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

Gauchotte, C. and O'Sullivan, G. and Davis, Simon and Kalin, R. (2009) Development of an advanced on-line position-specific stable carbon isotope system and application to methyl tert-butyl ether. Rapid Communications in Mass Spectrometry, 23 (19). pp. 3183-3193. ISSN 0951-4198

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Abstract

We present an advanced system for on-line position-specific carbon isotope analysis. The main limitation of on-line intramolecular isotope ratio measurements has been that optimal pyrolytic fragments are obtained mostly at temperatures where the analyte has not completely reacted. As a result of undetermined isotopic fractionation, the isotopic signatures of the pyrolysis products are not strictly equal to these of the equivalent moieties in the parent molecule. We designed a pyrolytic unit in which both temperature and reaction time are variable parameters, enabling determination of the enrichment factor of the pyrolysis at optimal temperature by construction of a Rayleigh plot. In the case of methyl tert-butyl ether (MTBE) presented here, a 'pre-pyrolysis' fractionation of MTBE leading to a depletion of 0.9 parts per thousand was discovered and the enrichment factor of the optimal pyrolysis reaction was determined at -1.7 parts per thousand. Absolute delta C-13 values of two functional groups of MTBE - the methoxy group and the 2-methylpropane group - could be determined with 95% confidence intervals of 0.4 parts per thousand and 0.5 parts per thousand, respectively.