Greule Markus, Moossen Heiko, Geilmann Heike, Brand Willi A, Keppler Frank
Institute of Earth Sciences, Heidelberg University, Im Neuenheimer Feld 234-236, 69120, Heidelberg, Germany.
Max-Planck-Institute for Biogeochemistry, Hans-Knoell-Str. 10, 07749, Jena, Germany.
Rapid Commun Mass Spectrom. 2019 Feb 28;33(4):343-350. doi: 10.1002/rcm.8355.
Stable hydrogen and carbon isotope ratios of methoxy groups (OCH ) 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 (CH I) 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.
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.
The H- and C 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.
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.
植物有机质中甲氧基（OCH）的稳定氢和碳同位素比值在生物地球化学、大气和食品研究中有许多潜在应用。到目前为止，通过同位素比率质谱法对植物甲氧基进行的大多数分析都采用液态碘甲烷（CH₃I）作为参考物质，以便将这些部分的稳定同位素测量值归一化为同位素δ尺度。然而，由于缺乏合适的参考物质，植物甲氧基稳定氢和碳同位素测量值的比较仍然受到阻碍。
我们研究了两种仅含一个甲氧基中的碳和氢的硫酸甲酯盐（HUBG1和HUBG2）作为甲氧基参考物质的适用性。首先，通过高温转换和元素分析仪同位素比率质谱法（HTC - 和EA - IRMS），以国际参考物质为对照校准整体化合物的稳定氢和碳同位素值。第二步，将这些值与使用气相色谱/同位素比率质谱法（GC/IRMS）测量得到的值进行比较，在分析之前将甲氧基转化为气态碘甲烷。
通过HTC - 和EA - IRMS测量并以通常国际尺度上的δ值表示，HUBG1的氢和碳同位素丰度分别为-144.5 ± 1.2 mUr（n = 30）和-50.31 ± 0.16 mUr（n = 14）。对于HUBG2，我们得到的值为-102. ± 1.3 mUr（n = 32）和 +1.60 ± 0.12 mUr（n = 16）。此外，通过GC/IRMS得到的值与HTC - 和EA - IRMS的值高度一致。
我们认为这两种硫酸甲酯都是将植物甲氧基碳同位素测量值归一化为同位素δ尺度以及进行实验室间校准的合适参考物质。对于稳定氢同位素测量，我们建议除了HUBG1和HUBG2之外，还需要其他参考物质来涵盖迄今为止报道的植物甲氧基的整个范围。
