ASTM D7039 Standard Test Method for Sulfur in Gasoline, Diesel Fuel, Jet Fuel, Kerosine, Biodiesel, Biodiesel Blends, and Gasoline-Ethanol Blends by Monochromatic Wavelength Dispersive X-ray Fluorescence Spectrometry
6. Apparatus
6.1 Monochromatic Wavelength Dispersive X-ray Fluorescence (MWDXRF) Spectrometer, equipped for X-ray detection at 0.5373 nm (5.373Å). Any spectrometer of this type can be used if it includes the following features, and the precision and bias of test results are in accordance with the values described in Section 15.
6.1.1 X-ray Source, capable of producing X rays to excite sulfur. X-ray tubes with a power >25W capable of producing Rh Lα, Pd Lα, Ag Lα, Ti Kα, Sc Kα, and Cr Kα radiation are recommended for this purpose.

6.1.2 Incident-beam Monochromator, capable of focusing and selecting a single wavelength of characteristic X rays from the source onto the specimen.

6.1.3 Optical Path, designed to minimize the absorption along the path of the excitation and fluorescent beams using a vacuum or a helium atmosphere. A vacuum of < 2.7 kPa (<20 Torr) is recommended. The calibration and test measurements must be done with identical optical paths, including vacuum or helium pressure.

6.1.4 Fixed-channel Monochromator, suitable for dispersing sulfur Kα X rays.

6.1.5 Detector, designed for efficient detection of sulfur Kα X rays.

6.1.6 Single-Channel Analyzer, an energy discriminator to monitor only sulfur radiation.

6.1.7 Removable Sample Cell, an open-ended specimen holder compatible with the geometry of the MWDXRF spectrometer and designed to use replaceable X-ray transparent film (see 6.1.8) to hold a liquid specimen with a minimum depth of 5 mm. The sample cell must not leak when fitted with X-ray transparent film. A disposable cell is recommended.

6.1.8 X-Ray Transparent Film, for containing and supporting the test specimen in the sample cell (see 6.1.7) while providing a low-absorption window for X rays to pass to and from the sample. Any film resistant to chemical attack by the sample, free of sulfur, and X-ray transparent can be used, for example, polyester, polypropylene, polycarbonate, and polyimide. However, samples of high aromatic content can dissolve polyester and polycarbonate films.

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 Calibration-Check Samples, for verifying the accuracy of a calibration. The check samples shall have known sulfur content and not be used in determining the calibration curve. A standard from the same reliable and consistent source of calibration standards used to determine the calibration curve is convenient to check the calibration.

7.3 Di-n-butyl Sulfide, a high-purity liquid with a certified sulfur concentration. Use the certified sulfur concentration when calculating the exact concentrations of sulfur in calibration standards. (Warning - Di-n-butyl sulfide is flammable and toxic. Prepared solutions may not be stable several months after preparation.)

NOTE 2 - It is essential to know the concentration of sulfur in the di-n-butyl sulfide, not only the purity, since impurities can 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.

7.4 Drift-Monitor Sample (Optional), to determine and correct instrument drift over time (see 10.4, 11.1, and 12.1). Various forms of stable sulfur-containing materials are suitable drift-correction samples, for example, liquid petroleum, solid, pressed powder, metal alloy, and fused glass. The count rate displayed by the monitor sample, in combination with a convenient count time (T), shall be sufficient to give a relative standard deviation (RSD) of < 1 % (see Appendix X1).

NOTE 3 - Calibration standards may be used as drift-monitor samples. Because it is desirable to discard test specimens after each determination, a lower cost material is suggested for daily use. Any stable material can be used for daily monitoring of drift.

NOTE 4 - The effect of drift correction on the precision and bias of this test method has not been studied.

7.4.1 Drift correction can be done automatically if the instrument embodies this option, although the calculation can be readily done by conventional methods of data reduction and processing.

7.5 Quality-Control (QC) Samples, for use in establishing and monitoring the stability and precision of an analytical measurement system (see Section 14). Use homogeneous materials, similar to samples of interest and available in sufficient quantity to be analyzed regularly for a long period of time.

NOTE 5 - Verification of system control through the use of QC samples and control charting is highly recommended.

NOTE 6 - Suitable QC samples can be prepared by combining retains of typical samples.

7.6 White Oil, use a high purity mineral oil and account for its sulfur content when calculating the sulfur concentrations of the calibration standards.

7.7 Helium, minimum purity 99.9 %, for use as an optical path.

7.8 Ethanol, use a high purity grade and account for its sulfur content when calculating the sulfur concentrations of the calibration standards. (Warning - Ethanol is flammable and harmful if swallowed or inhaled. It is an eye irritant and may cause skin irritation.)

7.9 2,2,4-Trimethylpentane (Isooctane), use a high purity grade and account for its sulfur content when calculating the sulfur concentration of the calibration standards. (Warning - Isooctane is flammable and harmful if swallowed or inhaled. It is an eye irritant and may cause skin irritation.)

7.10 Toluene, use a high purity grade and account for its sulfur content when calculating the sulfur concentration of the calibration standards. (Warning - Toluene is flammable and harmful if swallowed or inhaled. It is an eye irritant and may cause skin irritation.)

7.11 Polysulfide Oil, generally nonylpolysulfides containing a known percentage of sulfur diluted in a hydrocarbon matrix. (Warning - May cause allergic skin reactions.)

NOTE 7 - Polysulfide oils are high molecular weight oils that contain high concentrations of sulfur, as high as 50 weight percent.