ASTM D7371 test method for biodiesel content in diesel fuel oil
ASTM D7371 Standard Test Method for Determination of Biodiesel (Fatty Acid Methyl Esters) Content in Diesel Fuel Oil Using Mid Infrared Spectroscopy (FTIR-ATR-PLS Method)
4. Summary of Test Method
4.1 A sample of diesel fuel, biodiesel, or biodiesel blend is introduced into a liquid attenuated total reflectance (ATR) sample cell. A beam of infrared light is imaged through the sample onto a detector, and the detector response is determined. Wavelengths of the absorption spectrum that correlate highly with biodiesel or interferences are selected for analysis. A multivariate mathematical analysis converts the detector response for the selected areas of the spectrum from an unknown to a concentration of biodiesel.
4.2 This test method uses Fourier transform mid-IR spectrometer with an ATR sample cell. The absorption spectrum shall be used to calculate a partial least square (PLS) calibration algorithm.
5. Significance and Use
5.1 Biodiesel is a blendstock commodity primarily used as a value-added blending component with diesel fuel.
5.2 This test method is applicable for quality control in the production and distribution of diesel fuel and biodiesel blends containing FAME.
6. Interferences
6.1 The hydrocarbon composition of diesel fuel has a significant impact on the calibration model. Therefore, for a robust calibration model, it is important that the diesel fuel in the biodiesel fuel blend is represented in the calibration set.
6.2 Proper choice of the apparatus, design of a calibration matrix, utilization of multivariate calibration techniques, and evaluation routines as described in this standard can minimize interferences.
6.3 Water Vapor Interference - The calibration and analysis bands in A1.2 lie in regions where significant signals due to water vapor can appear in the infrared spectrum. This shall be accounted for to permit calibration at the low end concentrations.
NOTE 2 - Ideally, the spectrometer should be purged with dry air or nitrogen to remove water vapor. The purge should be allowed to stabilize over several hours before analytical work is pursued, due to the rapid changes in the air moisture content within the spectrometer during early stages of the purge. In cases where water vapor prevention or elimination is not possible using a purge, the operator should measure a reference background spectrum for correction of the ratioed spectrum for each sample spectrum measured. This operation is generally automated in today's spectrometer systems and the operator should consult the manufacturer of the spectrometer for specific instructions for implementing automated background correction routines. The spectrometer should be sealed and desiccated to minimize the affect of water vapor variations, and any accessory should be sealed to the spectrometer.
6.4 Fatty Acid Ethyl Esters (FAEE) Interference - The presence of FAEE in the composition of the biodiesel will result in an overall lower concentration measurement of biodiesel content. Outlier statistical results may be a useful tool for determining high concentration FAEE content (for additional FAEE information, see research report referenced in Section 15).
6.5 Undissolved Water - Samples containing undissolved water will result in erroneous results. Filter cloudy or water saturated samples through a dry filter paper until clear prior to their introduction into the instrument sample cell.
