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

Abstract

Rationale: Carbonate clumped isotope thermometry examines the thermodynamic preference of ¹³ C-¹⁸ O bonds to form within the carbonate crystal lattice. The ¹³ C¹⁸ O¹⁶ O 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 Δ_48offset , a measure of sample contamination, to within an acceptable range (<1.5 ‰). Lowering the PPQ temperature in purification of CM and ETH3 is associated with decreases in the δ¹³ C and δ¹⁸ O values as a result of isotopic fractionation_. We demonstrate that we can correct for fractionation at a trap temperature of -60°C.

Conclusions: Lowering the temperature of the Porapak Q trap to -60°C results in improved sample cleaning. It is possible to correct for fractionation in δ¹³ C and δ¹⁸ O values at lower PPQ trap temperatures using identically prepared standards. This result has important connotations for laboratories using similar sample preparation methods.