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Biodiesel purity expressed as fatty esters content is one of its most important quality parameters. Although several instrumental methods have been employed, gas chromatography with internal standard calibration has been the most used. Biodiesel is a very complex matrix, therefore finding a suitable internal standard (IS) is not straightforward. EN14103:2003 standard established methyl heptadecanoate (C17:0) as IS. Since this fatty ester is naturally present in some biodiesel feedstock as tallow, that standard was reviewed in 2011, and methyl nonadecanoate (C19:0) became the new IS. In turn, C19:0 produces a peak on the chromatogram that is difficult to resolve from methyl linoeate (C18:2) and metyl linolenate (C18:3) ones. In this paper, a GC-FID method for the determination of the esters content in methyl and ethyl biodiesel from canola and soy oils, using hexadecyl acetate as a new internal standard was validated. EN14103:2003 was used as reference method. Method selectivity, accuracy (t student parameter < 2.18 from both methods), repeatability (0.1 - 0.4%), intermediate precision (0.2 - 1.8%), and robustness (0.1 - 1.6%) were investigated and considered appropriate for the scope.

Biodiesel made from oils is rich in polyunsaturated fatty acids methyl esters, and, consequently, presents high tendency to oxidize. Biodiesel made from fats, on the other hand, is mainly comprised by saturated chains. Although it is highly stable, they easily crystalize at low temperatures. Blends from both can exhibit optimized properties. In this work, oxidative stability, expressed by induction period (IP), and cold filter plugging point (CFPP) of blends from palm, palm kernel, babassu, tallow or lard biodiesels with soy one were measured. Biodiesel samples were prepared by two steps transesterification methodology with KOH catalysis followed by H2SO4. IP increases, as the fat biodiesel content in the blend is higher, as expected. Blends comprised by 60% of fat biodiesel by volume had their IP increased from 1.5 (lard) to 2 times (palm kernel), compared to soy one. At this level, IP was found 8 h or higher, except for palm, which 80% was required. CFPP was observed at 8 °C or lower for the blends containing up to 60% of all the fat biodiesel, except for the palm one, which a 20% content must not be surpassed. Palm kernel 80% biodiesel blend reached the Brazilian specification, that is, IP higher than 12 h and CFPP lower than 5 °C.