Determination of Intramolecular δ¹³C from Incomplete Pyrolysis Fragments. Evaluation of Pyrolysis-Induced Isotopic Fractionation in Fragments from the Lactic Acid Analogue Propylene Glycol

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 (Deltadelta13C) and absolute (delta13C) 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 (Deltadelta13C) 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, alpha, 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(Deltadelta13C) <0.84 per thousand and SD(delta13C) <3.0 per thousand, respectively. The various fractionation scenarios produce an average delta(13)C range of 2 per thousand 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 delta13C(3) > delta13C(2) > delta13C(1). The validity of the method was determined by examining the difference in relative isotope ratios calculated through two independent methods: Deltadelta13C calculated directly using previous methods and Deltadelta13C extracted from absolute isotope ratios. Deviation between the two Deltadelta13C values for all positions averaged 0.1-0.2 per thousand, with the smallest deviation obtained assuming equal fractionation across all fragment positions. This approach applies generally to all compounds analyzed by pyrolytic PSIA.

Development of an advanced on‐line position‐specific stable carbon isotope system and application to methyl tert‐butyl ether

“…The criteria for the selection of the pyrolysis conditions for on‐line PSIA follow the notion of ‘isotopic fidelity’ introduced by Brenna and co‐workers 49–51. Isotopic fidelity describes the degree of correlation between the isotope ratio of a fragment and that of a specific moiety in the original compound 49. An ideal fragment has a fidelity of 100%, meaning that it originates entirely from a unique position or moiety.…”

“…With this initial design, molecular cracking occurs by non‐catalysed pyrolysis and as such the system is flexible and readily adaptable for a wide variety of volatile and semi‐volatile compounds compatible with gas chromatography. The range of its applications has so far included alkanes,47 toluene,47 fatty acid methyl esters,48 lactic acid,49 and four amino acids 50, 51. Low molecular weight organic acids have also been investigated through similar systems for applications in environmental52 and geoscience 53.…”

“…Because isotopic fractionation may occur during non‐quantitative reactions, the isotopic signatures of the pyrolysis products may not be equal to the isotopic signatures of the equivalent moieties in the parent molecule. Previous PSIA studies did not determine absolute intra molecular isotope ratios and the values were reported only as relative isotope ratios by comparison with chemically identical standards 49–51…”

“…The lack of absolute intra molecular isotope ratios limits the application of PSIA for Environmental Forensics because there is no means for interlaboratory comparisons of results. Brenna and co‐workers49 proposed a theoretical method to calculate intra molecular fractionation to overcome this problem. We report here, for the first time, the development of an on‐line PSIA system equipped with an improved pyrolysis furnace that allows for adjustable temperatures and residence times to facilitate experimental evaluation and calculation of the pyrolysis isotopic fractionation.…”

Mammalian DNA δ15N exhibits 40‰ intramolecular variation and is unresponsive to dietary protein level

“…However, the base C atoms are donated by six different sources, specifically amino acids, CO 2 , bicarbonate, and methyl groups from the one C pool, and with four bases and six sources, calculation of intramolecular isotope ratios from the δ 13 C values of the bases would be underdetermined. Analysis of intramolecular isotope ratios requires a different strategy, possibly involving pyrolysis methods similar to those that we have applied previously to several classes of compounds …”