ASTM D6591 Standard Test Method for Determination of Aromatic Hydrocarbon Types in Middle Distillates - High Performance Liquid Chromatography Method with Refractive Index Detection
4. Summary of Test Method
4.1 A known mass of sample is diluted in the mobile phase, and a fixed volume of this solution is injected into a high performance liquid chromatograph, fitted with a polar column. This column has little affinity for the non-aromatic hydrocarbons while exhibiting a pronounced selectivity for aromatic hydrocarbons. As a result of this selectivity, the aromatic hydrocarbons are separated from the non-aromatic hydrocarbons into distinct bands in accordance with their ring structure, that is, MAHs, DAHs, and T+AHs. At a predetermined time, after the elution of the DAHs, the column is backflushed to elute the T+AHs as a single sharp band.
4.2 The column is connected to a refractive index detector that detects the components as they elute from the column. The electronic signal from the detector is continually monitored by a data processor. The amplitudes of the signals (peak areas) from the sample aromatics are compared with those obtained from previously measured calibration standards in order to calculate percent m/m MAHs, DAHs, and T+AHs in the sample. The sum of the percentages by mass of DAHs and T+AHs is reported as the percent m/m POLY-AH. The sum of MAHs, DAHs, and T+AHs is reported as the total aromatic content (percent m/m) of the sample.
5. Significance and Use
5.1 The aromatic hydrocarbon content of motor diesel fuel is a factor that can affect exhaust emissions and fuel combustion characteristics, as measured by cetane number.
5.2 The United States Environmental Protection Agency (US EPA) regulates the aromatic content of diesel fuels. California Air Resources Board (CARB) regulations place limits on the total aromatics content and polynuclear aromatic hydrocarbon content of motor diesel fuel, thus requiring an appropriate analytical determination to ensure compliance with the regulations.
5.3 This test method is applicable to materials in the same boiling range as motor diesel fuels and is unaffected by fuel coloration. Test Method D1319, which has been mandated by the US EPA for the determination of aromatics in motor diesel fuel, excludes materials with final boiling points greater than 315°C (600°F) from its scope. Test Method D2425 is applicable to the determination of both total aromatics and polynuclear aromatic hydrocarbons in diesel fuel, but is much more costly and time-consuming to perform. Test Method D5186, currently specified by CARB, is also applicable to the determination of both total aromatics and polynuclear aromatic hydrocarbons in diesel fuel. Test Method D5186, however, specifies the use of supercritical fluid chromatography equipment that may not be readily available.
NOTE 2 - Test Method D5186 was previously specified by CARB as an alternative to Test Method D1319.
