Technical note: A fast and reproducible autosampler for direct vapor equilibration isotope measurements

<p>To investigate water movement in environmental systems, stable isotope (<span class="inline-formula">²</span>H and <span class="inline-formula">¹⁸</span>O) ratios of water are commonly used tracers. Analyzing the isotopic ratios of water in or adsorbed to substances like soil or plant tissue necessitates the extraction or equilibration of water prior to analysis. One such method, direct vapor equilibration, is popular due to its cost-effectiveness and straightforward sample processing. However, sample analysis requires significant manual labor, thereby limiting the number of samples that can be analyzed. This limitation is compounded by the fact that stored samples undergo evaporative isotopic changes over time. Moreover, manual measurements require many laborious procedural steps that can easily compromise reproducibility. The operator has to subjectively decide if the measurements are stable and then record the analyzer readings. To address these challenges, we have developed a system that automates the analysis process. Our autosampler for vapor samples, named VapAuSa, features a modular design that allows for up to 350 ports for direct vapor equilibration samples. These ports sequentially connect the prepared samples to a laser isotope analyzer, enabling continuous automated measurements. Within the accompanying software, measurement criteria can be specified, facilitating reproducible analysis. The developed system was tested by co-measuring 90 soil samples and 21 liquid water samples with known <span class="inline-formula"><i>δ</i></span> values. VapAuSa measurements have a negligible measurement bias (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>&lt;</mo><mn mathvariant="normal">1</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">13</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="56pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="84c4c517ec0a57ff59e5ade77a9a4f92"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-29-525-2025-ie00001.svg" width="56pt" height="14pt" src="hess-29-525-2025-ie00001.png"/></svg:svg></span></span> ‰ for both <span class="inline-formula"><i>δ</i>²</span>H and <span class="inline-formula"><i>δ</i>¹⁸</span>O) and similar measurement repeatability compared to manual analysis of identical samples (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup><mrow class="chem"><mi mathvariant="normal">H</mi></mrow><mo>=</mo><mo>±</mo><mn mathvariant="normal">4.5</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="58pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="a1756e064ae97bf3fb705c96a90e616e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-29-525-2025-ie00002.svg" width="58pt" height="14pt" src="hess-29-525-2025-ie00002.png"/></svg:svg></span></span> ‰ and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">18</mn></msup><mrow class="chem"><mi mathvariant="normal">O</mi></mrow><mo>=</mo><mo>±</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="49pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="5b12edb21efb09f699689b93d39aa9cd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-29-525-2025-ie00003.svg" width="49pt" height="14pt" src="hess-29-525-2025-ie00003.png"/></svg:svg></span></span>0.58 ‰ for VapAuSa measurements vs. <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup><mrow class="chem"><mi mathvariant="normal">H</mi></mrow><mo>=</mo><mo>±</mo><mn mathvariant="normal">5.7</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="58pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="8e74806c04be2ae4cec85869f56a2ae7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-29-525-2025-ie00004.svg" width="58pt" height="14pt" src="hess-29-525-2025-ie00004.png"/></svg:svg></span></span> ‰ and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">18</mn></msup><mrow class="chem"><mi mathvariant="normal">O</mi></mrow><mo>=</mo><mo>±</mo><mn mathvariant="normal">0.37</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="69pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="cb200124dbcb53e5f74aa1a6cdda1c53"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-29-525-2025-ie00005.svg" width="69pt" height="14pt" src="hess-29-525-2025-ie00005.png"/></svg:svg></span></span> ‰ for manual analysis). However, the increased sample throughput minimizes storage-induced isotopic changes. Moreover, VapAuSa triples sample throughput per week while also reducing the direct labor time to just 10 % of that required for manual processing.</p>