A Simple Method for Triple Stable Isotope Analysis of Cellulose, Sugar, and Bulk Organic Matter-Advances and Limitations

Rationale: Determining several isotope ratios in one analysis multiplies the information that can be retrieved from a sample in a cost-efficient way. The stable isotope ratios of hydrogen (δ²H), carbon (δ¹³C), and oxygen (δ¹⁸O) in organic compounds are highly relevant due to their complimentary hydroclimatic and physiological signals. Different types of organic material reflect different processes and integration times, like short term in leaf sugars and long term in tree ring cellulose, but currently, no simple method exists for their triple isotope analysis.

Methods: Here, we present a method that enables the isotopic analyses of the three elements H, C, and O in one run and is applicable to different types of carbohydrates and bulk organic matter. We discuss all steps required from water vapor equilibration necessary for obtaining reliable δ²H values of carbon-bound H to high-temperature conversion (HTC) of the sample to CO and H₂ and to the mass-spectrometric isotope-ratio analysis.

Results: We show that reliable triple isotope analysis is possible for a large range of samples, although it results in some reduction of precision compared to individual isotope analysis. Important considerations are the equilibration procedure, the type of autosampler, selection of HTC reactor, the influence of nitrogen in the sample, the verification of δ¹³C values obtained by HTC versus combustion, and the selection of reference materials.

Conclusions: By presenting a relatively simple triple-isotope method, we promote the use of multi-isotope studies in environmental sciences, which helps in addressing many important climate and ecological research challenges that we face today.