Reducing contamination parameters for clumped isotope analysis: The effect of lowering Porapak (TM) Q trap temperature to below-50 degrees C

Rationale Carbonate clumped isotope thermometry examines the thermodynamic preference of 13C–18O bonds to form within the carbonate crystal lattice. The 13C18O16O isotopologue in analyte CO2 has a natural abundance of 44.4 ppm necessitating stringent purification procedures to remove contaminant molecules that may produce significant isobaric effects within range of the mass 47 isotopologue. Strict purifications of analyte CO2 are thus required as well as reliable contamination indicators. Methods CO2 purification was carried out by vacuum cryogenic purification through a static trap packed with Porapak™ Q (PPQ). The correlation between mass excesses on m/z 47, 48 and 49 in CO2 produced by acid digestion of 12 natural samples was measured by isotope ratio mass spectrometry (IRMS). CO2 from two contaminated carbonate samples was then purified at PPQ trap temperatures between –25 and –65°C and measured by IRMS to determine changes in mass excesses on m/z 47, 48 and 49. Finally carbonate standards, Carrara marble (CM) and ETH3, were purified at PPQ trap temperatures of –35 and –60°C to identify isotopic fractionation associated with lowering trap temperature. Results The correlation between mass excesses on m/z 47, 48 and 49 is determined to be sample dependent. Lowering the PPQ trap temperature to –60°C has a 78% success rate in decreasing Δ48offset, a measure of sample contamination, to within an acceptable range (