABRE-enhanced 13C NMR and observe an optimal 13C{1H} signal-to-noise ratio (SNR) for a refocused INEPT approach, where hyperpolarisation is transferred from 1H to 13C. In addition, we exemplify SABRE-enhanced 2D 13C benchtop NMR through the acquisition of a 2D HETCOR spectrum of 260 mM of 4-methylpyridine at natural isotopic abundance in a total experiment time of 69 min. In theory, signal averaging for over 300 days would be required to achieve a comparable SNR for a thermally polarised benchtop NMR spectrum acquired of a sample of the same concentration at natural abundance.
Highlights
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
We investigate the feasibility of signal amplification by reversible exchange (SABRE)-hyperpolarised benchtop 13 C{1 H} NMR experiments with a focus on the optimisation of the pulse sequences used for hyperpolarised 13 C detection and an analysis of the challenges and opportunities provided by hyperpolarised 2D 13 C–1 H benchtop NMR spectroscopy
SABRE-hyperpolarised NMR spectra are acquired by dissolving H2 gas, enriched in the para state, in a solution containing the active SABRE catalyst and the target analyte within a weak polarisation transfer field (PTF) typically in the range of 0–20 mT [59,60]
Summary
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. It was through 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]
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