Ultra Low Sulfur Diesel (ULSD) Sulfur Test Method Variability: A Statistical Analysis of Reproducibility from the 2005 US EPA ULSD Round-Robin Test Program

Beginning June 1, 2006, 80% of the highway diesel fuel produced in the United States had to contain 15 ppm sulfur or less. To account for sulfur test method variability, the United States Environmental Protection Agency (US EPA) allowed a 2 ppm compliance margin, meaning that in an EPA enforcement action fuel measuring 17 ppm or less would still be deemed compliant since the true sulfur level could still be 15 ppm. Concern was voiced over the appropriateness of the 2 ppm compliance margin, citing recent American Society for Testing and Materials (ASTM) round-robin and crosscheck test program results that showed sulfur test lab-to-lab variability (reproducibility) on the order of 4 to 5 ppm depending on test method. A test program was implemented in July and August 2005 to determine what reproducibility would be in the post-2006 time frame when ULSD production would begin, testing of sub-15 ppm sulfur levels was commonplace, and EPA would require laboratories to qualify their sulfur test measurement methods. The contribution of calibration curve bias to reproducibility was also investigated. The results of the test program show that lab qualification of sulfur measurement methods, improvement in QA/QC procedures, and improvement in calibration standards has made the 2 ppm compliance margin achievable.