Increasing FTIR testing capacity following standard protocols using the Pearl: FAME Biodiesel case study

Several measurement standards exist for the quantification of fatty acid methyl esters (FAME) in biodiesel blends. One of these is EN 14078, which is an IR transmission measurement using liquid cell assemblies of various pathlengths for sampling in one of three concentration ranges.

For the lowest range of approximately 0.05% to 3% FAME (by volume) a pathlength of around 0.5 mm is typically required; for the second range of 3% to 20% FAME a cell of 0.1 mm pathlength will give suitable results for diluted samples. In the higher range of 20% or more FAME, further dilutions are required to produce absorbances in the linear range of the detector. Pathlengths of between 0.05 and 0.1 mm are required for this higher range, depending on the dilution performed [1].

Since Biodiesel blends are generally controlled by producers and limited by local regulation to blends of 5-7% FAME content by volume [2], the lower measurement ranges are the ones more commonly required for control testing of biodiesel blends. To maintain accurate measurements over a run of many samples, it is essential that the cell is cleaned thoroughly between samples, using solvents to remove any remaining biodiesel residues from the prior run. The pathlength of the cell must also remain fixed and reproducible. The Pearl™ accessory with its unique Oyster™ cell assemblies is one such cell.

One of our customers was keen to find out if the Pearl could be used in the place of a sealed cell in the EN 14078 standard. The standard applies a simple calibration to the carbonyl peak at circa 1745 cm^-1 that is present in FAME but not in ordinary diesel. Following the method set out in the standard we first prepared samples at known concentrations and then diluted them as required by the standard.

Spectra were recorded in parallel Oyster cells fitted with CaF₂ windows with nominal pathlengths of 50, 100 and 500 µm. The use of a parallel cell allows the exact pathlength to be calculated using the interference fringing method. An appropriate base fuel was used as a background to ensure no interference fringing was present in the final FAME spectra.

The IR spectra of mineral diesel and biodiesel against an empty cell background are shown in Figure 1 for reference. The spectra were recorded in a wedged cell to prevent the appearance of interference fringes in the final data. The band at 1747 cm^-1 is assigned to the C=O stretch in FAME and is completely absent in the case of mineral diesel, as expected. The presence of a distinct band for the analyte in a region free from solvent bands greatly simplifies the calibration process. The height of this peak is calculated from an appropriate baseline and plotted using the Beer-Lambert law.

Figure 2 shows spectra recorded in a 500 µm pathlength cell in the CO region. As expected, the carbonyl band increases with increasing FAME content. The calibration plot is shown in Figure 3. Also shown are calibration plots obtained for cells with pathlengths at 50 and 100 µm, using higher FAME contents. In all cases extremely high R² values were obtained demonstrating a good fit to the data. The calibration will vary depending on the raw material used and therefore the calibration should be constructed using standards closely matched to the unknown sample material. Since the exact pathlength will vary cell to cell it is advisable to recalculate the calibration graph whenever the Oyster cell is changed or use the exact pathlength to correct for changes.

The system has several advantages over the traditional liquid cell assembly used for FAME content determination: The Oyster cell is designed for rapid sample screening and easy cleaning ensuring a high sample throughput. Unlike a traditional cell that cannot be disassembled without altering the pathlength, the Oyster is designed to consistently form the same pathlength. This greatly simplifies cleaning, ensuring that errors due to incomplete cleaning are less likely to creep into your dataset.

Traditional cell designs suffer from the possibility of leakage which is disastrous; once the cell leaks it must be dismantled, cleaned, and reassembled followed by laborious recalibration. With the horizontal configuration of the Oyster cell the possibility of cell leakage is greatly reduced and so downtime for recalibration is minimised. The Pearl™ and Oyster system is a great addition to any routine testing lab where speed is of the essence. The following table sets out just some of the standards it can be used with.

References

• [1] EN 14078:2009 Determination of fatty acid methyl ester (FAME) content in middle distillates. Infrared spectrometry method
• [2] https://www.iea-amf.org/content/fuel_information/fatty_acid_esters/compatibility

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