A Comparison of Standard Test Methods for Determining FAME in Diesel by Infrared Spectroscopy - Ben Perston

As the importance of sustainable energy production increases, so too does the global commitment to using fuels from renewable biological sources. Biodiesel is one such renewable fuel. Consisting of fatty acids of methyl esters (FAME), it is produced from plant crops by transesterification – a reaction in which natural triglycerides are cleaved and reacted with methanol, producing glycerol and FAME. Rapeseed, soy, sunflower, palm and jatropha are just part of the range of feedstocks used globally in the production of biodiesel.

Biodiesel is intrinsically very low in sulphur, tends to be cleaner-burning than fossil fuel, and can reduce engine wear, all of which are desirable fuel properties. While many diesel engines can run on pure biodiesel, it is more commonly blended into commercial diesel fuel in ratios up to about 30% v/v. According to the ASTM D975 and EN 590 regulations, fuel sold as diesel (without specific biodiesel labelling) in the European Union and United States may contain up to 5%v/v FAME1,2.

FAME is not recommended for applications where fuel may be stored for a long time, due to concerns about its oxidative stability. As vegetable oils tend to have a degree of unsaturation (which is preserved during transesterification), chemical reactions can occur in which the double bonds slowly undergo oxidation and rearrangement, forming numerous primary and secondary oxidation products. This leads to an increase in the total acid number (TAN) of the fuel, and the formulation of insoluble particulates that can block filters. As such, FAME is not currently seen as a suitable ingredient for power generation, aviation, or marine fuels. However, the widespread use of FAME as fuel for terrestrial vehicles generates an increased risk of inadvertent contamination of mineral fuels. Methods of analysis for trace amounts of FAME are thus becoming increasingly important.

1 ASTM International D975-09: Standard Specification for Diesel Fuel Oils
2 BS EN 590:2004 Automotive fuels. Diesel. Requirements and test methods

This article discusses the use of FT-IR spectroscopy for quantification of FAME in fuels, and compares the two current standard test methods, EN 14078 and ASTM D7371, considering their relative merits for both blend and trace analysis.