ASTM D4294 for sulfur in petroleum and petroleum products
ASTM D4294 standard test method for sulfur in petroleum and petroleum products by energy-dispersive X-ray fluorescence spectrometry
6. Apparatus
6.1 Energy-dispersive X-ray Fluorescence Analyzer - Any energy dispersive X-ray fluorescence analyzer may be used if its design incorporates, as a minimum, the following features:
6.1.1 Source of X-ray Excitation, X-ray source with energy above 2.5 keV. (Warning - In addition to other precautions, if a radioactive source is used, it must be well shielded to international standard requirements and, therefore, not present any safety hazard. However, attention to the source is only to be carried out by a fully trained and competent person using the correct shielding techniques.)
NOTE 2 - Operation of analyzers using X-ray tube sources is to be conducted in accordance with the manufacturer's safety instructions and local regulations.
6.1.2 Sample Cell, providing a sample depth of at least 4 mm and equipped with a replaceable X-ray transparent plastic film window.
6.1.3 X-ray Detector, with sensitivity at 2.3 keV and a resolution value not to exceed 800 eV. A gas filled proportional counter has been found to be suitable to use.
6.1.4 Filters, or other means of discriminating between sulfur Ka radiation and other X-rays of higher energy.
6.1.5 Signal conditioning and data handling electronics that include the functions of X-ray intensity counting, a minimum of two energy regions (to correct for background X-rays), spectral overlap corrections, and conversion of sulfur X-ray intensity into percent sulfur concentration.
6.1.6 Display or Printer, that reads out in mass % sulfur.
7. Reagents and Materials
7.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
7.2 Di-n-Butyl Sulfide (DBS), a high purity standard with a certified analysis for sulfur content. Use the certified sulfur content when calculating the exact concentrations of the calibration standards (9.1.7). (Warning - Di-n-butyl sulfide is flammable and toxic.)
NOTE 3 - It is essential to know the concentration of the sulfur in the di-n-butyl sulfide, not the purity, since impurities may also be sulfur containing compounds.
7.3 Mineral Oil, White (MOW), ACS reagent grade or less than 2 mg/kg sulfur.
7.4 X-ray Transparent Film, any film that resists attack by the sample, is free of sulfur, and is sufficiently X-ray transparent may be used. Films found to be suitable are polyester, polypropylene, polycarbonate, and polyimide films.
7.4.1 Samples of high aromatic content may dissolve polyester and polycarbonate films. In these cases, other materials besides these films may be used for X-ray windows, provided that they do not contain any elemental impurities. An optional window material is polyimide foil. Although polyimide foil absorbs sulfur X-rays more than other films, it may be a preferred window material as it is much more resistant to chemical attack by aromatics and exhibits higher mechanical strength.
7.5 Sample Cells, resistant to sample attack and meet geometry requirements of spectrometer.
8. Sampling and Specimen Preparation
8.1 Samples shall be taken in accordance with the instructions in Practice D4057 or D4177, where appropriate. Samples should be analyzed immediately after pouring into a sample cell and allowing for the escape of the air bubbles caused by mixing.
8.2 If using reusable sample cells, clean and dry cells before use. Disposable sample cells are not to be reused. A new piece of X-ray film on a reused sample cell is required prior to analyzing the sample. Avoid touching the inside of the sample cell or portion of the window film in the cell or in the instrument window that is exposed to X-rays. Oil from fingerprints can affect the reading when analyzing for low levels of sulfur. Wrinkles in the film will affect the intensity of sulfur X-rays transmitted. Therefore, it is essential that the film be taut and clean to ensure reliable results. The analyzer will need recalibration if the type or thickness of the window film is changed.
8.3 Impurities or thickness variations, which may affect the measurement of low levels of sulfur, have been found in polyester films and may vary from lot to lot. Therefore, the calibration shall be verified after starting each new roll of film.
