ASTM D6334 Test Method for Sulfur in Gasoline by Wavelength Dispersive X-Ray Fluorescence
6. Apparatus
6.1 Wavelength Dispersive X-Ray Fluorescence Spectrometer (WDXRF), equipped for soft ray detection in the 5.37 Å range. For optimum sensitivity to sulfur, equip the instrument with the following:
6.1.2 Pulse-Height Analyzer, or other means of energy discrimination.
6.1.3 Detector, designed for the detection of long wavelength X-rays.
6.1.4 Analyzing Crystal, suitable for the dispersion of sulfur Ka X-rays within the angular range of the spectrometer employed. Pentaerythritol and germanium are the most popular, although materials, such as EDDT, ADP, graphite, and quartz, may be used.
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 may be suitable.
7. Reagents
7.1 Di-n-Butyl Sulfide (MW-146.30), a high-purity grade standard with a certified sulfur analysis.
7.2 Thiophene MW-84.14), a high-purity (98+ %) grade standard with a certified sulfur analysis.
7.3 2-Methylthiophene MW-98.17), a high purity (98+ %) grade standard with a standard sulfur analysis.
7.4 2,2,4-Trimethylpentane, (Isooctane), reagent grade, MW-114.23.
7.5 Methylbenzene, (Toluene), reagent grade, MW-92.14.
7.6 Drift Correction Monitor(s), (Optional), several different material have been found to be suitable for use as drift correction monitors. Examples of sulfur containing materials that meet these requirements are renewable liquid petroleum materials, semipermanent solids, pressed powder pellets, metal alloys, or fused glass disks. Bracket the calibration range with concentrations of monitor samples. The counting rate for each monitor is determined during calibration (see 9.7) and again at the time of analysis (see 10.1). These counting rates are used to calculate a drift correction factor (see 11.1).
7.7 Calibration Check Standards, one or more liquid petroleum or product standards of known sulfur content (which do not represent one of the samples prepared in Section 9) are used to verify the accuracy of the calibration curve.
7.8 Quality Control (QC) Sample, one or more stable liquid petroleum or product samples, which are used to verify that the measurement system is in control. Preferably the QC sample(s) should be representative of the samples typically analyzed. In cases where volatility of the QC sample(s) may affect the sample integrity, precautions need to be taken to minimize or eliminate sample losses prior to analysis to ensure that a stable and representative sample can be taken and analyzed over the period of intended use. It is permissible to use calibration standards for this purpose. Since standard samples are discarded after each determination, it is recommended that a lower cost material be used for daily calibration checks.
