QUANTIFICATION OF COMPOUND CLASSES IN COMPLEX-MIXTURES AND FUELS USING HPLC WITH DIFFERENTIAL REFRACTIVE-INDEX DETECTION

The chemical composition of liquid hydrocarbon fuels that affect engine combustion performance has historically been defined by three principal classes: saturates, monocyclic aromatics, and dicyclic aromatics. These classes may be separated by normal-phase analytical-scale high-performance Liquid chromatography and are conveniently observed as the response of a differential refractive index (DRI) detector. Obtaining accurate quantitative analysis is not straightforward because calibration standards for determining the response factor for each compound class are not available. This problem arises because the refractive index of each class varies from fuel to fuel and is not generally known accurately enough for quantitative analysis. However, the linearity of the detector response with analyte refractive index for a fixed volume of sample makes it possible to determine the relative compound class composition as well as the refractive index of each class from the peak areas measured with a DRI detector at the time of separation by HPLC. This is accomplished by obtaining the chromatograms of each sample using two different mobile phases of different refractive indexes, such as hexane and nonane in this study. The method has been applied to jet fuels in this paper, but the extension to other hydrocarbon fuels such as gasoline and diesel fuel is obviously possible.