High-precision position-specific isotope analysis.

Intramolecular carbon isotope distributions reflect details of the origin of organic compounds and may record the status of complex systems, such as environmental or physiological states. A strategy is reported here for high-precision determination of 13C/12C ratios at specific positions in organic compounds separated from complex mixtures. Free radical fragmentation of methyl palmitate, a test compound, is induced by an open tube furnace. Two series of peaks corresponding to bond breaking from each end of the molecule are analyzed by isotope ratio mass spectrometry and yield precisions of SD(delta-13C) < 0.4 per thousand. Isotope labeling in the carboxyl, terminal, and methyl positions demonstrates the absence of rearrangement during activation and fragmentation. Negligible isotopic fractionation was observed as degree of fragmentation was adjusted by changing pyrolysis temperature. [1-13C]methyl palmitate with overall delta-13C = 4.06 per thousand, yielded values of +457 per thousand for the carboxyl position, in agreement with expectations from the dilution, and an average of -27.95 per thousand for the rest of the molecule, corresponding to -27.46 per thousand for the olefin series. These data demonstrate the feasibility of automated high-precision position-specific analysis of carbon for molecules contained in complex mixtures.