Methyl sulfates as methoxy isotopic reference materials for δ¹³C and δ²H measurements

Abstract

Rationale Stable hydrogen and carbon isotope ratios of methoxy groups (OCH3) of plant organic matter have many potential applications in biogeochemical, atmospheric and food research. So far, most of the analyses of plant methoxy groups by isotope ratio mass spectrometry have employed liquid iodomethane (CH3I) as the reference material to normalise stable isotope measurements of these moieties to isotope–δ scales. However, comparisons of measurements of stable hydrogen and carbon isotopes of plant methoxy groups are still hindered by the lack of suitable reference materials. Methods We have investigated two methyl sulfate salts (HUBG1 and HUBG2), which exclusively contain carbon and hydrogen from one methoxy group, for their suitability as methoxy reference materials. Firstly, the stable hydrogen and carbon isotope values of the bulk compounds were calibrated against international reference substances by high‐temperature conversion‐ and elemental analyser isotope ratio mass spectrometry (HTC‐ and EA‐IRMS). In a second step these values were compared with values obtained by measurements using gas chromatography/isotope ratio mass spectrometry (GC/IRMS) where prior to analysis the methoxy groups were converted into gaseous iodomethane. Results The 2H‐ and 13C isotopic abundances of HUBG1 measured by HTC‐ and EA‐IRMS and expressed as δ‐values on the usual international scales are −144.5 ± 1.2 mUr (n = 30) and −50.31 ± 0.16 mUr (n = 14), respectively. For HUBG2 we obtained −102.0 ± 1.3 mUr (n = 32) and +1.60 ± 0.12 mUr (n = 16). Furthermore, the values obtained by GC/IRMS were in good agreement with the HTC‐ and EA‐IRMS values. Conclusions We suggest that both methyl sulfates are suitable reference materials for normalisation of isotope measurements of carbon of plant methoxy groups to isotope–δ scales and for inter‐laboratory calibration. For stable hydrogen isotope measurements, we suggest that in addition to HUBG1 and HUBG2 additional reference materials are required to cover the full range of plant methoxy groups reported so far.

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

“…The three wood reference materials HUBG3-5 calibrated within this work span a range of δ 2 H OCH3 values from -191 to -273 mUr. In combination with two recently investigated methyl sulfate salts HUBG1 and HUBG2 (Greule et al, 2019), the isotopic range of solid methoxy reference materials cover δ 2 H values from roughly -100 to -300 mUr (cf. Table 2).…”

“…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.…”

“…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.…”

“…For these reasons, we have recently investigated two methyl sulfate salts (HUBG1 and HUBG2), for their suitability to serve as methoxy reference materials (Greule et al, 2019).…”

“…These methyl sulfate salts have been demonstrated to be highly suitable to serve as reference materials to normalize δ 13 C OCH3 values, as they are treated in an identical manner as the Journal Pre-proof J o u r n a l P r e -p r o o f 5 sample and span a relatively wide range of δ 13 C values (HUBG1: -50.31 ± 0.16 mUr; HUBG2: +1.60 ± 0.12 mUr) covering most of the natural δ 13 C OCH3 values of terrestrial plant methoxy groups that have been reported so far (Greule et al, 2019). However, these methyl sulfate salts have a restricted range of δ 2 H OCH3 values (HUBG1: -144.5 ± 1.2 mUr; HUBG2: -102.0 ± 1.3 mUr) and thus are of limited use for normalizing δ 2 H OCH3 values of plant methoxy groups, which have δ 2 H OCH3 values in the range of -150 mUr to -300 mUr (Anhäuser et al, 2015;Greule et al, 2015Greule et al, , 2012Keppler and Hamilton, 2008;Riechelmann et al, 2017).…”