“…Stable isotope analyses require reference materials to normalize stable isotope ratios on the respective δ-scale (Brand et al, 2014;Carter and Barwick, 2011;Meier-Augenstein and Schimmelmann, 2019;Werner and Brand, 2001) Highest accuracy is achieved when workflows adhere to the principle of "identical treatment of sample and reference material" (IT) in preparation and analysis, which requires that samples are chemically analogous to standards of known isotopic composition (Meier-Augenstein and Schimmelmann, 2019;Schimmelmann et al, 2016;Werner and Brand, 2001) When selecting reference materials for IRMS analyses further criteria have to be considered as outlined in detail by several previous studies, such as isotopic homogeneity, unchanging stable isotopic composition over time, chemical similarity to the samples, easy preparation, storage and handling as well as no health risk (Brand et al, 2014;Carter and Barwick, 2011;Meier-Augenstein and Schimmelmann, 2019;Schimmelmann et al, 2016;Werner and Brand, 2001) Most published stable isotope values of methoxy groups analyzed by IRMS were normalized to the respective δ-scale using liquid CH 3 I as the reference material. However, this practice violates the above principles because liquid CH 3 I, unlike the sample, is typically not heated at 130°C with HI for derivatization (Greule et al, 2019). Moreover, δ 2 H and δ 13 C values of commercially available CH 3 I have restricted ranges (δ 2 H: -66 to -179 mUr; δ 13 C: -46 to -70 mUr) (Feakins et al, 2013b;Greule et al, 2019;Keppler et al, 2007) that do not bracket typical δ 2 H OCH3 and δ 13 C OCH3 values of plant methoxy groups.…”