25 Analytical Instrumentation use of acetonitrile (a known carcinogen).
Defernez et al have shown that low-fi eld (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 fi gure.
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.
Figure 1: A sample of the various X-nuclei that it’s possible to detect on X-Pulse.
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 benefi ts from early hands-on NMR exposure to deepen understanding of the analytical technique 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.
Figure 2: Stejskal-Tanner plots of integrals obtained from PFGSE data of LiPF6 in DMC, 50:50 EC:DMC, and PC. 1 orange 7
Li data is shown in blue, 19 F in grey, and 31 Table 1: Parameters measured by NMR for three electrolyte systems: LiPF6 H data is shown in dark orange and light P in green. All data were acquired on a single X-Pulse broadband benchtop NMR spectrometer. in DMC, in 50:50 EC:DMC, and in PC.
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 fi elds, 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 fi eld 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 fi eld set-up, benchtop NMR can also free up the more expensive-to-run magnets to be used in a more effi cient way.
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. 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 offi cial 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
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, the thermodynamic barrier for rotation of the carbon-nitrogen bond can be calculated. With a benchtop instrument, NMR
WWW.PETRO-ONLINE.COM
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52
