ASTM D7620 Standard Test Method for Determination of Total Sulfur in Liquid Hydrocarbon Based Fuels by Continuous Injection, Air Oxidation and Ultraviolet Fluorescence Detection
7. Apparatus
7.1 Furnace - An electric furnace held at a temperature sufficient to pyrolyze the entire sample (typically 1050 °C +/- 25 °C) and oxidize sulfur to SO2.
7.2 Combustion Tube - A quartz combustion tube constructed to allow the direct injection of a continuous flow of sample into the heated oxidation zone of the furnace. The oxidation section shall be large enough to ensure complete combustion of the sample. Fig. 2 illustrates a typical combustion tube (Note 2).
NOTE 2 - Other combustion tube configurations are acceptable if precision and accuracy are not degraded.
7.3 Flow Control - The apparatus shall be equipped with suitable flow control apparatus capable of maintaining a constant supply of air.
7.4 Drier Tube - The apparatus shall be equipped with a mechanism for the removal of excessive water vapor. The oxidation reaction produces water vapor which must be eliminated prior to measurement by the detector. This may be accomplished with a membrane drying tube, or a permeation dryer, that utilizes a selective capillary action for water removal.
7.5 UV Fluorescence Detector - A quantitative detector capable of measuring the energy emitted from the fluorescence of sulfur dioxide by UV light.
7.6 Millilitre Syringe - A disposable 1 mL syringe capable of accurately delivering a controlled and constant flow of calibration and sample materials. The syringe shall accommodate a disposable tip to aid the filling of the syringe and a disposable septum seal to accommodate penetration and sample flow.
7.7 Sample Inlet System - An automatic sample injection device that is compatible with a disposable 1 mL syringe is required. The injector shall allow the introduction of an appropriate continuous flow of sample into a combustion tube carrier stream, which directs the sample into the oxidation zone at a controlled and repeatable rate.
7.8 Strip Chart Recorder - Equivalent electronic data logger, integrator or recorder (optional).
7.9 Balance - With a precision of +/- 0.01 mg (optional).
8. Reagents and Materials
8.1 Purity of Reagents - Reagent grade chemicals shall be used in tests. Unless otherwise indicated, it is intended that all reagents shall 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 Air - Filtered (with a 2 µm filter).
8.3 Iso-octane, Toluene, Xylenes - Reagent grade. (Warning - Organic solvents are flammable.)
8.4 Thiophene - FW84.14, Sulfur content 38.10 % (m/m).
8.4.1 Other sources of sulfur and diluent materials may be used if precision and accuracy are not degraded.
8.4.2 Apply the appropriate correction for chemical impurity.
8.5 Sulfur Stock Solution - 1000 µg S/mL. Prepare a stock solution by accurately weighing 0.2624 g +/- 0.013 g of thiophene into a tared 100 mL volumetric flask. Dilute to volume with selected solvent. This stock may be further diluted to desired sulfur concentration.
8.5.1 Working standards should be reblended on a regular basis depending upon frequency of use and age.
NOTE 3 - Typically, stock solutions have a useful life of about 3 months.
8.6 Quality Control (QC) Samples - Preferably, these are portions of one or more hydrocarbon materials that are stable and representative of the samples of interest. These QC samples may be used to check the validity of the testing process as described in Section 16.
9. Hazards
9.1 Consult current OSHA regulations, suppliers' Material Safety Data Sheets, and local regulations for all materials used in this test method.
9.2 High temperature is employed in this test method. Exercise extra care when using flammable materials near the oxidative pyrolysis furnace.
9.3 Due to the types of samples analyzed in this test method, chemical resistant gloves should be worn when performing this test method.
