ASTM D6445 Test Method for Sulfur in Gasoline
ASTM D6445 Standard Test Method for Sulfur in Gasoline by Energy-Dispersive X-ray Fluorescence Spectrometry
5. Apparatus
5.1 Energy-dispersive X-ray Fluorescence Analyzer - The analyzer needs to have sufficient sensitivity to measure the concentration of sulfur at 500 mg/kg with a one standard deviation value due to counting statistics no greater than 10 mg/kg under optimized conditions. Any energy dispersive X-ray fluorescence analyzer may be used if its design incorporates, as a minimum, the following features:
5.1.1 Source of X-ray Excitation - X-ray tube with energy above 2.5 keV.
NOTE 1 - Operation of analyzers using X-ray tubes is to be conducted in accordance with the manufacturer’s safety instructions and federal state and local regulations.
5.1.2 Sample Cell, providing a sample depth of at least 4 mm and equipped with replaceable X-ray transparent film window.
5.1.3 X-ray Detector, with a resolution value not to exceed 800 eV at 2.3 keV. A gas filled proportional counter has been found suitable to use.
5.1.4 Filters, or other means of discriminating between sulfur Ka radiation and other X rays.
5.1.5 Signal conditioning and data handling electronics that include the functions of X-ray intensity counting, spectral overlap corrections, and conversion of sulfur X-ray intensity into mg/kg sulfur concentration. It is also imperative that the instrument has the capability to monitor counts for at least one energy region distinct from the sulfur region to allow compensation for variations in spectral background (that is, calculation of net intensities).
5.1.6 Display or Printer, that reads or prints out in mg/kg or masspercent sulfur.
6. Matrix Effects
6.1 Matrix effects refer to changes in measured intensity of sulfur caused by concentration variations of the elements in a sample. These variations directly influence X-ray absorption and change the measured intensity of each element. For example, performance enhancing additives, such as oxygenates in gasoline, can affect the apparent sulfur reading. These types of interferences are always present in X-ray fluorescence analysis and are completely unrelated to spectral interferences.
6.2 Many modern instruments have the capability to correct for matrix effects by ratioing measured sulfur intensities to that of X-ray radiation scattered from the sample (for example, scattered X-ray tube lines). This can be an effective method for compensating for matrix differences between samples and standards, although it can result in some degradation of the measurement precision. It is the user's responsibility, however, to ensure that the matrix corrections applied are accurate. It is recommended that these are checked by analyzing standard reference materials and that the software corrections offered by the manufacturer not be accepted at face value. In addition, corrections should be verified for new formulations.
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. The concentration should be known to at least three significant figures or nearest 1 mg/kg, whichever is higher.
7.2 Di-n-Butyl Sulfide (DBS), a high purity standard, minimum 96 % purity, with a certified analysis for sulfur content. Use the certified sulfur content when calculating the exact concentrations of the calibration standards (see 10.1).
NOTE 2 - Warning: Di-n-butyl sulfide is flammable and toxic.
7.3 Thiophene, sulfur content 37.72 mass %, 99 % purity.
7.4 2-Methylthiophene, 32.00 % sulfur, 98 % purity.
NOTE 3 - Purity on the label for di-n-butyl sulfide, thiophene, and 2-methylthiophene is only a nominal value. It is essential to know the concentration of sulfur in the sulfur standard, not the purity, since impurities may also be sulfur containing compounds.
7.5 Isooctane (2,2,4-trimethylpentane), with a certified analysis for sulfur content or checked by Test Method D3120 or equivalent test method as containing less than 3 mg/kg sulfur.
7.6 Toluene, with a certified analysis for sulfur content or checked by Test Method D3120 or equivalent test method as containing less than 3 mg/kg sulfur.
7.7 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. Typical film thicknesses range from 1.5 to 8 µm. Film thickness will affect the transmission of X rays and the films resistance to chemical attack.
7.7.1 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 film. While polyimide film 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.
7.8 Sample Cells, resistant to sample attack and meeting geometry requirements of spectrometer. Disposable cells are preferred.
