Determining the Source Oil of Biodiesel with Infrared Spectroscopy

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®
Spectrumâ„¢ 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.
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1. Equivalent results can be obtained using the PerkinElmer Frontier FT-IR system.
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