24 Analytical Instrumentation
FROM R&D TO QC, MAKING NMR ACCESSIBLE FOR EVERYONE: PUTTING NMR SPECTROSCOPY AT THE HEART OF THE ANALYTICAL CHEMISTRY LAB
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For many decades, access to nuclear magnetic resonance (NMR) spectroscopy has been an intrinsic requirement for any analytical chemistry lab. Due to its ability to elucidate molecular structure and track reaction dynamics, this technique is recognised throughout industry and academia as a powerful, non-destructive, non-invasive method, and is taught in undergraduate chemistry curricula. But how can NMR be employed on the bench in almost any laboratory environment?
Access to NMR has often been diffi cult, with the requirement for substantial space, special facilities and expert users, limiting the usability of the technique in synthetic chemistry and industrial labs.
However, recent advances in benchtop NMR instrumentation, brings this technique out of the basement or centrally managed facility, and into the heart of any laboratory. Benchtop NMR instruments are a smaller, cryogen-free version of traditional high fi eld instruments, and they even enable data collection in a fume hood. Oxford Instruments X-Pulse adds broadband capability to benchtop NMR.
This capability allows operators to use just one benchtop instrument and rapidly tune between most NMR active chemical nuclei.
This enables instrument sharing across many applications, future-proofs R&D capabilities and signifi cantly expands the range of challenges solved across education, academia, and industry.
For industry, the benefi ts of small footprint, mobile, benchtop NMR are very clear: shorter preparation times, automation capabilities, fewer staff specialists, and a reduced space demand to name a few. This means time effi ciencies and cost savings versus high fi eld NMR instruments, which incur signifi cant initial capital investment and often require dedicated rooms to operate in. Benchtop NMR fi ts right alongside existing instruments in a lab or can be quickly transported on a trolley between labs. As benchtop NMR is cryogen-free, it is much cheaper to run than traditional high fi eld instruments. Our analysis showed that one can conservatively save over US$9,000 per year in running cost just from the cryogens alone.
Any business looking to invest in high fi eld NMR needs to answer these questions:
• Can one establish a reliable and continuous supply of liquid helium and liquid nitrogen?
• How will they be stored safely on site?
• Is there someone on site that is qualifi ed to handle them in a safe manner?
As well as the on-going, and ever rising cost of cryogens, there are also the peripheral logistics of buying and installing a high fi eld instrument to consider.
Research and Development
While early applications largely focused on basic one- dimensional 1H NMR spectra, modern spectrometers offer many additional capabilities beyond structural elucidation. One important example is the ability to measure self-diffusion coeffi cients, to extract physical information about a sample including:
• molecular size PIN OCTOBER / NOVEMBER 2024 • viscosity
• ionic conductivity and transference (e.g., in lithium-ion battery electrolytes)
A benchtop NMR spectrometer equipped with pulsed fi eld gradient (PFG) hardware can use techniques such as the PFG spin echo (PFGSE) experiment to determine diffusion coeffi cients of sample components by measuring change in NMR signal as a function of the PFG strength. Adding variable temperature capability allows the study of sample thermal behaviour under a range of expected working conditions for the samples.
Pulsed fi eld gradients are applied to the sample and vary in intensity. As the gradient strength increases, the signal is attenuated due to the changing phase difference between molecules after the fi rst and second gradient pulses. The attenuation can be related to the diffusion constant using the Stejskal-Tanner equation [1].
This method can be used particularly in battery research and development where measuring the self-diffusion, cationic transference, and ionic conductivity of the various lithium salts in different electrolytes can quantify performance and aid the design.
As an example, lithium hexafl uorophosphate, Li[PF6 ] was studied
in three different electrolyte solvents: dimethyl carbonate (DMC); a 50:50 mixture of ethylene carbonate (EC) and DMC; and propylene carbonate (PC). The PFGSE spectra are shown in Figure 2. This reveals the differences in diffusion behaviour among components of a single electrolyte solution, as well as differences among the same species in different solvent conditions. Because 31
P is part of the same [PF6]− ion as 19 two should diffuse at the same rate.
In fact, the diffusion behaviours were identical within the precision of the method. However, as expected, the diffusion behaviour of the smaller Li+ ion differed signifi cantly from that of the larger [PF6
]− ions. In addition, the markedly different quantitative results in Table 1 for the same Li+ and [PF6 ]− ions
in the three solvent systems, demonstrate the importance of solvent choice in battery design. Conductivity differed by approximately a factor of three between electrolytes using PC and DMC solvents, while cation transference changed far less. Moreover, the difference in diffusion behaviour for DMC as a pure solvent, compared to the 50:50 mixture with EC, demonstrates the effects of environment on solvent molecule. By utilising the portability of benchtop NMR, these electrolyte characterisations can happen in the lab, accelerating and enhancing development capabilities from the start.
F, the
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