ASTM D7039 Standard Test Method for Sulfur in Gasoline, Diesel Fuel, Jet Fuel, Kerosine, Biodiesel, Biodiesel Blends, and Gasoline-Ethanol Blends by Monochromatic Wavelength Dispersive X-ray Fluorescence Spectrometry
3. Summary of Test Method
3.1 A monochromatic X-ray beam with a wavelength suitable to excite the K-shell electrons of sulfur is focused onto a test specimen contained in a sample cell (see Fig. 1). The fluorescent Kα radiation at 0.5373 nm (5.373 Å) emitted by sulfur is collected by a fixed monochromator (analyzer). The intensity (counts per second) of the sulfur X rays is measured using a suitable detector and converted to the concentration of sulfur (mg/kg) in a test specimen using a calibration equation. Excitation by monochromatic X rays reduces background, simplifies matrix correction, and increases the signal/background ratio compared to polychromatic excitation used in conventional WDXRF techniques. (Warning - Exposure to excessive quantities of X-ray radiation is injurious to health. The operator needs to take appropriate actions to avoid exposing any part of his/her body, not only to primary X rays, but also to secondary or scattered radiation that might be present. The X-ray spectrometer should be operated in accordance with the regulations governing the use of ionizing radiation.)
4. Significance and Use
4.1 This test method provides for the precise measurement of the total sulfur content of samples within the scope of this test method with minimal sample preparation and analyst involvement. The typical time for each analysis is five minutes.
4.2 Knowledge of the sulfur content of diesel fuels, gasolines, and refinery process streams used to blend gasolines is important for process control as well as the prediction and control of operational problems such as unit corrosion and catalyst poisoning, and in the blending of products to commodity specifications.
4.3 Various federal, state, and local agencies regulate the sulfur content of some petroleum products, including gasoline and diesel fuel. Unbiased and precise determination ofsulfur in these products is critical to compliance with regulatory standards.
5. Interferences
5.1 Differences between the elemental composition of test samples and the calibration standards can result in biased sulfur determinations. For samples within the scope of this test method, elements contributing to bias resulting from differences in the matrices of calibrants and test samples are hydrogen, carbon, and oxygen. A matrix-correction factor (C) can be used to correct this bias; the calculation is described in Annex A1. For general analytical purposes, the matrices of test samples and the calibrants are considered to be matched when the calculated correction factor C is within 0.98 to 1.04. No matrix correction is required within this range. A matrix correction is required when the value of C is outside the range of 0.98 to 1.04. For most testing, matrix correction can be avoided with a proper choice of calibrants. For example, based on the example graph in Annex A1 (Fig. 2), a calibrant with 86 mass % carbon and 14 mass % hydrogen can cover non-oxygen containing samples with C/H ratios from 5.4 to 8.5. For gasolines with oxygenates, up to 2.3 mass % oxygen (12 mass % MTBE) can be tolerated for test samples with the same C/H ratio as the calibrants.
5.2 Fuels containing large quantities of oxygenates, such as biodiesel, biodiesel blends, and gasoline-ethanol blends, can have a high oxygen content leading to significant absorption of sulfur Kα radiation and low sulfur results.
5.2.1 Biodiesel and biodiesel blends may be analyzed using this test method by applying correction factors to the results or using calibration standards that are matrix-matched to the test sample (see Table 1). Correction factors may be calculated (see Annex A1), or obtained from Table 2 if the sample has been measured on a mineral oil calibration curve.
5.2.2 Gasoline-ethanol blends may be analyzed using this test method by applying correction factors to the results or using calibration standards that are matrix matched to the test sample (see Table 1). Correction factors may be calculated (see Annex A1), or obtained from the correction tables. Use Table 3 if the sample has been measured on a mineral oil calibration curve, or use Table 4 if the sample has been measured on an ethanol calibration curve. Ethanol-based calibrants can be used for gasoline-ethanol blends. Ethanol-based calibrants are recommended for gasoline-ethanol blends containing more than 50 % (by volume) ethanol.
5.3 Other samples having interferences as described in 5.1 may be analyzed using this test method by applying correction factors to the results or by using calibration standards that are matrix matched to the test sample (see Table 1). Correction factors may be calculated as described in Annex A1.
5.4 To minimize any bias in the results, use calibration standards prepared from sulfur-free base materials of the same or similar elemental composition as the test samples. When diluting samples, use a diluent with an elemental composition the same or similar to the base material used for preparing the calibration standards.
5.4.1 A base material for gasoline can be approximately simulated by mixing 2,2,4-trimethylpentane (isooctane) and toluene in a ratio that approximates the expected aromatic content of the samples to be analyzed.
