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The influence of different referencing methods on the accuracy of δ13C 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 δ13C values traceable to Vienna Pee Dee Belemnite (VPDB), the international zero point for 13C/12C measurements, certified and compatible with gas chromatography (GC)/IRMS instruments may compromise the accuracy of δ13C determinations.

Methods

We evaluated the influence of methods for the calibration and normalization of raw δ13C 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 δ13C values obtained with the three methods of normalization were statistically compared with those obtained with elemental analyzer (EA)/IRMS, since the δ13C 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 CO2 calibration and the GC column have a major effect on the δ13C measurements, leading to a bias of almost 2–3 ‰ in the δ13C 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 CO2 calibration, chromatography and normalization presented in this work demonstrated several sources of traceability and accuracy loss for the determination of δ13C 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 δ13C values. This is important not only for bioethanol fuel samples, but also for many analytes commonly analyzed by IRMS. Copyright © 2015 John Wiley & Sons, Ltd.

Authors:   Laura A. Neves, Janaína M. Rodrigues, Romeu J. Daroda, Paulo R. M. Silva, Alexandre A. Ferreira, Donato A. G. Aranda, Marcos N. Eberlin, Maíra Fasciotti
Journal:   Rapid Communications in Mass Spectrometry
Volume:   29
edition:   21
Year:   2015
Pages:   1938
DOI:   10.1002/rcm.7298
Publication date:   30-Sep-2015
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