Analysis of biodiesel/petroleum diesel blends with comprehensive two-dimensional gas chromatography

Comprehensive two-dimensional gas chromatography (GCxGC) is used to analyze petroleum diesel, biodiesel, and biodiesel/petroleum diesel blends. The GCxGC instrument is assembled from a conventional gas chromatograph fitted with a simple, in-line fluidic modulator. A 5% phenyl polydimethylsiloxane primary column is coupled to a polyethylene glycol secondary column. This column combination generates chromatograms where the fatty acid methyl esters (FAMEs) found in biodiesel occupy a region that is also populated by numerous cyclic alkanes and monoaromatics found in petroleum. Fortunately, the intensities of the petroleum hydrocarbon peaks are far lower than the intensities of the FAME peaks, even for blends with low biodiesel content. This allows the FAMEs to be accurately quantitated by direct integration. The method is calibrated by analyzing standard mixtures of soybean biodiesel in petroleum diesel with concentrations ranging from 1 to 20% v/v. The resulting calibration curve displays excellent linearity. This curve is used to determine the concentration of a B20 biodiesel/petroleum diesel blend obtained from a local retailer. Excellent precision and accuracy are obtained.