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In recent years stable isotopes have begun to replace radioisotopes as tracers in metabolic studies. However, these are relatively expensive in the enrichments and quantities required by either conventional gas chromatography/mass spectrometry (GC/MS) or stable isotope ratio analysis (SIRA), the former having good sensitivity but low precision and the latter low sensitivity with more than adequate precision. We describe here a technique for 13C which achieves an appropriate balance between precision and sensitivity for such studies. A combustion interface is placed between the output from a capillary gas chromatograph and a dual collector SIRA mass spectrometer. Compounds eluting from the gas chromatograph are converted into discrete pulses of COz gas and intensities of "COZ and lzCOz are measured simultaneously. Typically, 13C relative abundances can be obtained from 0.8 nmol of a Clo compound (8 nmol COz) with a precision (1 SD) of better than 1% (0.0011 at.% 13C). This permits a reduction by at least a factor of 10 in the quantity of tracer required compared with conventional methods.

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The isotopic composition of vanillin was investigated in order to distinguish synthetic wood lignin vanillin and natural vanilla-extracted vanillin. Natural vanillin is enriched in deuterium and carbon-13 as compared to synthetic vanillin. This enrichment probably results from differences in the synthetic pathways of this compound. In particular, carbon isotope fractionation is associated with photosynthesis; this fractionation lowers the 13C/12C ratio by about 20 0/00, relative to atmospheric carbon dioxide, for the Calvin cycle in photosynthetic carbon dioxide fixation, which occurs in trees. The fractionation factor is even lower, about 13 0/00, for the orchid Vanilla planifolia Andrews in which the Crassulacean acid metabolic pathway probably operates.