ASTM D2622 for sulfur in petroleum products
ASTM D2622 standard test method for sulfur in petroleum products by wavelength dispersive X-ray fluorescence spectrometry
6. Apparatus
6.1 Wavelength Dispersive X-Ray Fluorescence Spectrometer (WDXRF), equipped for X-ray detection in the wavelength range from about 0.52 nm to about 0.55 nm (specifically at 0.537 nm). For optimum sensitivity to sulfur, the instrument should be equipped with the following items:
6.1.1 Optical Path, vendor specified, helium preferred, ambient air or nitrogen are inferior.
6.1.2 Pulse-Height Analyzer, or other means of energy discrimination.
6.1.3 Detector, for the detection of X-rays with wavelengths in the range of interest (from about 0.52 nm to about 0.55 nm).
6.1.4 Analyzing Crystal, suitable for the dispersion of sulfur Ka and background X-rays within the angular range of the spectrometer employed. Germanium or pentaerythritol (PET) are generally found to be acceptable. Other crystals may be used, consult with the instrument vendor.
6.1.5 X-ray Tube, capable of exciting sulfur Ka radiation. Tubes with anodes of rhodium, chromium, and scandium are most popular although other anodes can be used.
NOTE 3 - Exposure to excessive quantities of high energy radiation such as those produced by X-ray spectrometers is injurious to health. The operator needs to take appropriate actions to avoid exposing any part of their 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.
6.2 Analytical Balance, capable of weighing to the nearest 0.1 mg and up to 100 g.
7. Reagents
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 material with a certified analysis for sulfur content. Use the certified sulfur content and the material purity when calculating the exact concentrations of the calibration standards (see 9.1). (Warning - Di-n-butyl sulfide is flammable and toxic. Prepared solutions may not be stable several months after preparation.)
NOTE 4 - It is essential to know the concentration of sulfur in the di-n-butyl sulfide, not only the purity, since impurities may also be sulfur containing compounds. The sulfur content may be determined via mass dilution in sulfur-free white oil followed by a direct comparison analysis against NIST (or other primary standards body) reference materials.
7.3 Drift Correction Monitor(s) (Optional), Several different materials have been found to be suitable for use as drift correction monitors. Appropriate drift monitor samples should be permanent materials that are stable with respect to repeated exposure to X-rays. Stable liquids like polysulfide oils, glass or metallic specimens are recommended. Liquids, pressed powders, and solid materials that degrade with repeated exposure to X-rays should not be used. Examples of sulfur containing materials that have been found to be suitable include a renewable liquid petroleum material, a metal alloy, or a fused glass disk. The monitor's counting rate, in combination with count time, shall be sufficient to give a relative counting error of less than 1 %. The counting rate for the monitor sample is determined during calibration (see 9.4) and again at the time of analysis (see 10.1). These counting rates are used to calculate a drift correction factor (see 11.1).
7.3.1 Drift correction is usually implemented automatically in software, although the calculation can readily be done manually. For X-ray instruments that are highly stable, the magnitude of the drift correction factor may not differ significantly from unity.
7.4 Polysulfide Oil, generally nonyl polysulfides containing a known percentage of sulfur diluted in a hydrocarbon matrix. (Warning - May cause allergic skin reactions.)
NOTE 5 - Polysulfide oils are high molecular weight oils that contain high concentrations of sulfur, as high as 50 weight percent. They exhibit excellent physical properties such as low viscosity, low volatility, and durable shelf life while being completely miscible in white oil. Polysulfide oils are readily available commercially. The sulfur content of the polysulfide oil concentrate is determined via mass dilution in sulfur-free white oil followed by a direct comparison analysis against NIST (or other primary standards body) reference materials.
7.5 Mineral Oil, White (MOW), ACS Reagent Grade containing less than 2 mg/kg sulfur or other suitable base material containing less than 2 mg/kg sulfur. When low level (<200 mg/kg) measurements are anticipated, then the sulfur content, if any, of the base material needs to be included in the calculation of calibration standard concentration (see 9.1). When the sulfur content of the solvent or reagent is not certified, verify the absence of sulfur. Use the purest grades for the preparation of calibration standards. It is also important to measure the C/H ratio (see Section 12 and Fig. 1).
7.6 X-ray Transparent Film - Any film that resists attack by the sample, is free of sulfur, and is sufficiently X-ray transparent can be used. Film types can include polyester (for example, Mylart), polypropylene, polycarbonate, and polyimide. However, samples of high aromatic content can dissolve polypropylene and polycarbonate films.
7.7 Helium Gas, minimum purity 99.9 %.
7.8 Counting Gas, for instruments equipped with flow proportional counters. The purity of the counting gas should be in agreement with the specification provided by the instrument manufacturer.
7.9 Sample Cells, compatible with the sample and the geometry requirements of the spectrometer. Disposable cells are preferred over reusable ones for ultra low (<50 mg/kg) sulfur levels.
7.10 Calibration Check Samples, portions of one or more liquid petroleum or product standards of known or certified sulfur content (including polysulfide oils, di-n-butyl sulfide, thiophenes, etc.) and not used in the generation of the calibration curve. The check samples shall be used to determine the precision and accuracy of the initial calibration (see 9.5).
7.11 Quality Control Samples, stable petroleum or product samples or solids representative of the samples of interest that are run on a regular basis to verify that the system is in statistical control (see Section 13).
NOTE 6 - Verification of system control through the use of QC samples and control charting is highly recommended. It is recognized that QC procedures are the province of the individual laboratory.
NOTE 7 - Suitable QC samples can often be prepared by combining retains of typical samples if they are stable. For monitors, solid materials are recommended. QC samples must be stable over long periods.
