Biodiesel is a clean-burning alternative fuel, produced from renewable resources such as vegetable oils and animal fats. The official definition consistent with federal and state laws and the original equipment manufacturer (OEM) guidelines identifies biodiesel as mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats which conforms to ASTM D 6751 specifications for use in diesel engines.

Biodiesel can be used as a pure fuel or blended at any level with petroleum diesel to create a biodiesel blend that can be used in compression-ignition engines with little or no modification.

This alternative fuel is constantly increasing in popularity, with the National Biodiesel Board reporting a rise in sales volume in the United States from 500,000 gallons in 1999 to 75 million gallons in 2005.

All fuels and fuel additives must be registered with the U.S. EPA and be subjected to the health effects regulations contained within 40 Code of Federal Regulations Part 79. Biodiesel is the only alternative fuel to be legally registered with the EPA and to have fully completed the health effects testing requirements of the 1990 Clean Air Act Amendments. Furthermore, pure biodiesel (B100) has been designated as an alternative fuel by the U.S. DOE and the U.S. Department of Transportation.

In order for biodiesel to be sold as a fuel or blending stock, it must meet a set of requirements defined in ASTM D 6751 and EN 14214 standards, which specify the maximum allowable concentrations of contaminants in B100. Biodiesel must be produced to these strict specifications to ensure optimum engine performance as well as safe engine operation.

A range of techniques can be used for both the quality control and the identification of labeled blend levels, including gas chromatography (GC), infrared spectrometry (IR) and inductively coupled plasma (ICP).

Gas Chromatography: The Official Methods
› Free and total glycerin (EN14105 / ASTM D 6584) Free and bonded glycerin reflects the quality of biodiesel. Low levels of total glycerin ensure high conversion of the oil while high levels of glycerin and glycerides can cause injector deposit, a clogged fueling system and affect cold weather operation. The analysis of glycerin, and mono, di and triglycerides requires a non-discriminative injection system able to transfer both volatile and heavy compounds.

The regulations mentioned earlier specify that GC analysis, with automatic on-column injector and high-temperature oven program from 50 degrees to 380 degrees Celsius (122 degrees to 716 degrees Fahrenheit) is capable of testing the content of free and total glycerin in B100. A non-polar column phase is required. Calibration is achieved by the use of two internal standards (IS) and four reference compounds, namely 1,2,4-butanetriol (IS1) for glycerine, tricaprin (IS2) for mono, di and tri-glycerides, and glycerin, mono-olein, di-olein and tri-olein. Thermo Fisher Scientific's trademarked Trace GC Ultra provides successful determinations of free and total glycerin, either equipped with automatic on-column injector or with programmed temperature vaporizing (PTV) operated in simulated on-column mode.

› Esters and linolenic methyl ester (EN 14103) The cetane number of biodiesel depends on the distribution of fatty acids in the original oil. An accurate characterization of fatty acid methyl esters (FAME) in biodiesel blends is essential for a more accurate calculation of the cetane index. This method requires a GC analysis with split/splitless or PTV injector and wax column for a detailed separation of FAME. The analysis provides verification that the esters content in B100 is greater than 96.5 percent by mass while also allowing the characterization of FAME composition. Calculation of the percentage of FAME is achieved with internal standard calibration.

When FAME is analyzed, the use of PTV backflush option permits to vent the heavier fraction (triglycerides) out of the injection system, preventing wax column contamination and preserving the column performance for a longer period of time. This inlet option ensures easier injector maintenance, since the injector can be opened, leaving the carrier flowing into the column. Air is also prevented from flowing into the column.

› Residual methanol (EN 14110) Monitoring residual methanol in B100 is a matter of safety since even small amounts of this material can reduce the flash point. Moreover, residual methanol can affect fuel pumps, seals and elastomer, and result in poor combustion properties. Regulations require a headspace GC method to be used in order to verify the content of residual methanol in B100. Either polar or non-polar columns are allowed.

There are two options of GC systems. First, a dual GC system can be set up to comply with all of the three official methods for the analysis of glycerin, FAME and methanol. Two Trace GC Ultra units, one dedicated to EN 14105 and one running EN 14103 and EN 14110, along with a single autosampler serving both the GCs, can deliver tremendous productivity and significant costs savings.

Secondly, based on the orthogonal separation principle, comprehensive multi-dimensional GC (GCxGC) increases separation power by more than a factor of 10 with respect to conventional GC, providing an effective solution to characterize biodiesel blends. The GCxGC approach provides triple information at once. It achieves petrodiesel compositional characterization determining the biodiesel percentage content in the blend and details the FAME composition delivering valuable information such as the type of oil used, the origin of the biodiesel and the cetane index, plus pour point evaluation. Quantitative results have been demonstrated to be reliable and highly repeatable.

Infrared Spectroscopy
Issues that arise during the analysis of biodiesel have a number of sources. For example, production facilities and terminal services need to ensure quality for such things as tranesterification and glycerol removal while testing labs and regulatory agents must ensure the labeled blend levels are present. The former are generally concerned with high FAME content materials (B100), while the latter may be exposed to a wide range of FAME content, from B2 and B20 to B100. IR has been shown to provide a rapid, precise and accurate tool for this analysis.

By coupling a Fourier transform infrared spectrometer equipped with a standard potassium bromide beam splitter and a standard mid-infrared detector with an attenuated total reflection accessory, it is possible to collect the required data, with tremendous advantages.


Figure 1. Calibration result for a 45 degree ARK ATR biodiesel blend.


Figure 2. Calibration result for a 60 degree ARK ATR biodiesel blend.

The ASTM method recommends partial least square (PLS) be used for the analysis of biodiesel. Essentially, PLS models the variation in sample and matrix, so a well-defined set of standards is required to cover the range of matrix effects. Figures 1 and 2 show that the process is very well modeled even with a low number of PLS factors. Prediction of a series of unknown samples when using the 60 degree zinc selenide plate gave answers within plus or minus 0.07 or better over the low range (45 degree plate), and plus or minus 0.1 or better over the high range.


Figure 3. Calibration transfer through ACLS. The top PLS shows the original calibration (0) and the second bench predicted from it (+). The bottom result shows calibration after inoculation of the original method.

The key concern for testing labs is that the variability in petroleum diesels from region to region must be modeled for the analysis to be useful. Transfer of the calibration between spectrometers can be addressed with the augmented classical least squares (ACLS) algorithm developed by Sandia National Laboratories. Figure 3 shows data taken from two spectrometers, one being used to predict the other. A small offset is visible. The ACLS algorithm removes this offset cleanly. Thus, once a calibration is built on one spectrometer, methods can be transferred to another spectrometer by "inoculating" the method with data from the second unit, as is frequently done in Fourier transform near-infrared applications.

The methods developed thus far have focused only on the FAME content, in keeping with the work being outlined by the ASTM D02 committee (ASTM Committee on Biodiesel Analysis by Fourier transform infrared (FT-IR) and attenuated total reflectance (ATR)). However, FT-IR can be used to analyze many other components, including free fatty acids or glycerol, with proper calibration. A generalized method could be developed given a proper set of standards.

Inductively Coupled Plasma
The elemental analysis of biodiesel and biodiesel blends has become increasingly important over the past decade due to the steady trend to promote low-sulfur and low-emission diesel fuels in the European Union and other parts of the world. The elemental analysis of the biodiesel feedstock and the final blend is essential to maintain quality standards within the EN 14214 regulations of maximum elemental content as well as reducing detrimental environmental effects.


Table 1.

The maximum currently allowable concentrations for the various elemental impurities are easily achievable with biodiesel production methods and are set out in Table 1.

ICP spectrometry has long been a cost-effective tool for rapid, multi-element analyses of a variety of liquids and materials, including organics. However, elemental analysis of biodiesel presents certain challenges to ICP instrumentation however, which must be met to produce credible results-not least of which is the volatility and plasma loading of diesel and biodiesel. Therefore, the instrument requires:

› Good organic capabilities-preferably the ability to analyze undiluted biodiesel at ambient temperatures, reducing evaporation and dilution errors while increasing sensitivity
› Robust, rapid response radio frequency generator to handle the high plasma loading from organic sample introduction
› High resolution optics for peak separation
› High saturation resistance for the detector to resist blooming due to high carbon and diatomic carbon emissions
› Rapid, multi-element analysis enables high throughput.

ICP can be used to meet all the above criteria. Since the standards are normally artificially prepared from organo-metallic stock standards and then matrix-matched as closely as possible, an internal standard is commonly used to overcome density, volatility and nebulization differences between the standards and the samples. For example, Yttrium can be added to each sample individually in the same quantity. Alternatively, the internal standards can also be automatically added on line using the Thermo Scientific iCAP 6000's multi-channel pump and the internal standard mixing kit.

With the significant benefits of excellent sensitivity and multi-element capability, ICP is an ideal technique to analyze biodiesel and its precursors for all the required elemental analyses. In addition, simultaneous ICP allows other elements such as copper, silicon and zinc to be analyzed concurrently enabling a comprehensive, cost-effective elemental analysis.

The definition of pure biodiesel along with physical and chemical property limits is established by official quality specifications with which the composition of pure biodiesel must comply either for its pure use or prior to blending with mineral diesel. A range of analytical tools can be applied for the analysis and quality control of biodiesel, including GC, IR and ICP.

For more information, contact Thermo Fisher Scientific at analyze@thermofisher.com or (800) 532-4752.

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