Three wood isotopic reference materials for δ²H and δ¹³C measurements of plant methoxy groups

Abstract

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 (δ²H_(OCH₃) and δ¹³C_(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 δ²H_(OCH₃) but also δ¹³C_(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 δ²H_(OCH₃) and δ¹³C(OCH₃) values of this CH₃I were calibrated against international reference substances by high-temperature conversion- and elemental analyzer isotope ratio mass spectrometry. The δ²H_(OCH) and δ¹³C_(OCH₃) values of HUBG3 and HUBG4 at 1σ uncertainty calibrated to the VSMOW and VPDB isotopic δ-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 δ²HOCH value of HUBG5 is −191.7 ± 0.8 mUr. Whilst the δ²H_(OCH₃) values of the three wood samples span a relatively wide range (~−280 to −190 mUr) suitable for normalization of δ²H_(OCH₃) values of most plant samples that have been reported so far, δ¹³C_(OCH₃) values are restricted to a composition (~−30 mUr) typical of wood methoxy groups. We suggest that the three investigated wood materials HUBG3–5 are ideally suited for long-term usage, inter-laboratory 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 δ²H_(OCH₃) and δ¹³C_(OCH₃) values to the respective δ-scales.

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