a p p l icatio n Note FT-IR Spectroscopy Determining the Source Oil of Biodiesel with Infrared Spectroscopy Introduction Biodiesel is a renewable fuel produced from a wide range of naturally occurring fats and oils by a transesterification reaction in which the triglycerides are broken down and fatty acid methyl esters (FAMEs) are formed. The fatty acid distribution of the original oil is retained in the biodiesel, thus the physical and chemical properties of the biodiesel have some dependence on the feedstock used. One property particularly influenced by the feedstock is the cloud point, the temperature at which solid crystals start to form creating a cloudy suspension and potentially blocking fuel filters. For operation in cool climates, a low cloud point is essential. A wide range in cloud points is seen among biodiesel samples from various feedstocks: a sample of palm biodiesel may have a cloud point around 15 °C; biodiesel from rapeseed oil may be closer to -10 °C. The reason for the difference is that the saturated fatty acid chains that predominate in palm oil pack more readily to form crystals. A rapid method for identifying the origin of a biodiesel sample is therefore desirable. Infrared spectroscopy is particularly suitable for the identification of materials, even when the differences between the materials are subtle variations in complex mixtures. In this note we demonstrate that biodiesel from several common feedstocks can be distinguished on the basis of absorption bands arising from double bonds in the fatty acid chains. Experimental Samples were obtained of palm, soy, and rapeseed biodiesel. The infrared spectra were measured using a PerkinElmer® Spectrum™ 100 FT-IR spectrometer1 equipped with a singlebounce diamond UATR accessory. Results and discussion The FT-IR spectra are plotted in Figure 1. Clear differences are visible in the intensities of bands related to alkene functional groups. The intensities of these bands increase in the order palm < rapeseed < soy. These observations can be explained by considering the distribution of fatty acid chains present in each oil. Table 1 presents some typical distributions for these three oils. Palm oil is composed largely of lauric and myristic acid chains, which are completely saturated; some unsaturation is present as oleic and linoleic acid chains. Rapeseed and soy oil, on the other hand, have high proportions of unsaturated fatty acid chains. Consistent with the FT-IR data, the ratio of double bonds per chain to average chain length (a quantity approximately proportional to the volume density of double bonds) increases in the order palm < rapeseed < soy. Table 1. Typical fatty acid distributions for palm, soy, and rapeseed oil (source: www.accustandard.com). The bottom three rows are weighted averages over the distributions for each oil. Chain Length DB Palm % chains Soy Rapeseed % chains % chains Caprylate 8 0 7 0 0 Caprate 10 0 5 0 0 Laurate 12 0 48 0 0 Myristate 14 0 15 0 1 Palmitate 16 0 7 6 4 Stearate 18 0 3 3 3 Arachidate 20 0 0 3 3 Behenate 22 0 0 0 3 Lignocerate 24 0 0 0 3 Oleate 18 1 12 35 45 Erucate 22 1 0 0 20 Linoleate 18 2 3 50 15 Linolenate 18 3 0 3 3 Average chain length 13.28 17.94 19.04 Average DBs per chain 0.18 1.44 1.04 DB fraction 1.4% 8.0% 5.5% Conclusions Figure 1. FT-IR spectra of the three biodiesel samples, showing the bands useful for discrimination. These bands are all due to vibrational modes of alkenes and increase in intensity with the average double bond density in the fatty acid chains. The FT-IR spectrum of biodiesel takes only a few seconds to measure with a diamond ATR accessory, and contains readily accessible information about the extent of unsaturation in the fatty acid chains. This information is directly related both to the source oil used and to properties such as the cloud point of the fuel. FT-IR spectroscopy, thus, provides a very simple way to check the provenance of a biodiesel sample. PerkinElmer, Inc. 940 Winter Street Waltham, MA 02451 USA P: (800) 762-4000 or (+1) 203-925-4602 www.perkinelmer.com 1. Equivalent results can be obtained using the PerkinElmer Frontier FT-IR system. For a complete listing of our global offices, visit www.perkinelmer.com/ContactUs Copyright ©2011, PerkinElmer, Inc. All rights reserved. PerkinElmer® is a registered trademark of PerkinElmer, Inc. All other trademarks are the property of their respective owners. 009163A_01 Printed in United Kingdom