Institut für Lebensmittelchemie, Biozentrum J.W. Goethe-Universität, Marie-Curie-Str. 9, 60439 Frankfurt/Main, Germany European Food Research and Technology (2008), V227(3), pp767-776, doi: 10.1007/s00217-007-0785-7 Feed additives: authenticity assessment using multicomponent-/multielement-isotope ratio mass spectrometry Markus Greule, Clarissa Hänsel, Ulrike Bauermann and Armin Mosandl Since the ban on growth-promoting antibiotics throughout the EU essential oils are used increasingly as feed additives. In order to ensure a reliable authenticity assessment of those oils appropriate methods were developed. 2H/1H-, 13C/12C- and 18O/16O isotope ratio mass spectrometric analysis of carvacrol from origanum and savory oil, thymol from thyme oil and trans-anethole from fennel oil in combination with IRMS-analysis of minor components and diVerential analysis is reported.