Determination of FAME in Biodiesel Blends Using FT-IR

Biodiesel, which consists of fatty acid methyl esters (FAME), is a renewable alternative to petrodiesel. Mostly, vegetable oils that are subjected to a base-catalyzed transesterification reaction with methanol are used to synthesize biodiesel. The end product is a clear yellow liquid having a viscosity like that of standard diesel fuel. Since biodiesel has excellent lubricating properties, it reduces the wear of fuel injection pumps as well as engines. Owing to its high oxygen content, carbon black particle emissions are clearly reduced when compared to petrodiesel. Other advantages of biodiesel are its high biodegradability, its low toxicity, and the low sulfur content.

While biodiesel can be used undiluted (B100) in suitable engines, it is usually blended with petrodiesel up to a maximum concentration of about 30% v/v (B30). Regulations in the US and Europe specify that blends with a biodiesel content of up to 5% may be sold as normal diesel fuel with no special labeling. The FAME content has to be controlled at different locations during the production process of diesel. The actual concentration of biodiesel can be effortlessly determined by FT-IR spectroscopy as the fatty acid methyl esters have a prominent infrared absorption at around 1745 cm^-1. This absorption band is typical of organic esters and does not occur in petrodiesel that mainly consists of aromatic hydrocarbons and alkanes. To obtain precise results down to concentrations of about 0.02%, it is best to perform the measurement in transmission utilizing a liquid cell.

Figure 1. ALPHA II spectrometer with transmission cell.

The Bruker FAME in diesel analysis kit enables the quantification of the FAME content in diesel in accordance with DIN EN 14078. The measurement is carried out by means of a liquid flow-through cell in combination with a FT-IR spectrometer like the ALPHA. The kit includes a dedicated software-wizard that directs the user through the analysis procedure such as measurement, data evaluation, and reporting. Its intuitive and simple user interface allows even spectroscopically inexperienced personnel to operate it.

Instrumentation

An ALPHA spectrometer is used to measure the IR-spectra for the calibration, by using a liquid flow-through cell with spacers of 100 μm and 500 μm, respectively. The ALPHA, which has a footprint of a lab notebook and a weight of only 7 kg, is a very robust and very compact spectrometer. It is equipped with wear-free components, a rugged aluminum case, a durable diode laser, and Bruker´s patented RockSolid™ interferometer. It features an optional touchscreen PC with a dedicated touch optimized software that is ideal for FTIR beginners and still offers full functionality for infrared experts.The ALPHA spectrometer along with the flow-through cell is shown in Figure 1. The FAME in diesel analysis kit can also be used together with a spectrometer of the VERTEX or INVENIO series.

Methods

The FAME content of a diesel blend can be accessed through the carbonyl band around 1745 cm^-1. Since there are no overlapping bands of the diesel fuel, the analysis can be carried out through a univariate linear function derived from the Lambert-Beer law. The spectra of samples with different FAME concentrations are shown in Figure 2. A 100 μm flow through cuvette was used to perform the measurements which clearly show the linear trend of the absorbance. The calibration curve is shown in the picture inset. A good linearity with a correlation coefficient of 0.9999 and a standard deviation of 0.017% m/m is seen within the calibration range of 0 to 5% m/m. Following sample dilution, higher sample concentrations can certainly be measured.

It is possible to achieve an even higher sensitivity, where concentrations between 0.02 and 1% m/m can be measured- with a path length of 500 μm. Some sample spectra with FAME contents in the range of 0 to 0.5% are shown in Figure 3. Concentrations as low as 0.02% can be measured as the corresponding spectrum (green in Figure 3) is obviously differentiated from that of pure diesel (blue in Figure 3). The inset of Figure 3 illustrates the calibration curve with an upper limit of 1%. Again the standard deviation is considerably better with a value of 0.003% m/m and the correlation coefficient has an excellent value of 1.0000.

Figure 2. Spectra and calibration curve (Fit vs. True, % m/m) of diesel-FAME blends measured with a 100 μm flow-through cuvette.

Figure 3. Spectra and calibration curve (Fit vs. True, % m/m) of diesel-FAME blends measured with a 500 μm flow-through cuvette.

Example Application

The Bruker FAME in diesel analysis kit comes with a dedicated software-wizard (“FAME-wizard”). The wizard is assisting the user in many ways:

• It helps to calculate the result of the measurement when the sample was diluted
• It guides the user through the measurement and evaluation process
• It helps to transfer the calibration to another measurement setup
• The Wizard also allows for the configuration and control of an autosampler
• The path length of the flow-through cell can be detected automatically, and the appropriate calibration is selected accordingly.

To illustrate the use of the wizard, an example application where the FAME content of a diesel blend is controlled is shown. In the test sample, it is essential to verify that the blend contains a minimum of 30% m/m of FAME. To perform the measurement, 25 ml of the blend are diluted with petrodiesel in a volumetric flask of 250 ml. The measurement is subsequently carried out in three easy steps: First, the “Measure Background” button is clicked to measure a background spectrum (see Figure 4 point 1). Next, a 5 ml syringe is used to fill the liquid cell. The cell is placed into the sample compartment and the “Measure Sample” button (point 2) is pressed.

The sample details and information about the dilution of the sample (250 ml flask, 25 ml sample) are entered. If the checkbox “Diluted” is active, the result is automatically corrected for sample dilution. Finally, on clicking “Continue”, the measurement is started (see point 3).

Figure 4. FAME-wizard with measurement dialog.

The measurement including the background can be executed under four minutes. During the measurement, the spectrum is automatically corrected for device-specific influences. Additionally, the cell’s thickness is automatically determined and the suitable calibration is selected. As shown in Figure 5, the dilution-corrected values and the raw values of the measurement are displayed in % (m/m and v/v) and g/L. The measurement result indicates a concentration of 30.7% m/m FAME i.e. the sample is within the specification. Furthermore, a DIN EN 14078 compliant PDF-Report will be automatically generated and printed, if required.

Figure 5. FAME-wizard with the measurement result.

Summary

A fast and accurate analysis of the FAME content in diesel according to DIN EN 14078 can be carried out by using a Bruker FT-IR spectrometer system with a flow-through cell. Thanks to a user-friendly measurement procedure, simple sample preparation, and a robust instrumental setup, even untrained users can perform the analysis.

In general, the IR-analysis takes not more than a few minutes for each sample. Combined with an autosampler, fully automated measurement of even larger sample numbers is possible.

This information has been sourced, reviewed and adapted from materials provided by Bruker Optics.

For more information on this source, please visit Bruker Optics.