Use of radiocarbon analyses for determining levels of biodiesel in fuel blends -- Comparison with ASTM Method D7371 for FAME

Abstract

A simplified radiocarbon (14C) analytical method was investigated for measuring the biodiesel (FAME) content of diesel fuel blends. That method involves mixing a fuel sample with a fluor and then analyzing it in a liquid scintillation counter (LSC) without further processing. The method is referred to as the direct LSC method because it analyzes the samples directly rather than employing the sample preparation procedures (including sample combustions) typically used in radiocarbon analyses. Biodiesel produced from four different feedstock materials was mixed with petrodiesel to produce fuel blends containing 2–20% FAME, which were subsequently analyzed using the direct LSC method. The fuel blends were also analyzed by several laboratories using FTIR in accordance with ASTM Method D7371, as well as by alternate IR-based procedures being considered for inclusion in that method. Accurate results could be obtained using the direct LSC method if color quench within the samples was sufficiently alleviated. When using counting times of 360min, results obtained on canola, soy, and white grease biodiesel blends were accurate to within 0.3% (absolute) or better, while results on the coconut biodiesel blends were accurate to within 1.0% (absolute) or better. Results from the direct LSC analyses were comparable to those obtained by Method D7371 for the canola, soy, and white grease biodiesel blends, and were much more accurate than Method D7371 for coconut biodiesel blends. The results reported by different laboratories using Method D7371 varied by 1% (absolute) or more for all the samples, and by 2% (absolute) or more for samples containing 15–20% FAME. Therefore, the practice of reporting results to two decimal places does not appear to be justified in view of the data scatter observed between laboratories.