ASTM D7220 standard test method for sulfur in automotive fuels
ASTM D7220 Standard Test Method for Sulfur in Automotive Fuels by Polarization X-ray Fluorescence Spectrometry
7. Apparatus
7.1 Polarization X-ray Fluorescence Analyzer - A Polarized Excitation Energy Dispersive XRF spectrometer may be used if its design incorporates as a minimum, the following features:
7.1.1 Source of X-ray Excitation, X-ray end window tube with Pd or Rh anode and Beryllium window, in combination with polarizing X-ray optics. The polarizer must polarize Pd or Rh La radiation.
7.1.2 Sample Cell, providing a sample depth of at least 4 mm and equipped with replaceable X-ray transparent film window.
7.1.3 X-ray Detector, with a resolution value not to exceed 175 eV at 5.9 keV (10.000cps). A Si drift chamber has been found suitable for use. Using a detection system with this minimum spectral resolution has been shown to eliminate the potential effect of interference from chlorine on sulfur should either salt contamination, or chlorine from other sources (for example, recycled vegetable oils) occur.
7.1.4 He-flush, the system must allow flushing of the optical path with helium (see 8.6).
7.1.5 Signal Conditioning and Data Handling Electronics, including the functions of X-ray intensity counting, spectra handling by background subtraction and deconvolution, calculation of overlap corrections and conversion of sulfur X-ray intensity into mg/kg sulfur concentration.
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 - 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 (see 11.1). (Warning - Di-n-butyl sulfide is both flammable and toxic.)
NOTE 1 - It is essential to know the concentration of sulfur in the di-n-butyl sulfide, not the purity, since impurities may also be sulfur containing compounds.
8.3 Drift Correction Monitor(s) (Optional) - Several different materials have been found to be suitable for use as drift correction monitors. Examples of sulfur containing materials that have been found to be suitable include a renewable liquid petroleum material or a fused glass disk. The monitor's count rate, in combination with count time, shall be sufficient to give a relative counting error of less than 1 %. The count rate for the monitor sample is determined during calibration (see 11.4) and again at the time of analysis (see 12.1). These counting rates are used to calculate a drift correction factor (see 13.1).
8.3.1 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.
8.4 White Oil (Light Paraffin Oil) or Matrix-Matching Blank Sample, with a certified content of less than 0.2 mg/kg sulfur. If only one matrix is to be analyzed (for example, diesel) accuracy of results may be improved by using a matrix matched diluent. In these cases, the matrix matched diluent should match approximately the C/H ratio and oxygen content of the material to be analyzed.
8.5 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, with film thickness of between 2 to 6 µm. Films can include polyester, polypropylene, polycarbonate, and polyimide. Typically polycarbonate with a thickness of 5 to 6 µm is used. However, samples of high aromatic content can dissolve polyester and polycarbonate films. It is important that samples, standards, and blanks be measured using the same batch of film to avoid bias.
8.6 Helium Gas, minimum purity 99.9 %.
8.7 Sample Cells, compatible with the sample and the geometry requirements of the spectrometer. Disposable cells are recommended.
8.8 Calibration Check Samples, portions of one or more liquid petroleum or product standards of known sulfur content and not used in the generation of the calibration curve. The check samples shall be used to determine the accuracy of the initial calibration (see 11.5).
8.9 Quality Control Samples, stable petroleum or product samples representative of the samples of interest that are run on a regular basis to verify that the system is in statistical control.
NOTE 2 - 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. Suitable QC samples can often be prepared by combining retains of typical samples.
9. Sampling, Test Specimens, and Test Units
9.1 Samples shall be taken in accordance with the instructions in Practices D4057 or D4177 when applicable.
9.2 When reusable sample cells are used, clean and dry cells before each use. Disposable sample cells shall not be reused. For each sample, an unused piece of X-ray film is required for the sample cell. Avoid touching the inside of the sample cell, the portion of the window film in the cell, or the instrument window that is exposed to X-rays. Oil from fingerprints can affect the reading when determining low levels of sulfur. Wrinkles in the film will affect the intensity of the sulfur X-rays transmitted. Therefore, it is essential that the film be taut and clean to ensure reliable results. When handling the window film, avoid touching the central part (the part that actually forms the optical window) as this can lead to contamination from sweat, grease, or other petrochemical products. Similarly, discard any film that has been exposed to the atmosphere (for example, hanging outside of the film roll dispensing box). Also, when opening a new roll of film, discard the first metre, since some films are packaged in plastic bags that contain sulfur. The analyzer may need recalibration if the type or thickness of the window film is changed. After the sample cell is filled, make a small vent hole in the lid of the sample cell. Place the sample in the cell using techniques consistent with good practice for the particular instrument being used. Although sulfur radiation will emerge from only a small distance into the sample, scatter from the sample cell and the sample can vary. Laboratory personnel shall ensure that the sample cell is filled above a minimum depth, beyond which additional sample does not significantly affect the count rate. Generally, fill the sample cell to a minimum of three-fourths of the cell's capacity.
9.3 If the instrument is equipped with a replaceable secondary/safety window, it must be replaced with every sample to ensure there is no cross-contamination. When changing the window, follow the precautions given in 9.2.
9.4 Impurities or thickness variations, which may affect the determination of low levels of sulfur, have been found in polyester films and may vary from lot to lot. Therefore, the test method shall be verified after starting each new roll or batch of film.
9.5 When connecting a new helium gas cylinder, always run a blank measurement to ensure the helium gas line is purged of air.
