Two-point normalization using internal and external standards for a traceable determination of δ13C values of fatty acid methyl esters by gas chromatography/combustion/isotope ratio mass spectrometry

Isotope ratio mass spectrometry (IRMS) is a technique that measures subtle differences in the abundances of stable isotopes in samples or specific molecules. One important group of molecules is the fatty acids methyl esters (FAME), due to its importance to areas such as food control, geochemistry and, more recently, bio-based fuels such as biodiesel. The technique of choice for FAME analysis is gas chromatography (GC), but the lack of GC compatible stable isotope certified reference materials may cause inaccuracy in isotopic values determination. In this work, we proposed a simple methodology to accurately determine the δ13C values in Jatropha curcas FAMEs as a test case, by using a two-point normalization using internal and external standards to correct the isotopic fractionation from the transesterification reaction and from the GC/C/IRMS analysis itself. We observed that the transesterification process may lead up to 2 mUr of deviation of δ13C values for the internal standard (FAME C22:0). After the two point normalization, the determined δ13C average value for the FAMEs of 9 different samples of J. curcas was δ13CVPDB = − 29.51 ± 1.03 mUr, while the bulk value obtained with EA/IRMS was −28.97 ± 0.43 mUr, a typical signature of C3-plants. This work also demonstrates that major quality procedures must indeed be applied for accurate determinations of isotopic values, especially in GC analysis, following the identical treatment principle.