The influence of different referencing methods on the accuracy of delta super(13)C value measurement of ethanol fuel by gas chromatography/combustion/isotope ratio mass spectrometry

Rationale Brazil is the largest producer of sugar cane bioethanol in the world. Isotope ratio mass spectrometry (IRMS) is the technique of choice to certify the origin/raw materials for ethanol production, but the lack of certified reference materials (CRMs) for accurate measurements of delta super(13)C values traceable to Vienna Pee Dee Belemnite (VPDB), the international zero point for super(13)C/ super(12)C measurements, certified and compatible with gas chromatography (GC)/IRMS instruments may compromise the accuracy of delta super(13)C determinations. Methods We evaluated the influence of methods for the calibration and normalization of raw delta super(13)C values of ethanol samples. Samples were analyzed by GC/C/IRMS using two different GC columns. Different substances were used as isotopic standards for the working gas calibration. The delta super(13)C values obtained with the three methods of normalization were statistically compared with those obtained with elemental analyzer (EA)/IRMS, since the delta super(13)C results obtained using EA are traceable to VPDB via the NBS 22 reference material. Results It was observed that both the isotopic reference material for CO sub(2) calibration and the GC column have a major effect on the delta super(13)C measurements, leading to a bias of almost 2-3 ppt in the delta super(13)C values. All three methods of normalization were equivalent in performance, enabling an improvement in the GC/C/IRMS accuracy, compared with the EA/IRMS reference values for the samples. Conclusions All the methods of CO sub(2) calibration, chromatography and normalization presented in this work demonstrated several sources of traceability and accuracy loss for the determination of delta super(13)C values in ethanol fuel samples by GC/C/IRMS. This work has also shown the importance of using proper CRMs traceable to VPBD that should be compatible and certified using GC/C/IRMS, ideally in a wide range of delta super(13)C values. This is important not only for bioethanol fuel samples, but also for many analytes commonly analyzed by IRMS.