• Journal Article

Determination of intramolecular delta-C13 from incomplete pyrolysis fragments. evaluation of pyrolysis-induced isotopic fractionation in fragments from the lactic acid analogue propylene glycol

Citation

Wolyniak, C. J., Sacks, G. L., Metzger, S., & Brenna, T. (2006). Determination of intramolecular delta-C13 from incomplete pyrolysis fragments. evaluation of pyrolysis-induced isotopic fractionation in fragments from the lactic acid analogue propylene glycol. Analytical Chemistry, 78(8), 2752-2757. DOI: 10.1021/ac0522198

Abstract

Intramolecular carbon isotope ratios reflect the source of a compound and the reaction conditions prevailing during synthesis and degradation. We report here a method for determination of relative (??13C) and absolute (?13C) intramolecular isotope ratios using the volatile lactic acid analogue propylene glycol as a model compound, measured by on-line gas chromatography-pyrolysis coupled to GC-combustion-isotope ratio mass spectrometry. Pyrolytic fragmentation of about one-third of the analyte mass produces optimal fragments for isotopic analysis, from which relative isotope ratios (??13C) are calculated according to guidelines presented previously. Calibration to obtain absolute isotope ratios is achieved by quantifying isotope fractionation during pyrolysis with an average fractionation factor, ?, and evaluated by considering extremes in isotopic fractionation behavior. The method is demonstrated by calculating ranges of absolute intramolecular isotope ratios in four samples of propylene glycol. Relative and absolute isotope ratios were calculated with average precisions of SD(??13C) <0.84‰ and SD(?13C) <3.0‰, respectively. The various fractionation scenarios produce an average ?13C range of 2‰ for each position in each sample. Relative isotope ratios revealed all four samples originated from unique sources, with samples A, B, and D only distinguishable at the position-specific level. Regardless of pyrolysis fractionation distribution, absolute isotope ratios showed a consistent pattern for all samples, with ?13C(3) > ?13C(2) > ?13C(1). The validity of the method was determined by examining the difference in relative isotope ratios calculated through two independent methods: ??13C calculated directly using previous methods and ??13C extracted from absolute isotope ratios. Deviation between the two ??13C values for all positions averaged 0.1?0.2‰, with the smallest deviation obtained assuming equal fractionation across all fragment positions. This approach applies generally to all compounds analyzed by pyrolytic PSIA. <br>