ABSTRACT
Quantitative isotopic (13)C NMR at natural abundance has been used to determine the site-by-site (13)C/(12)C ratios in vanillin and a number of related compounds eluted from silica gel chromatography columns under similar conditions. Head-to-tail isotope fractionation is observed in all compounds at the majority of carbon positions. Furthermore, the site-specific isotope deviations show signatures characteristic of the position and functionality of the substituents present. The observed effects are more complex than would be obtained by simply summing the individual effects. Such detail is hidden when only the global (13)C content is measured by mass spectrometry. In particular, carbon positions within the aromatic ring are found to show site-specific isotope fractionation between the solute and the stationary phase. These interactions, defined as non-covalent isotope effects, can be normal or inverse and vary with the substitution pattern present.
Subject(s)
Carbon Isotopes/chemistry , Chromatography, Liquid/methods , Nuclear Magnetic Resonance, Biomolecular/methods , Benzaldehydes/chemistry , Coumaric Acids/chemistry , Deuterium/chemistry , Models, Chemical , Oxygen Isotopes/chemistry , Silica Gel , Silicon Dioxide/chemistryABSTRACT
Continuous flow isotope ratio mass spectrometry (CF-IRMS) has been used to analyze samples of gilthead sea bream (g.s.b.) (Sparus aurata) of known geographical origin (four countries on the Mediterranean Sea) from wild and farmed sources (40 farmed and 10 wild). delta(13)C and delta(15)N values have been measured on muscle samples as these are the most informative parameters of the diet of the animals. Both stable isotopes are indicators of the origin of the fish: delta(13)C giving a tool to distinguish between wild and farmed g.s.b. and delta(15)N being more informative on the geographical origin of the fish (this fact could be related more to differences in feed mixtures given to farmed fish than to geographical reasons). The proposed methodology offers a cost- and time-effective alternative to other analytical techniques in identifying wild and farmed fish.
