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D5186-24 – Standard Test Method Technical Guide
Supercritical Fluid Chromatography (SFC) provides a detailed characterization of hydrocarbon types in middle distillates, specifically targeting aromatic compounds. This test method, designated D5186-24, standardizes the quantification of monoaromatic and polynuclear aromatic hydrocarbons (PAH) in motor diesel fuels and blend stocks, supporting compliance efforts, product development, and process optimization.
📐 Scope and Concentration Limits
This standard is specifically applicable to motor diesel fuels and blend stocks. The validated concentration ranges are tightly defined to ensure analytical integrity: total aromatic content must fall between 1 % and 75 % by mass, while polynuclear aromatic hydrocarbon content must be between 0.5 % and 50 % by mass. The method includes relative bias correlations to Test Methods D1319 (Fluorescent Indicator Adsorption) and D6591 (HPLC with RI Detection), but these correlations are strictly limited to these concentration ranges and to fuels free of biodiesel.
⚠️ Critical Restriction — Biodiesel Interference: This method and its associated bias correlations were developed exclusively for diesel fuels that do not contain biodiesel. The presence of biodiesel causes severe interference with the SFC analysis and invalidates the provided correlation equations.
🔬 Terminology and Method Principle
D5186-24 clearly defines the target analytes based on molecular structure. Mononuclear aromatic hydrocarbons contain exactly one aromatic ring (e.g., benzene, alkyl-benzenes, indans, tetralins). Polynuclear aromatic hydrocarbons contain two or more rings, which may be fused (naphthalene, phenanthrene) or separate (biphenyl). The method operates by exploiting the unique solvating power of a mobile phase above its critical temperature and critical pressure, allowing efficient group-type separation of the aromatic classes from the saturates matrix.
The standard also designates SI units as the standard for all results, with inch-pound units provided solely for informational context.
📊 Performance and Quality Assurance Metrics
The reliability of the test method is supported by referencing established statistical quality practices. The relative bias studies comparing D5186 to D1319 and D6591 to D5186 are conducted in accordance with Practice D6708. Ongoing laboratory quality control relies on Practice D6299.
| 🟦 Analyte Class | 📏 Applicable Range (% by mass) |
|---|---|
| Total Aromatic Content (Monoaromatic + PAH) | 1 % – 75 % |
| Polynuclear Aromatic (PAH) Content | 0.5 % – 50 % |
| 📐 Reference Standard | 🎯 Analytical Technique | ⚡ Role in Standard |
|---|---|---|
| D1319 | Fluorescent Indicator Adsorption (FIA) | Relative bias comparison for total aromatics |
| D6591 | HPLC with Refractive Index Detection | Relative bias comparison for PAH content |
| D6708 | Statistical Assessment of Bias | Framework for correlation equations |
✅ Standard Unit System: Per Section 1.4 of the standard, the values stated in SI units are the regulatory standard for this test method.
❓ Frequently Asked Questions
🔍 What types of samples are covered by D5186-24?
The standard covers motor diesel fuels and blend stocks. It is designed to measure total aromatic and polynuclear aromatic hydrocarbon content using Supercritical Fluid Chromatography.
💡 What are the upper and lower limits for aromatic content measurement?
Total aromatic content must be between 1 % and 75 % by mass. Polynuclear aromatic (PAH) content must be between 0.5 % and 50 % by mass for the method to be fully applicable.
⚡ How does biodiesel impact the results obtained by this method?
The standard explicitly warns that biodiesel interferes with the analysis. The correlation equations provided for comparing results with D1319 and D6591 are valid only for fuels that do not contain biodiesel.
📌 Which related ASTM standards support the statistical validity of D5186?
Practice D6708 is used to assess the expected agreement (relative bias) between D5186 and other methods. Practice D6299 provides the framework for statistical quality assurance in the analytical laboratory.