ASTM D6550 Test Method for Determination of Olefin Content of Gasolines
ASTM D6550 Standard Test Method for Determination of Olefin Content of Gasolines by Supercritical-Fluid Chromatography
4. Summary of Test Method
4.1 A small aliquot of the fuel sample is injected onto a set of two chromatographic columns connected in series and transported using supercritical carbon dioxide (CO2) as the mobile phase. The first column is packed with high-surface-area silica particles. The second column contains either high-surface-area silica particles loaded with silver ions or strong-cation-exchange material loaded with silver ions.
4.2 Two switching valves are used to direct the different classes of components through the chromatographic system to the detector. In a forward-flow mode, saturates (normal and branched alkanes, cyclic alkanes) pass through both columns to the detector, while the olefins are trapped on the silver-loaded column and the aromatics and oxygenates are retained on the silica column. Aromatic compounds and oxygenates are subsequently eluted from the silica column to the detector in a back-flush mode. Finally, the olefins are back-flushed from the silver-loaded column to the detector.
4.3 A flame-ionization detector (FID) is used for quantitation. Calibration is based on the area of the chromatographic signal for olefins, relative to standard reference materials, which contain a known mass % of total olefins as corrected for density.
5. Significance and Use
5.1 Gasoline-range olefinic hydrocarbons have been demonstrated to contribute to photochemical reactions in the atmosphere, which result in the formation of photochemical smog in susceptible urban areas.
5.2 The California Air Resources Board (CARB) has specified a maximum allowable limit of total olefins in motor gasoline. This necessitates an appropriate analytical test method for determination of total olefins to be used both by regulators and producers.
5.3 This test method compares favorably with Test Method D1319 (FIA) for the determination of total olefins in motor gasolines. It does not require any sample preparation, has a comparatively short analysis time of about 10 min, and is readily automated. Alternative methods for determination of olefins in gasoline include Test Methods D6293 and D6296.
