ASTM D4420
Understanding ASTM D4420: Determination of Aromatics in Finished Gasoline by Gas Chromatography
Overview
The ASTM D4420 standard outlines a method for quantifying benzene, toluene, C8, C9 and heavier aromatics, along with total aromatics in finished motor gasoline and gasoline blending components. This method leverages gas chromatography (GC) for precise measurement within specified concentration ranges. Notably, this method's applicability to gasoline containing oxygenates, such as ethers and alcohols, remains undetermined.
Scope and Application
The method is designed to quantify the concentrations of benzene (0.1 to 5 vol%), toluene (1 to 20 vol%), C8 aromatics (3 to 25 vol%), C9 and heavier aromatics (5 to 30 vol%), and total aromatics (10 to 80 vol%) in finished motor gasoline. It also applies to gasoline blending components. This protocol offers an alternative to D 3606 for benzene and toluene analysis alone.
Key Components and Procedure
The technique involves a dual-column chromatographic system with a thermal conductivity detector. The analysis begins with injecting a measured sample volume into a column with a polar liquid phase. Non-aromatics are vented, while aromatics proceed to a second column with a nonpolar phase, separating based on boiling points. Quantitative results derive from comparing recorded peak areas to known standards.
Apparatus
Chromatograph: Equipped with a vaporizing injector, backflush system, and a thermal conductivity detector.
Columns: Two, one polar for separating non-aromatics and another nonpolar for aromatics.
Recorder and Integrator: For recording and analyzing data.
Reagents and Calibration
High-purity reagents, including helium, benzene, toluene, xylenes, and isooctane, are used. A calibration mixture of known aromatic concentrations in isooctane sets the benchmark for sample analysis.
Safety
Due to the flammable and toxic nature of many chemicals involved, adherence to safety guidelines is paramount. The standard specifies precautionary statements for handling these substances.
Test Method overview
Sampling
Samples should be collected and handled as per ASTM D4057, ensuring minimal exposure to air to prevent evaporation of light ends.
Calibration
Calibration involves preparing a blend of known aromatic content in isooctane. This mixture is chromatographed, and the response factors for benzene, toluene, C8, and C9+ aromatics are calculated based on peak areas.
Chromatographic Analysis
The sample is chromatographed under the same conditions as the calibration mixture. The areas under the peaks for benzene, toluene, C8, and C9+ aromatics are measured, and their concentrations are calculated using the previously determined response factors.
Reporting
Results are expressed as liquid volume percent to the nearest 0.1%, detailing the concentrations of benzene, toluene, C8 aromatics, C9+ aromatics, and total aromatics.
Precision and Bias
The precision of the test method has been established through interlaboratory testing, yielding specific repeatability and reproducibility values for each aromatic component. However, the method's bias cannot be determined due to the absence of a reference material with known aromatic concentrations. The calibration process aims to mitigate instrumental bias, though matrix effects could introduce variability.
Conclusion
ASTM D4420 provides a rigorous framework for analyzing aromatics in gasoline, crucial for quality control and assessing potential health impacts. Despite its precision, the method's applicability to oxygenated gasolines requires further investigation, and matrix effects may influence accuracy. Nonetheless, this method remains a valuable tool for the petroleum industry, enabling the detailed characterization of gasoline compositions.