Position-Specific Isotope Analysis of Xanthines: A 13C Nuclear Magnetic Resonance Method to Determine the 13C Intramolecular Composition at Natural Abundance
详细信息 查看全文
- 作者：Didier G. Diomande ; Estelle Martineau ; Alexis Gilbert ; Pierrick Nun ; Ariaki Murata ; Keita Yamada ; Naoharu Watanabe ; Illa Tea ; Richard J. Robins ; Naohiro Yoshida ; G茅rald S. Remaud
- 刊名：Analytical Chemistry
- 出版年：2015
- 出版时间：July 7, 2015
- 年：2015
- 卷：87
- 期：13
- 页码：6600-6606
- 全文大小：242K
- ISSN：1520-6882
文摘
The natural xanthines caffeine, theobromine, and theophylline are of major commercial importance as flavor constituents in coffee, cocoa, tea, and a number of other beverages. However, their exploitation for authenticity, a requirement in these commodities that have a large origin-based price-range, by the standard method of isotope ratio monitoring by mass spectrometry (irm-MS) is limited. We have now developed a methodology that overcomes this deficit that exploits the power of isotopic quantitative 13C nuclear magnetic resonance (NMR) spectrometry combined with chemical modification of the xanthines to enable the determination of positional intramolecular 13C/12C ratios (未13Ci) with high precision. However, only caffeine is amenable to analysis: theobromine and theophylline present substantial difficulties due to their poor solubility. However, their N-methylation to caffeine makes spectral acquisition feasible. The method is confirmed as robust, with good repeatability of the 未13Ci values in caffeine appropriate for isotope fractionation measurements at natural abundance. It is shown that there is negligible isotope fractionation during the chemical N-methylation procedure. Thus, the method preserves the original positional 未13Ci values. The method has been applied to measure the position-specific variation of the 13C/12C distribution in caffeine. Not only is a clear difference between caffeine isolated from different sources observed, but theobromine from cocoa is found to show a 13C pattern distinct from that of caffeine.