Advancing Sulfur Detection with Sindie R4: Precision Redefined for Gasoline and Diesel Analysis

Leslie McHenry

Introduction

Experiment

The following setup conditions and procedure were used for the internal pilot study. 

Setup:

• 3 operators
• 3 Sindie R4 analyzers
• 3 sample sets each containing 7 gasoline and 7 diesel samples with:
• 0.08, 0.125, 0.25, 0.5, 1, 2, 3 ppm nominal sulfur concentrations
• The pilot study was performed at one location

Procedure:

• Each operator calibrated their analyzer according to ASTM D7039 using six sulfur in mineral oil standards in the range of 0-500 ppm. See Figure 1 for calibration data from operator 1.
• A minimum of ten aliquots of each pilot study sample were measured under repeatability conditions by each operator.
• Diesel samples were reported as measured.
• Gasoline samples were matrix corrected using correction factors from ASTM D7039 Table 3 (correction factor = 0.9895 for 0% ethanol in gasoline measured on a mineral oil calibration).

Figure 1. Operator 1 Calibration Data

Results

Table 1 shows the reported data for the 1 ppm sulfur in diesel and gasoline samples (additional data for other concentrations is available upon request). See Tables 2 and 3 for a summary of the pilot study statistics (in blue) as it compares to the current ASTM D7039 method (in gray).

Although the repeatability criterion was met, as this study was performed at one location, it did not meet ASTM PLOQ and reproducibility criteria. Therefore, instead of a single PLOQ, the output is multiple LLOQs. Additionally, the term intermediate precision is used instead of reproducibility (though calculated in the same manner). See the callout box to understand the differences in how these terms are used.

Table 1. Reported Data for 1 ppm Sulfur in Diesel and Gasoline Samples

*GESD (Generalized Extreme Studentized Deviate) outlier (ASTM Practice D7915) - not included in final statistics

Table 2. Pilot LLOQ vs. Current ASTM D7039 Method PLOQ

*Estimates are not significantly different using Hartley’s Test using 5 degrees of freedom for each data set 

Table 3. Pilot Precision vs. Current ASTM D7039 Precision

*Using a factor of 0.7, the projected reproducibility can be estimated from the pilot data 

Product Highlight: Sindie R4

Analyze Sulfur with Unmatched Limits of Detection

Easier to use than ever, Sindie R4 is our most advanced sulfur analytical solution for compliance with ASTM D2622, ASTM D7039, and ISO 20884 methods, enabling complete flexibility for your analytical needs. Advanced R4 optics provide extremely low limits of detection and allow for cycle time flexibility to save hours per day in testing time.

 

Discover Sindie R4

Discussion

As seen in Table 1 for the 1 ppm samples, data was consistent with average concentrations ranging from 0.95 to 1.12 ppm and relative standard deviations in the 5-8% range. This is important, as it illustrates measurement capability below that of the current method scope of 3.2 ppm.

Additionally, regarding pilot statistics vs. the existing method, there is a conclusion that can be drawn despite the terminologies not matching up. For example, it is to be expected that an individual laboratory LLOQ will be less than a method PLOQ, but the question is, can it be demonstrated over multiple operators that it is significantly less than the method PLOQ? Yes, all three operators demonstrated a ≤1 ppm LLOQ for both sulfur in diesel and gasoline samples; therefore, it should be possible to lower the method PLOQ to 1 ppm for this apparatus, though a full Interlaboratory Study (ILS) will be needed to confirm this.

Precision estimates of the pilot study were non-inferior to current methodology precision. The term non-inferior is carefully chosen instead of the more optimistic better as differences in how they were obtained do matter. Even repeatability must be cautiously compared, as the pilot was over a reduced range, whereas method repeatability was assessed over a larger range starting at a higher concentration. Intermediate precision was calculated the same way as reproducibility, but because the study was performed in one location (due to it being pre-product release), it is no surprise that the intermediate precision is better than reproducibility. However, using a factor of 0.7, the projected reproducibility was estimated to also be non-inferior to method reproducibility. Results are promising, though a full ILS will be needed for a more definitive precision determination of the new apparatus.

PLOQ and precision results of this new ASTM D7039 study will also be of interest to ASTM D2622 and ISO 20884 users. This is because Sindie R4 is also compliant with ASTM D2622 and ISO 20884, and if improved performance is demonstrated for one method, it can be inferred that this analyzer will outperform existing D2622/ISO 20884 compliant Sindie analyzers as well.

As a result of the presentation to the ASTM XRF Task Group of Subcommittee 3 on Elemental Analysis within Committee D02 for Petroleum Products, Liquid Fuels, and Lubricants, permission was given by the subcommittee chair to open a work item (WK 93373) and register an interlaboratory study (ILS 1986) to estimate precision and lower the scope of the D7039 test method for the new apparatus.

The ILS will span the existing scope of D7039 materials and is expected to contain 28 samples (in blind duplicate) in the nominal range of 0.08-3000 ppm sulfur. Samples will be split into a gasoline set and a distillate set, and participants will be expected to correct for matrix differences, especially oxygen content. Timing will depend on Sindie R4 user adoption, but the ILS is expected to be performed in late 2025/early 2026. If you'd like to learn more about this study or Sindie R4, reach out to one of our experts.

About the Author