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arabica) and robusta (C. canephora), with arabica accounting for 60-70% of the world market. Arabica is generally considered to be of higher quality and sells on world commodity markets for about twicethe price of robusta. There is therefore the potential for economic fraud, with unscrupulous traders adding robusta to arabica and still labelling the product ‘100% Arabica’. Analytical methods are needed to detect the presence of robusta coffee in products labelled as arabica. The official method for testing for the presence of robusta in arabica is DIN 10779, which is a complex HPLC method that takes over 7 hours and involves the use of acetonitrile (a known carcinogen).
Defernez et al have shown that low-field (benchtop) NMR can be used instead. These researchers found that 16-O-methylcafestol (16-OMC) could be detected providing a faster, more sustainable method to detect adulteration of arabica by robusta.
A surveillance study of retail purchased ‘100% Arabica’ coffees using this method found that 6 out of 60 samples displayed signals commensurate with adulteration at levels of 3–30% w/w2
.
16-OMC has several clear peaks in the NMR spectrum, but one in particular, at 3.16 ppm, is in a region where there are no interfering peaks from other compounds present in the samples. Figure 3 illustrates where this peak appears in relation to spectra of various coffee samples. The peak at 3.16 ppm is labelled (i) in the figure.
as well as delivering structural information for the synthesised molecules and compounds. Having benchtop NMR in the lab speeds up data acquisition, so students can analyse what they have synthesised, just minutes after making it. To prepare students for a future in chemistry it is essential that they are well versed in NMR, which is employed widely throughout the chemical and biotechnology research industries.
For education, NMR can now be a much more hands on experience. To learn structural elucidation, often students are handed paper copies of spectra that have been at least partially processed for them, sometimes with challenging-to-read axes. They perform the analysis with the knowledge that all the information they require to determine the structure has been provided to them. This process is not a reality in a research environment where researchers interact with the data electronically, and where they may need to identify and run more complex spectra to complete their analysis. Using benchtop NMR, even these more complex spectra are readily available in the lab. Two-dimensional experiments are now commonplace in most instruments and the most used ones such as COSY, TOCSY, and HSQC come as standard on the X-Pulse. The extra dimension of data provided by these experiments accelerates structure determination of small molecules. You can read more about structure elucidation using X-Pulse on the Oxford Instruments website.
Furthermore, if samples do require higher magnetic fields, then benchtop NMR becomes an invaluable pre-screening tool to determine if the product or sample you’re investigating really does warrant the time and money required for high field evaluation. This pre-screening functionality can also be useful to NMR facility managers. As well as taking up much less space than a traditional high field set-up, benchtop NMR can also free up the more expensive-to-run magnets to be used in a more efficient way.
Figure 3: NMR spectra of 16-OMC and various coffee samples from Derfernez et al [2].
Although the trace amounts vary from sample to sample, they are generally equivalent to an addition of approximately 1% of robusta into arabica, so this is the lower limit of detection of adulteration.
The complete statistical analysis generated indicates that arabica coffees adulterated with robusta at the 1% w/w level will be detected in about half of all cases. This rises to 90% at the 2% level, and at the 3% level it is unlikely that any adulterated sample will pass undetected.
The developed method has a total preparation and measurement time of 90 minutes per sample, which is over 4 times faster than the HPLC method. By bringing NMR out of the custom-built laboratory and on to the benchtop QA/QC turnaround times are greatly improved.
Education
Synthetic chemistry education benefits from early hands-on NMR exposure to deepen understanding of the analytical technique
Figure 4: ¹H NMR spectra of 3-Dimethylaminoacrolein in CDCl₃, over a temperature range of +2 to +48°C.
Using the benchtop NMR variable temperature accessory, students can watch the progression of dynamic changes in molecular structure using NMR to reinforce learning concepts from both molecular structure and NMR. In Figure 4, the cis-trans isomerisation of the amino-methyl groups in 3-dimethylaminoacrolein is tracked. The NMR spectra show that as the sample group signals coalesce into a single signal. At the molecular level, increasing the sample temperature increases the rotational energy of the molecule, which speeds up the rotation of the methyl groups until they are spinning so fast that they become equivalent. From this,
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