The effects of Porapak™ trap temperature on δ 18 O, δ 13 C, and Δ 47 values in preparing samples for clumped isotope analysis

Abstract

Rationale: The clumped isotope paleothermometer, a new proxy widely applicable in studies of paleoclimate, tectonics, and paleontology, relates the abundance of doubly substituted isotopologues of carbonate‐derived CO 2 to the temperature of formation of the carbonate phase. As this technique becomes more widely used, more is discovered about the effects of everyday laboratory procedures on the clumped isotopic composition of CO 2 gas. Methods: Preparation of CO 2 for clumped isotope analysis requires the removal of isobaric contaminants prior to measurement, achieved dynamically by passing the CO 2 through a gas chromatography column using a helium carrier gas or cryogenically pumping CO 2 through a static trap filled with Porapak™ Q (PPQ) material. The stable and clumped isotopic compositions of carbonate standards prepared at PPQ trap temperatures between −40°C and −10°C were measured by isotope ratio mass spectrometry to evaluate potential artifacts introduced by the static PPQ trap method. Results: The stable isotopic composition of carbonates run at temperatures below −20°C was fractionated, despite achieving >99% retrieval of gas at temperatures as cold as −30°C. The δ 13 C and δ 18 O values decreased by ~0.01 and ~0.03 ‰/(°C below −20°C). The raw Δ 47 values decreased by 0.003–0.005 ‰/(°C below −20°C), but the final reference‐frame‐corrected values (Δ 47‐RFAC ) were unaffected as long as the carbonate samples and standard gases were prepared identically. Conclusions: Preparing carbonate samples for clumped isotope analysis using a PPQ trap that is too cold can result in erroneous stable isotopic compositions. New and existing labs using the static PPQ trap cleaning procedure should determine the ideal PPQ trap temperature for their particular system through monitoring not only yield through the PPQ trap, but also stable isotopic composition at various PPQ trap temperatures. Copyright © 2015 John Wiley & Sons, Ltd.