ASTM D7620 Standard Test Method for Determination of Total Sulfur in Liquid Hydrocarbon Based Fuels by Continuous Injection, Air Oxidation and Ultraviolet Fluorescence Detection
4. Summary of Test Method
4.1 A small, very controlled flow of hydrocarbon sample is continuously injected during measurement. It is introduced via a syringe into a high temperature combustion tube containing air where sulfur is oxidized to sulfur dioxide (SO2). Water produced during the sample combustion is removed, as required, and the sample combustion gases are next exposed to ultraviolet (UV) light. The SO2 absorbs the energy from the UV light and is converted to excited sulfur dioxide (SO2*). Fluorescence emitted from the excited SO2* as it returns to a stable state SO2 is detected by a photomultiplier tube and the resulting signal is a measure of the sulfur contained in the sample. (Warning - Exposure to excessive quantities of ultraviolet light is injurious to health. The operator shall avoid exposing their body, especially their eyes, not only to direct UV light but also to secondary or scattered radiation that is present.)
4.2 Fig. 1 illustrates a basic block diagram describing sulfur determination. Sample collection and conditioning, sample introduction, detection system and data handling are depicted.
5. Significance and Use
5.1 Some process catalysts used in refining can be poisoned when trace amounts of sulfur bearing materials are contained in the feedstocks. There are also government regulations as to how much sulfur is permitted to be present in commercial transportation fuels. This test method can be used to determine sulfur in process and downstream distribution streams. It can also be used for purposes of screening and quality control of finished hydrocarbon fuel products.
6. Interferences
6.1 Halogens above 0.35 % (mass/mass) will interfere with accurate sulfur determination.
6.2 Bound nitrogen at concentration greater than 150 mg N/kg can cause a 1 mg S/kg positive bias.
6.3 Excessive moisture produced during the combustion step will interfere if not removed prior to the detector.
