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Applications in education • Selective excitation
There is the opportunity with benchtop NMR to explore advanced techniques, such as selective excitation of a single resonance in a spectrum (Figure 1). This can then be used further in more advanced experiments such as the one-dimensional total correlation spectroscopy (1D TOCSY), which only shows all the signals coupled with each other for further data insights. (See Figure 2)
• Structure elucidation
Within its limits, even structure elucidation is possible with benchtop NMR, which determines the structure of unknown molecules. It is often presumed that only high-fi eld instruments can elucidate the structure of a molecule and that low-fi eld NMR spectroscopy is restricted to the structure verifi cation of small and simple molecules. Modern benchtop NMR systems are proving that it is possible to use this technique in academia and that the elucidation of small molecular structures is no longer the sole province of the commercial laboratory. Students can determine the structure of unknown molecules with extensive experimental libraries and dedicated software tools such as Bruker CMC-se (see Figure 3).
• Monitoring reactions
Benchtop NMR allows for the precise monitoring of reactions in the lab. For example, the majority of undergraduate chemistry labs have synthesised aspirin from the simple starting materials of salicyclic acid and acetic anhydride. This reaction can be followed on a benchtop spectrometer, checking the starting materials to make sure they are pure, monitoring the reaction over time and verifying the fi nal product purity (Figure 4).
Figure 1. Comparison of 1-dimensional 1H NMR spectra recorded on a 1 M ibuprofen sample at 80 MHz (Fourier 80, Bruker BioSpin) to demonstrate the effect of selective excitation. While the fully selected spectrum (blue) shows all proton signals of the sample, in the selectively excited spectrum (red, region of selective excitation indicated by the black arrow) only the selected signals can be observed.
Figure 4. The synthesis of aspirin using salicylic acid (a) and acetic anhydride (b) as educts observed over time using 80 MHz 1D 1H NMR spectra (Fourier 80, Bruker BioSpin).
Wide range of applications
Despite their lower fi eld strength compared with modern high-fi eld systems, the advanced electronics and methodology of modern benchtop NMR spectrometers make these instruments ideally suited to high-throughput chemical analysis, far superior to that of early low-fi eld spectrometers.
Figure 2. Comparison of 1-dimensional 1H NMR spectra recorded on a santonin sample at 80 MHz (Fourier 80, Bruker BioSpin). While the fully selected spectrum (red) shows all proton signals of the sample, in the 1D TOCSY spectrum (blue, region of selective excitation indicated by the black arrow) only signals deriving from spins coupled to the excited one can be observed.
NMR in its benchtop form does not only become broadly accessible to chemistry students but reaches beyond. A prosperous application area is dealing with the analysis of food and beverages, highly interesting for biochemists and nutritional science students who might be yet to experience the power that NMR can provide. For example, the quantifi cation of the fat content in milk, the differentiation of arabica versus robusta coffee beans (Figure 5) and the detection of fraudulent low-quality sunfl ower oil mixed in high-quality olive oil are interesting applications.
Figure 5. Students can analyse extracts from coffee beans to fi nd out whether it is arabica or robusta and determine whether their coffee sample contains any caffeine using 1D 1H spectra recorded at 80 MHz (Fourier 80, Bruker BioSpin).
Simple interface
Figure 3. The structure of santonin (30 mg in DMSO) has been elucidated automatically by TopSpin CMC-se software (Bruker BioSpin) using a pre-defi ned set of spectra recorded at 80 MHz (Fourier 80, Bruker BioSpin) only. The 1D 1H (top left), 1D 13C (bottom right) and 2D HSQC-me (multiplicity-edited Heteronuclear Single Quantum Coherence) spectra are shown.
Software designed for the operation of modern benchtop NMR systems is user-friendly and some may provide easy access to vast experiment libraries and functionalities known from high-fi eld NMR systems. This is well-suited for students and encouraging for those interested in using NMR as a technique when choosing a career path, taking confi dence and knowledge from early NMR studies into a professional laboratory.
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