GB/T 23801-2021 Determination of fatty acid methyl ester content in middle distillates — Infrared spectroscopy method

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GB/T 23801-2021, Determination of fatty acid methyl ester content in middle distillates — Infrared spectroscopy method, describes the determination of the volume fraction of fatty acid methyl esters (FAME) in middle distillates by infrared spectroscopy.

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Scope: This document describes the method for determining the volume fraction of fatty acid methyl esters (FAME) in middle distillates by infrared spectroscopy. Applicable to the determination of samples containing FAME conforming to the requirements of Annex C of GB 25199—2017. Applicable to the determination of middle distillate samples with a FAME volume fraction of 0.05 % to 50 %. Samples with a FAME volume fraction above 50 % may also be determined by this method after dilution, but their precision has not been investigated. If interfering components such as esters and carbonyl-containing compounds are present, the results may be high. NOTE: To convert mass concentration (g/L) to volume fraction, a fixed value of 883.0 kg/m³ is used for the density of FAME. WARNING: Personnel using this document shall have practical experience of regular laboratory work; the use of this document may involve certain hazardous materials, equipment and operations, and this document does not indicate all possible safety problems; users are responsible for taking appropriate safety and health measures and for ensuring compliance with the conditions specified in the relevant national regulations.

Principle: The infrared spectrum of the sample is measured; if necessary the sample is diluted with a solvent free of FAME to a suitable concentration, the maximum absorbance of the ester band in the range 1745 cm⁻¹ ± 5 cm⁻¹ is read, and the FAME content of the sample is calculated from an established calibration curve. The mass concentration (g/L) can be converted to volume fraction (%) by means of the density of FAME (883.0 kg/m³). Different measuring ranges and corresponding dilution factors should be selected for different FAME concentration ranges of samples. A measuring mode requiring no dilution and direct injection should be chosen wherever possible, for example a long-path cell for samples in range A or a short-path cell for samples in range B. To obtain good measurement precision, the lower the FAME concentration in the sample, the more strictly the measurement process needs to be controlled.

Apparatus: Infrared spectrometer: dispersive or interferometric type, wavenumber range 4000 cm⁻¹ to 400 cm⁻¹, absorbance units between 0.1 AU and 1.1 AU, spectral resolution better than 2 cm⁻¹, wavelength accuracy better than 0.1 cm⁻¹, wavelength precision better than 0.01 cm⁻¹, signal-to-noise ratio better than 10000:1. 6.2 Sample cell: transmission cell; KBr, CaF₂, NaCl, ZnSe and other materials may all be used. In humid regions a ZnSe cell is recommended. The path length of the cell may be selected according to the concentration range of the sample being measured. The choice of cell should avoid optical interference affecting the measurement results. Example: a solution with a FAME mass concentration of 3 g/L (volume fraction 0.34 %) measured in a cell of 0.5 mm path length gives a maximum absorbance of about 0.4 AU near 1745 cm⁻¹. 8.3.2.1: the number of scans for spectrum measurement shall not be less than 16.

Reagents and reference materials: General: unless otherwise stated, the purity of all chemical reagents used in the tests is not less than 99 %. 5.1 FAME for calibration: conforming to the requirements of Annex C of GB 25199—2017, with a FAME mass fraction of not less than 96.5 %. 5.2 Solvent free of FAME: may be used as the sample dilution solvent or as the reference solvent for infrared background measurement; a middle distillate solvent may be used. The solvent shall have good mutual solubility with the sample to be tested and shall not distort the spectral measurement. A solvent free of FAME means a middle distillate solvent that has no absorption band at the typical ester band of FAME. 5.3 Cleaning solvent: anhydrous ethanol, n-pentane or cyclohexane, etc. Preparation of calibration solutions (8.2.2): prepare calibration solutions according to the concentration ranges of ranges A, B, C and D; each calibration solution shall be prepared by separate weighing; the set of calibration solutions of known concentration used to establish the calibration curve in each concentration range shall not be fewer than 5; the specific preparation is to accurately weigh the FAME for calibration into a volumetric flask and dilute to the mark with solvent free of FAME. NOTE (8.2.2.2): This method calculates the FAME content by measuring the C=O absorption band of fatty acid methyl esters with carbon numbers C8 to C22 by infrared spectroscopy; the accuracy of the measurement results is related to the matching of the carbon chain or the consistency of molecular weight between the sample to be tested and the calibration solutions. If the fatty acid methyl esters in the sample have shorter carbon chains while the calibration solutions have longer carbon chains, the results may be high, and vice versa. Therefore, to reduce bias, the FAME used for the calibration solutions should have an average molecular weight close to that of the FAME in the sample to be tested. 8.2.2.3: the solvent free of FAME may serve as an additional point of the calibration curve, assigned a content of 0. To avoid propagation of error, this additional point shall not be prepared by dilution.

Key conditions: Measurement wavenumber: maximum absorbance of the ester band within 1745 cm⁻¹ ± 5 cm⁻¹. Baseline: draw a tangent between 1670 cm⁻¹ and 1820 cm⁻¹ to establish the measurement baseline (8.3.2.4). The optimum absorbance range is about 0.1 AU to 1.1 AU, and the absorbance shall always be within the linear detection range (8.1.1.1). The number of scans shall not be less than 16; the measurement parameters of the calibration solutions and the sample shall be kept completely consistent (8.3.2.1). Selection of cell path length (8.1.3): Range A: FAME volume fraction about 0.05 % to 1 %, a 0.5 mm path length cell is recommended; Range B: FAME volume fraction about 0.5 % to 3 %, a 0.1 mm path length cell is recommended; Range C: FAME volume fraction about 3 % to 20 %, dilute the sample 1:5 and use a 0.1 mm path length cell; Range D: FAME volume fraction about 20 % to 50 %, dilute the sample 1:10 and use a 0.1 mm path length cell. NOTE: At a FAME volume fraction of 3 %, range B is used for the determination; at a FAME volume fraction of 20 %, range C is used. Range A: measured in a long-path cell, no dilution of the sample; range B: measured in a short-path cell, no dilution of the sample; range C: the sample is diluted to a suitable concentration and measured in a short-path cell; range D: the sample is diluted by a higher dilution factor than range C to a suitable concentration and measured in a short-path cell (8.1.1.1). When measuring samples in range A, the spectrum of the reference solvent may be used as the reference spectrum (8.3.2.2).

Calculation and results: Calculation of FAME content as mass concentration (g/L): calculate the FAME content X_S in the sample according to equation (4): X_S = (E_corr − b) / a; where a is the slope of the calibration curve regression equation; b is the intercept of the calibration curve regression equation; X_S is the mass concentration of FAME in the sample, in grams per litre (g/L). 9.2 Conversion between mass concentration and volume fraction: from the measured mass concentration X_S (g/L) and the density of FAME 883.0 kg/m³ (15 °C), calculate the volume fraction Y_S (%) of the sample according to equation (5): Y_S = 100 × X_S / 883.0. Example: the mass concentration of FAME in a sample is 23.5 g/L, which converts to a volume fraction of 2.66 % by equation (5). 10 Expression of results: report the FAME volume fraction (%) of the sample, rounded to 0.01 % for ranges A and B and to 0.1 % for ranges C and D.

Quality control: General: judge the reliability of the test results as specified below (95 % confidence level). 11.2 Repeatability r: the difference between the results of two repeated determinations of the same sample by the same operator, in the same laboratory, using the same instrument and following the same method, shall not exceed the repeatability values given in Table 1. 11.3 Reproducibility R: the difference between two single, independent test results obtained by different operators, in different laboratories, using different instruments and following the same method on the same sample shall not exceed the reproducibility values given in Table 1. Table 1 Repeatability and reproducibility: | Measuring range | Repeatability r (volume fraction/%) | Reproducibility R (volume fraction/%) | |---|---|---| | Range A: volume fraction 0.05 % to 1 % | 0.0126X₁+0.0079 | 0.0499X₂+0.0231 | | Range B: volume fraction 0.5 % to 3 % | 0.0126X₁+0.0079 | 0.0499X₂+0.0231 | | Range C: volume fraction 3 % to 20 % | 0.0166X₁−0.0195 | 0.0793X₂−0.0413 | | Range D: volume fraction 20 % to 50 % | 0.0032X₁+0.4187 | 0.0632X₂−0.0036 | NOTE 1: X₁ is the mean of the two repeated determination results; X₂ is the mean of the two single, independent test results. NOTE 2: At a FAME volume fraction of 3 %, the precision is determined according to range C. NOTE 3: At a FAME volume fraction of 20 %, the precision is determined according to range D.

Normative references: GB/T 4756 Petroleum liquids — Manual sampling GB 25199—2017 B5 diesel fuel GB/T 27867 Petroleum liquids — Automatic pipeline sampling NOTE: The above documents become indispensable provisions of this document through normative reference in the text. For dated references, only the edition corresponding to that date applies to this document; for undated references, the latest edition (including all amendments) applies to this document.

Verification note: [To be verified] and [source unclear] mark points that still need to be checked against the original standard text; they do not mean the values have been verified. Do not use these values or procedures directly as a basis for work.

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