EXPLANATION
The determination of the boiling range distribution of gasoline by gas chromatographic simulated distillation (CG-SD) provides an insight into the composition of the components from which the gasoline was blended. This knowledge is useful for the control of refinery processes and for the blending of finished gasoline. This method has better precision than the conventional distillation by Test Method D86. Additionally, this test method provides more accurate and detailed information about the composition of the light ends. The distillation data produced by this test method are similar to that which would be obtained from a cryogenic, true boiling point (15 theoretical plates) distillation. The method is applicable to petroleum products and fractions with a final boiling point of 280° C or lower, as measured by this test method. This test method is designed to measure the entire boiling range of gasoline and gasoline components with either high or low vapor pressure and is commonly referred to as Simulated Distillation (SimDis) by gas chromatographers. This test method has been validated for gasolines containing ethanol. Gasolines containing other oxygenates are not specifically excluded, but they were not used in the development of this test method. This test method can estimate the concentration of n-pentane and lighter saturated hydrocarbons in gasoline.
TEST SUMMARY
The sample is vaporized and transported by carrier gas into a nonpolar, wide-bore capillary gas chromatographic column. The column temperature is raised at a reproducible, linear rate so as to elute the hydrocarbon components in boiling point order for measurement by a flame ionization detector. Conditions are selected such that n-pentane and lighter saturated hydrocarbons in the calibration mixture are resolved discretely. Linear correlation between hydrocarbons boiling point and retention time is established using a known mixture of hydrocarbons covering the boiling range expected in the sample. Area slices are converted to volume using theoreticalhydrocarbonvolumeresponsefactors. Oxygenated samples require experimental determinations of oxygenate response factors.
INTERFERENCES - Ethanol or other oxygenates may coelute with hydrocarbons present in the sample. Since the response of the oxygenates is substantially different from the response of the hydrocarbons, response factors are used to correct the area slice for the elution interval of oxygenates. Concentration of n-pentane and lighter saturated components may be estimated from the analysis. However, early eluting olefins present in the gasoline samples may coelute with these compounds.
TEST PRECISION
The currently available precision data are based on limited interlaboratory test results. Further data will be developed in the future. Based on the results from three laboratories, analyzing the same sample, following repeatability standard deviation was obtained:
For gases, the repeatability values are as follows:
The reproducibility of this test method is being determined. The bias in results of this test method cannot be determined because the boiling range distribution is defined by this test method.