ASTM D7212 Standard Test Method for Low Sulfur in Automotive Fuels by Energy-Dispersive X-ray Fluorescence Spectrometry Using a Low-Background Proportional Counter
7. Apparatus
7.1 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:
7.1.1 Source ofX-ray Excitation, X-ray source with titanium target and primary filtration so that excitation is by essentially monochromatic radiation of 4.51 keV.
7.1.2 Low Background Proportional Counter (see 3.1.1), with sensitivity at 2.3 keV.
7.1.3 Multi-channel Analyzer, with a channel width of 10eV or less, covering the energy range from 0 to 10 keV.
7.1.4 Signal Conditioning and Data Handling Electronics, that include the functions of X-ray intensity counting, a minimum of four energy regions (to correct for background X rays and spectral overlap), spectral overlap corrections, and conversions of sulfur X-ray intensity into sulfur concentration.
7.1.5 Display or Printer, that reads out in mg/kg sulfur.
7.1.6 Removable Sample Cell, providing a sample depth of at least 4 mm and equipped with a replaceable X-ray transparent plastic film window.
7.1.7 Helium Purged Optical Path, to maximize sensitivity and minimize spectral overlap from argon in air. The helium shall be at least 99.9 % purity.
8. Reagents and Materials
8.1 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.
8.2 Di-n Butyl Sulfide (DBS), a high purity standard with certified analysis for sulfur content. Use the certified sulfur content when calculating the exact concentrations of the calibration standards. (Warning - Di-n-butyl sulfide is flammable and toxic).
8.3 Mineral Oil, White (MOW), with sulfur content less than 0.2 mg/kg as certified by a suitable analytical method, like Test Method D4045.
8.4 X-ray Transparent Film, any film that resists attack by the sample, is free of sulfur and other interfering elements (see 6.1), and is sufficiently X-ray transparent may be used.
NOTE 2 - Polycarbonate with thickness of 5 or 6 µm most closely matches these requirements and eight of nine participants in the interlaboratory study used one ofthese films, while the other laboratory used 3 µm polyester.
8.4.1 Samples of high aromatic content may dissolve polycarbonate film and polypropylene has a tendency to absorb some hydrocarbons and may stretch during a long measurement time.
NOTE 3 - One laboratory has shown that polycarbonate resists a mixture of 25 mass % isooctane and 75 mass % toluene for 80 min.
8.4.2 In these cases high purity polyester is acceptable, but overall, polycarbonate offers the best combination of spectral purity and resistance to stretching. Other materials may be used for X-ray windows, provided that they do not contain any elemental impurities and match the listed films for X-ray transmission.
8.5 Sample Cells, resistant to sample attack and meet the geometry requirements of spectrometer.
8.6 Calibration Check Samples, portions of one or more automotive fuel standards ofknown sulfur content and not used in the generation of the calibration line. The check samples shall be used to determine the accuracy ofthe initial calibration (see 11.3).
8.7 Quality Control (QC) Samples, preferably portions of one or more automotive fuel materials that are stable and representative of the samples of interest.
