Reproducible measurements of the δ2H composition of non-exchangeable hydrogen in complex organic materials using the UniPrep2 online static vapour equilibration and sample drying system

Abstract

Non-exchangeable hydrogen-isotope (δ2Hn) measurements of complex organic samples are used in forensics to determine sample authenticity, traceability, and provenance. However, δ2Hn assays of organics are usually complicated by uncontrolled “exchangeable hydrogen” and residual moisture contamination; hence, δ2Hn assays are persistently incomparable amongst laboratories. We introduce a revised technical solution (UniPrep2) to control hydrogen-isotope exchange and for robust online sample drying and vapour equilibration. The UniPrep2 device is coupled to a high-temperature thermochemical elemental analyser and continuous-flow isotope-ratio mass spectrometer. This technical solution empowers isotope analysts to:•Conduct reproducible controlled vapour equilibrations of complex organic samples and standards to determine the δ2Hn values by controlling hydrogen-isotope exchange.•Conduct online vacuum-oven evacuation with extensive helium drying without exposure to air to reabsorb or exchange hydrogen with ambient water vapour.The protocol describes the operation of the UniPrep2 device and the step-by-step procedures needed to obtain accurate and precise δ2Hn values for a wide range of organic sample types. Two analytical approaches are described in detail; the Dual-Vapour Equilibration (DVE) approach, intended for determining δ2Hn for a complex organic environmental sample where matrix equivalent H isotope reference materials are not available, and the Comparative Equilibration (CE) approach, which is intended for routine high-throughput analyses of complex organic samples where at least two matrix-equivalent organic isotope reference materials with consensus δ2Hn values are being used. These standard operating procedures are envisioned to be a sound basis for advancing hydrogen-isotope analysis for different organic environmental matrices and studies.