The high price of petroleum-based fuel makes it an attractive target for fraudsters and thieves. On the small scale, employers who hold stocks of fuel, perhaps for a transport fleet, regularly report losses due to theft by their employees, with year on year increases seen in the EU.
On the larger scale, cheaper substitute fuels are used illegally by motorists. For instance, red diesel is being discovered increasingly in the engines of motor vehicles in the UK. This fuel is intended for use in off-road agricultural vehicles and is approximately 33% of the price of regular fuels due to a lower government tax rate. It is named red diesel due to the presence of a red dye added before distribution to help distinguish it from other fuels.
The practice of dyeing has become relatively common throughout the world. Aviation fuels are marked with a dye to ensure that aircraft are fuelled with the right fuel to avoid accidents. Gasoline (petrol) and diesel are also dyed to prevent fraud and help to provide evidence of fuel theft.
In order to protect the fuel tax system and prevent fraud, sensitive methods must be in place to detect dyes in fuels. This is especially the case when dyed fuels have been illegally treated to remove the dyes, so that cheaper fuels can be sold as more expensive ones to increase the revenue stream and subsequent profits. In these cases, the concentration of dyes will be very low.
There are a number of reported methods for analysing fuel dyes, including HPLC with detection by mass spectrometry, UV-visible spectroscopy and diode array detection. A further variation has been developed by scientists in Brazil and Germany, two countries where fuel dyeing has been established.
Maria Valnice Boldrin Zanoni from Sao Paulo State University, Frank-Michael Matysik from the University of Regensburg and Magno Aparecido Goncalves Trindade, who is affiliated to both universities, used HPLC with electrochemical detection. It was devised especially for the two anthraquinone dyes Solvent Blue 14 and Solvent Blue 35, and the azo dye Solvent Red 24.
Initially, the method was tested and optimised with standard solutions of the three dyes plus the azo dye Solvent Orange 7 and the anthraquinone dye quinizarin. The compounds were separated on a reversed-phase C8 column using an isocratic mixture of acetonitrile and ammonium acetate. The peak shapes and tails were improved and the retention times reduced by stepping the flow rate during the separation.
Dye elution was complete within 12 minutes, although quinizarin was not detected under any conditions. The remaining four dyes were detected with an electrochemical detector that had a glassy working electrode, an Ag/AgCl reference electrode and a platinum wire auxiliary electrode. The optimum applied potential was set to +0.8 V, at which the peak currents were at or near their maximum values and the background current was small and reproducible. The stepped chromatographic flow rate had no effect on the background current.
The method was applied to gasoline samples that were spiked with the dyes. Some extra peaks observed in the chromatograms, which increased the background current, were attributed to residual gasoline components. They also co-eluted with Solvent Orange 7, which ruled that dye out of the procedure.
The gasolines were subjected to two sample preparation procedures. A simple evaporation procedure to remove the hydrocarbon components gave poor recoveries (60 and 40%, respectively) for Solvent Blue 35 and Solvent Red 24 and good recovery for Solvent Blue 14.
An SPE method improved the recoveries when it was used in conjunction with the standard additions method for quantitation, raising those for Solvent Blue 35 and Solvent Red 24 to more than 78%. The detection limits were less than 0.1 mg/L.
The research team declared that HPLC with electrochemical detection by anodic oxidation will be suitable for the analysis of selected fuel dyes in a simple and rapid procedure, which will help to combat the adulteration and fraudulent sale of various types of petroleum-based fuels.
Related links:
* Electroanalysis 2010 (Article in Press): "Sensitive determination of water insoluble dyes used as marking of commercial petroleum products using high-performance liquid chromatography with electrochemical detection"
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