By Sergio Alejandro Ortiz Restrepo
Nuclear Magnetic Resonance (NMR) is a key method for the quantitative analysis of complex mixtures. In many cases, proton NMR spectroscopy is the method of choice as protons are ubiquitous present in nature and quantitative experiments can be conducted in a short time. Currently, most of such investigations are usually done using NMR devices working at high-magnetic fields to ensure high accuracy. Yet, the use of such equipment outside academia is usually hampered by high purchase and operating costs, the requirement of special facilities, highly qualified personnel, and regular maintenance.
As an alternative to the high-field, benchtop NMR devices are currently gaining a lot of popularity due to the affordable prices, compact size, easy operation, and less maintenance. However, the accuracy of the quantitative composition analysis of complex mixtures by proton low-field NMR spectroscopy can be compromised due to the lower sensitivity and resolution of spectra in comparison with the corresponding high-field NMR spectra.
Current research topics from our laboratory illustrating strategies for quantitative composition analysis of various complex mixtures by proton low-field NMR spectroscopy will be presented with demonstration on industrially relevant samples. We expect that the presented approaches bear high potential for many other practical questions and applications. 1) A first example will focus on natural gas whose price is determined by its composition. A novel methodology for the composition quantification of such samples up to 200 bar using high-pressure (HP) benchtop proton NMR spectroscopy will be presented [1-4]. It makes use of a recently developed HP setup with a dedicated HP sapphire tube and the Indirect Hard Modelling (IHM) for the analysis of the heavily overlapping proton signatures of various hydrocarbons presented in the natural gas. In order to ensure high-quality data of the recorded spectra, various experimental strategies were implemented including single-scan acquisition, spectra alignment, and a refined design of the HP tube. For example, the use of this new HP tube boosted the signal-to-noise ratio by around 80% as compared with the original HP tube [4]. The implementation of these strategies improved the robustness and the accuracy of the NMR component quantification leading to an excellent agreement with the results from the Gas Chromatography. 2) Another class of samples of practical relevance is represented by plasticizers, which account for about one third of the worldwide produced additives while the forecasts predict a further increase in their amount in the coming years. Plasticizers are very important for almost all polymer formulations and have a key role for polyvinyl chloride (PVC). Plasticizers are large molecules, which are in most cases only mixed with the PVC material to increase its flexibility. With passing of time or due to external factors, plasticizers migrate out with possible health or environmental issues. In addition, the use of plasticizers is currently strongly regulated and thus reliable and cost-effective tools are urgently needed also for controlling purposes. In this context, we introduce proton low-field NMR spectroscopy as a simple and low-cost method for quantitative plasticizers analysis [5]. Despite the strong overlapping of the signals of the studied plasticizers in the recorded proton NMR spectra, the chemical recognition and quantification of the individual plasticizers in liquid mixtures using the IHM approach was successful. It could be shown that quantification limits as small as 0.35 wt% plasticizer in PVC could be achieved. 3) Glycols are important chemicals in many fields of activities, with water strongly controlling their chemical and physical properties, even if present only in tiny amounts. So far, Karl-Fischer titration is the gold method for the quantification of water content in aqueous solutions. Here, we demonstrate that proton NMR spectroscopy has a high potential for being use as a fast and green alternative to Karl-Fisher. This was proved by quantifying the water content in aqueous ethylene glycol (EG) and triethylene glycol (TEG) mixtures containing water up to about 11 wt%. As an alternative to IHM, lineshape analysis was used for this purpose. The introduced procedure leads to an excellent agreement with the Karl-Fisher results in the whole range of studied water contents [6].
References
[1] A. Duchowny, O. Mohnke, H. Thern, P. M. Dupuy, H. C. Widerøe, A. Faanes, A. Paulsen, M. Küppers, B. Blümich and A. Adams, Composition analysis of natural gas by combined benchtop NMR spectroscopy and mechanistical mutivariate regression, Energy Rep. 8 (2022), 3661–3670, https://doi.org/10.1016/j.egyr.2022.02.289.
[2] A. Duchowny, P. M. Dupuy, H. C. Widerøe, O. J. Berg, A. Faanes, A. Paulsen, H. Thern, O. Mohnke, M. Küppers, B. Blümich and A. Adams, Versatile high-pressure gas apparatus for benchtop NMR: Design and selected applications, J. Magn. Reson. 329 (2021), 107025, https://doi.org/10.1016/j.jmr.2021.107025.
[3] S. A. Ortiz Restrepo et al., “Refined natural gas composition analysis with benchtop proton NMR spectroscopy”, (in preparation).
[4] A. Duchowny, S. A. Ortiz Restrepo, M. Adams, R. Thelen, A. Adams, Refined high-pressure tube design for improved resolution in high-pressure NMR spectroscopy, Analyst 147 (2022),3827-3832, https://doi.org/10.1039/D2AN00926A.
[5] A. Duchowny, S. A. Ortiz Restrepo, S. Kern, A. Adams, Quantification of PVC plasticizer mixtures by compact proton NMR spectroscopy and Indirect Hard Modeling, Anal. Chim. Acta 1229 (2022), 34038, https://doi:10.1016/j.aca.2022.340384.
[6] S. A. Ortiz Restrepo, A. Adams, Fast quantification of water content in glycols by 1H NMR spectroscopy, Talanta 2253 (2023) 123973, https://doi.org/10.1016/j.talanta.2022.123973.