13
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 • viscosity
• ionic conductivity and transference (e.g., in lithium-ion battery electrolytes)
same species in different solvent conditions. Because 31 the same [PF6]− ion as 19
P is part of F, the two should diffuse at the same rate.
Figure 2: Stejskal-Tanner plots of integrals obtained from PFGSE data of LiPF6 in DMC, 50:50 EC:DMC, and PC. 1
in dark orange and light orange 7 grey, and 31
Li data is shown in blue, 19 broadband benchtop NMR spectrometer.
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
Figure 1: A sample of the various X-nuclei that it’s possible to detect on X-Pulse.
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
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.
Table 1: Parameters measured by NMR for three electrolyte systems: LiPF6 in DMC, in 50:50 EC:DMC, and in PC.
H data is shown F in
P in green. All data were acquired on a single X-Pulse
QA/QC
As well as providing a research and development tool, benchtop NMR offers powerful quality assurance/quality control (QA/QC) properties. One important example is in the food industry where NMR can identify adulterants or impurities in a spectrum. One industry that greatly benefi ts from this is coffee. Coffee is one of the most widely traded commodities in the world. The trade is made up of two main varieties, commonly known as arabica (C.
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