Three Wood Isotopic Reference Materials For Delta H-2 And Delta C-13 Measurements Of Plant Methoxy Groups

Methoxy groups (OCH3) of plants show specific stable carbon and hydrogen isotope patterns that are used for applications in biogeochemical, atmospheric, paleoclimatic and food research. The method of choice for determining stable hydrogen and carbon isotope values of methoxy groups (delta H-2(OCH3) and delta C-13(OCH3) values) is the conversion to gaseous iodomethane (CH3I) and subsequent measurement by stable isotope ratio mass spectrometry. However, comparative measurements particularly for stable hydrogen isotopes of plant methoxy groups are limited due to the lack of suitable reference materials. We have prepared three batches of powdered wood samples (birch HUBG3, beech HUBG4, and tineo HUBG5) collected from different geographical locations to serve as long-term reference materials for normalization of delta H-2(OCH3) but also delta C-13(OCH3) values. Methoxy contents of the three wood samples range between 4.7 and 5.4%. Methoxy groups from subsamples of the three homogenized wood samples were quantitatively converted to CH3I and delta H-2(OCH3) and delta C-13(OCH3) values of this CH3I were calibrated against international reference substances by high-temperature conversion-and elemental analyzer isotope ratio mass spectrometry. The delta H-2(OCH3) and delta C-13(OCH3) values of HUBG3 and HUBG4 at 1 sigma uncertainty calibrated to the VSMOW and VPDB isotopic 8-scale, respectively are 272.9 1.5 mUr and 29.40 0.13 mUr; and -239.1 +/- 1.4 mUr and -30.17 +/- 0.13 mUr, respectively. In addition, the calibrated delta H-2(OCH) value of HUBG5 is -191.7 +/- 0.8 mUr. Whilst the delta H-2(OCH3) values of the three wood samples span a relatively wide range (similar to-280 to -190 mUr) suitable for normalization of delta H-2(OCH3) values of most plant samples that have been reported so far, 813CocH3 values are restricted to a composition (similar to-30 mUr) typical of wood methoxy groups. We suggest that the three investigated wood materials HUBG3-5 are ideally suited for long-term usage, interlaboratory comparison, and together with the two recently reported methyl sulfate salts (HUBG1 and HUBG2) complete a new set of solid methoxy reference materials that cover almost the full range of plant methoxy groups reported so far. Therefore, we recommend replacing liquid CH3I and instead use the solid reference materials set HUBG1-5 for normalizing delta H-2(OCH3) and delta C-13(OCH3) values to the respective 8-scales.