ASTM D7806-20
Standard Test Method for Determination of Biodiesel (Fatty Acid Methyl Esters) and Triglyceride Content in Diesel Fuel Oil Using Mid Infrared Spectroscopy (FTIR Transmission Method)
ASTM D7806 covers the determination of fatty acid methyl ester (FAME) biodiesel and triglyceride (TAG) concentrations in traditional diesel and renewable diesel fuel blends using a portable mid-infrared spectrometer.
The method applies to biodiesel concentrations from 1 to 40 Vol. %. Additionally, it applies to biodiesel concentrations from 1 Vol% to 27 Vol% in samples where triglycerides are detected (from 1 to 10 Vol. %). Triglycerides from 2 to 10 Vol. % may be determined in diesel having biodiesel concentrations from 1 Vol. % to 27 Vol. %. FAME and triglyceride may be determined outside these stated ranges, but stated precision might not apply.
This method distinguishes biodiesel from oil but it cannot be applied to differentiate methyl esters (FAME) from fatty acid ethyl esters (FAEE).
Fatty Acid Methyl Esters are long straight chain esters derived from vegetable oil and animal fat that undergo the so-called transesterification catalytic process with methanol is a very low sulfur material and one important component of new middle distillate fuel formulations with the purpose of reducing SOx and NOx emissions. These compounds often refer as B100. B100 is blended with Diesel fuels to produce the different blends B5, B10, B20 which is the volume % of FAMES following the letter “B”. Standard Specification for Biodiesel D6751 controls the composition, quality and properties of these valuable distillates for fuel formulations.
Reference Documents:
ASTM Standards:
D975 Standard Specifications for Diesel Fuel Oils
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D1298 Test Method for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method
D4052 Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
D4307 Practice for Preparation of Liquid Blends for Use as Analytical Standards
D5854 Practice for Mixing and Handling of Liquid Samples
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
D6299 Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance
D6751 Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels
E168 Practices for General Techniques of Infrared Quantitative Analysis
E1655 Practice for Infrared Multivariate Quantitative Analysis
E2056 Practice for Qualifying Spectrometers and Spectrophotometers for Use in Multivariate Analysis, Calibrated Using Surrogate Mixtures
D7418 Set-up and Operation of Fourier Transform Infrared (FT-IR) Spectrometers for In-Service Oil Condition Monitoring
D1655 Standard Specifications for Aviation Turbine Fuels
Summary of Test Method
This method utilizes a Fourier transform mid infrared spectrometer fitted with a transmission sample cell with a specified path length. A beam of infrared light is passed through the sample and the motion of the spectrometer mirrors enables determination of the sample absorption spectrum. Specific regions of the spectrum where FAME and triglycerides show strong absorption are used for the analysis. Users should prepare the sample according to instrument manufacturer instructions after the correct validation and instrumental checks of the instrument.
Figure 1. Eraspec FT-IR Spectrometer
Significance and Use:
Apparatus preparation and calibration: Once the system has been assembled, leak tested, heated and stabilized per the instrument manufacturer, typical operating conditions D4629 are stablished and adjusted as per the manufacturer’s guidelines.
Biodiesel is a fuel commodity primarily used as a value-added blending component with diesel fuel. This test method is applicable for quality control in the production and distribution of diesel fuel and biodiesel blends containing FAME. The use of triglycerides in fuels is not approved for transportation applications within any ASTM specification. This test method allows the quantification of triglyceride concentration in biodiesel blends, thus enabling detection of out-of-specification blending. This test method is fast, simple to run, inexpensive and requires no sample preparation.
Report
Report the nitrogen results as mg N/kg of the sample. Report to the nearest 0.01 mgN/kg. Repeatability—The difference between two test results obtained by the same operator with the same apparatus under constant operating conditions on identical test material would, in the long run, in the normal and correct operation of the test method, provide the following repeatability values: 0.006 at the 0.03 mg N/kg level and 0.06 at the 1 mg N/kg level. Reproducibility—The difference between two single and independent test results obtained by different operators working in different laboratories on identical test material would, in the long run, in the normal and correct operation of the test method, exceed the following values in only one case in twenty, where X = the average of the two test results.
Interferences
The presence of carbonyl containing compounds including carboxylic acids, ketones and aldehydes, as well as fatty acid ethyl esters (FAEE), interfere with the FAME determinations.
The hydrocarbon composition of the diesel fuel has a minor impact on the calibration model. The proper choice of the evaluation routine as described in this standard can minimize interferences from the hydrocarbon composition of the diesel fuel.
Water Vapor Interference—The calibration and analysis bands as described in the Procedure section 12 of the D7608 standard method show regions where significant signals due to water vapor can appear in the infrared spectrum. This shall be accounted for to permit calibration and measurements at the low-end concentrations.
NOTE 1 – 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 effect of water vapor variations, and any accessory should be sealed to the spectrometer.
High Lauric Acid Methyl Esters Interference—Samples with high lauric acid methyl ester (i.e. Palm biodiesel) content are known to cause a bias when used in the calibration model. Unless the method is being used to calibrate specifically and exclusively for samples containing high levels of lauric acid methyl esters, feedstocks containing high levels of these compounds should be avoided.
Apparatus
ERASPEC is rugged, robust and portable, can operate off line and at line. Primarily it determines the overall absorption spectrum of the sample in the Middle Infrared region, approximately from 450 up to 6500 cm -1. After a mathematical transformation of the firstly obtained raw interferogram spectrum illustrated in Fig.2, signal is transformed in plots of frequencies vs. absorption intensities. Since all hydrocarbon types absorb in the Mid-IR in a characteristic manner following Beer – Lambert’s Law, the absorption spectrum of bands and intensities can be integrated and related to chemical composition, physical properties and product quality parameters.
The resolution of these systems is typically 4 cm–1 and the noise of the single beam spectrum at 100 % transmission shall be less than 0.3 % peak-to-peak in the region from 1725 cm–1 to 1765 cm–1.
Figure 2. Diagram of the FT_IR Spectrometer
Reagents
Standards for Calibration, Qualification, and Quality Control Check Standards used in this test method are mainly commercial blends, not high purity standards.
B100 (Neat biodiesel) used for calibration, qualification and quality control standards must be D6751 compliant. If the origin of FAME is known, this type shall be used for calibration. If the type of biodiesel is not known, use soy methyl ester or the FAME most commonly used in the geographical area where the samples are obtained.
The triglycerides shall be food grade soy oil, available for purchase commercially. Other types of triglycerides may be used provided they are liquid at room temperature. Consult D7806 especially Appendix A2 for further discussion.
Middle distillate fuel used for calibration, qualification and quality control standards must be D975 compliant, free of biodiesel or biodiesel precursors compounds and representative of the petroleum base stocks anticipated for blends to be analyzed (i.e., crude source, 1D, 2D, blends, winter/summer cuts, etc.). See appendix A2.2 of D7806 for further discussion on calibration fuels.
Sampling and Sample Handling
Samples should be taken following procedures described in Standard Practices D4057 & D4177 wherever the case might be. Protect samples from extreme temperatures.
Avoid the use of leaking containers that might affect the sample integrity. Equilibrate sample temperature prior to perform an analysis and reseal the container for future needs.
Calibration of the Apparatus
The calibration of the instrument must proceed according to the procedure described in Annex A1 of D7806. This calibration may be performed by the instrument manufacturer prior to delivery of the instrument to the end user.
Quality Control Checks
Each Laboratory should have a quality control program to ensure all the measurements are in statistical control as defined in Standard Practice D6299.
Each day that the instrument is to be used, confirm it is in statistical control by measuring the biodiesel concentration using the procedure outlined in D7806 (Section 12) on at least one quality control sample of known biodiesel content.
Standard(s) of known biodiesel and triglyceride concentration shall be prepared by mass and converted to volume % using the measured density as outlined in D7806 (Section 13.1). At least one standard shall be prepared for each calibration range. Additional standards including 0 volume % may also be prepared and used for quality control checks.
Standard(s) should be prepared in sufficient volume to allow for a minimum of 30 quality control measurements to be made on one batch of material. Properly package and store the quality control samples to ensure that all analyses of quality control samples from a given lot are performed on essentially identical material.
Procedure
Determination of biodiesel and triglyceride content should proceed according to the Instruction Manual of the manufacturer. Eraspec comes pre calibrated from the factory so the operator should equilibrate the temperature of the sample to the ambient temperature. Eraspec automatically also performs a background spectrum used in the calculations. The validation of the instrument and the revision of the IQT parameters should be obtained prior to the introduction of the QC reference standard.
Rinsing of the absorption cell introduction of sample and the rest of the D7806 analysis steps is performed automatically by the Eraspec after the settings parameters and method had been selected.
Calculations
Conversion to Volume % of Biodiesel – To convert the calibration and qualification standards to volume % use Eq.:
Vb = Mb (Df / Db)
where:
Vb… biodiesel volume % Mb… biodiesel mass % Df… relative density at 15.56 °C of the calibration or qualification standard being tested as determined by practice D1298 or test method D4052 Db… B100 biodiesel blend stock relative density at 15.56°C of the calibration or qualification standard being tested as determined by practice D1298 or test method D4052.
Eraspec has a built in D4052 Density Module which makes this conversion extremely useful and simultaneous in every run. Results are printed separately and the feed to the calculation software is carried out automatically.
Conversion to Volume % of Triglyceride—To convert the calibration and qualification standards to volume %, use Eq.:
Vt = Mt (Df / Dt)
where:
Vt = volume % of triglycerides
Mt = triglyceride mass %
Df = relative density at 15.56°C of the calibration or qualification standard being tested as determined by Practice
D1298 or Test Method D4052.
Dt = triglyceride relative density at 15.56°C of the calibration or qualification standard being tested as determined by Practice D1298 or Test Method D4052.
It is important to note that D7806 is most accurate when the biodiesel used in the calibration is derived from the same source as the biodiesel in the samples being analyzed. If the biodiesel used in the calibration is derived from a different source than the biodiesel in the sample being analyzed, adjusting the factory calibration with an additional 2-point calibration is allowed and recommended.
Report
Report Volume % biodiesel and volume % triglycerides by test method D7806, to the nearest 0.1%.
Precision and Bias
The precision of this test method, which was determined by statistical examination of the results from an interlaboratory study involving 16 labs and 22 samples, reads as follows:
Repeatability (r) – The difference between successive test results obtained by the same operator with the same apparatus under constant operating conditions on identical test samples would, in the long run, and in the normal and correct operation of the test method, exceed the following values only in one case in twenty, where X = the average of two test results.
FAME in Diesel / No TAG measured (Vol. %)
r = 2.349E-02 * X1.1752
FAME in Diesel / TAG (range 1 Vol. % to 10 Vol. %)
r = 4.581E-02 * X 1.1789
TAG in Diesel / biodiesel (range 1 Vol. % to 27 Vol. %)
r = 4.202E-02 * X 1.4209
Reproducibility (R) – The difference between two single and independent results obtained by different operators working in different laboratories on identical test material would, in the long run, in the normal and correct operation of the test method, exceed the following values in only 1 case in 20, where x represents the average of the two consecutive test results.
FAME in Diesel / No TAG measured (Vol. %)
R = 0.2102 * X 0.705
FAME in Diesel / TAG (range from 1 Vol. % to 10 Vol. %)
R = 9.041E-02 * X1.1789
TAG in Diesel / biodiesel (range from 1 Vol. % to 27 Vol. %)
R = 0.1398 * X1.4209
See example tables for precision for FAME’s and TAG’s in Diesel in D7806.
Bias – Because no suitable reference materials were included in the interlaboratory test program, no statement of bias is being made.
References
1. D7806 Standard Test Method Determination of Biodiesel (Fatty Acid Methyl Ester) hand Triglyceride Content in Diesel Fuel Oil Using Mid-Infrared Spectroscopy (FTIR Transmission Method)
2. ERALYTICS Eraspec – Operation Manual
