“.. benchtop low-field NMR spectrometers can be a versatile tool for determining biodiesel content”
The majority of freight trucks, trains, buses, boats, and farm and construction vehicles are fuelled with diesel. This is largely because of its good thermodynamic efficiency making it a cost-effective means for powering transport. Following incentives from many governments to switch from petrol to the more environmentally-friendly diesel, it is also now used in many cars and small trucks. Indeed, diesel fuel obtained from petroleum is the most successful fuel across the globe.
However, it was subsequently revealed that the emissions from diesel engines have a devastating impact on human health. Diesel exhaust fumes contain high levels of nitrogen oxides and dioxides and particulate matter, the inhalation of which lead to tens of thousands of premature deaths each year. Having encouraged the use of diesel cars, a solution to reduce the damaging diesel emissions was urgently needed1.
Biodiesel, which is derived from the reaction of vegetable oil or animal fat with an alcohol, has very similar physicochemical properties to petrodiesel, but is associated with reduced emission of pollutants, sulphur oxides and carbon monoxide. It can also be used in standard diesel engines, unlike vegetable and waste oils that require engine conversion2. Biodiesel thus provides a realistic alternative to reduce fossil fuel combustion and lower the risk of damage to health.
Diesel emissions can be reduced by blending diesel fuel with biodiesel and legislations are in place to introduce a minimum level of biodiesel in petrodiesel-biodiesel blends. Consequently, it is necessary to have standard testing methods for determining the biodiesel content in diesel−biodiesel blends in order to enforce such legislation. Several methodologies, mostly based on infrared spectroscopy, are currently used but recently a more portable and cost-effective analytical technique has been proposed3.
The technique is based on time domain nuclear magnetic resonance (NMR) spectroscopy, which measures the time required for nuclei to return to equilibrium after excitation. Since time-domain NMR uses low magnetic fields it can be performed using affordable bench-top NMR spectrometers. Furthermore, since permanent magnets can be used, there is no need for extensive cooling by liquid gasses.
Time domain NMR spectroscopy is widely used across a range of industries and has proved to be a reliable, convenient and rapid methodology for qualitative and quantitative analyses. In addition, it is non-destructive and does not require sample preparation.
The new methodology based on time domain NMR spectroscopy was conducted on several fuel samples with varying biodiesel content using a Bruker Model mq-20 NMR spectrometer with a permanent magnet. The resultant spectra showed a strong correlation with biodiesel content and both univariate and multivariate analyses provided an accurate determination of biodiesel content.
Although the latest technique was not as precise as the current Fourier-transform infrared spectroscopy standards, it offers the advantage of obtaining results rapidly using readily available portable bench-top NMR spectrometers. It therefore represents a promising tool for convenient, cost-effective, in-situ determination of biodiesel content in fuel samples.