Carbonate clumped isotope analyses with the long-integration dual-inlet (LIDI) workflow: scratching at the lower sample weight boundaries.

Abstract

Clumped isotope analyses (Δ47 ) of carbonates by dual inlet (DI) mass spectrometry require long integration times to reach the necessary high precision due to the low abundance of the rare isotopologue 13 C18 O16 O. Traditional DI protocols reach this only with large amounts of sample and/or a large number of replicates as a large portion of the analyte gas is wasted. We tested an improved analytical workflow that significantly reduces the sample sizes and total analysis time per sample while preserving precision and accuracy. We implemented the LIDI (long-integration dual-inlet) protocol to measure carbonates in micro-volume mode using a Kiel IV carbonate device coupled to a Thermo Scientific 253 Plus isotope ratio mass spectrometer without the new 1013 ohm amplifier technology. The LIDI protocol includes a single measurement of the sample gas (600s integration) followed by a single measurement of the working gas (WG) with the same integration time. The Δ47 measurements of four calcite standards over a period of 5weeks demonstrate excellent long-term stability with a standard deviation of ±0.021 to ±0.025 ‰ for the final values of the individual aliquots. The Δ47 analyses of a coral, four foraminifera and a calcite precipitated in the laboratory demonstrate that 14 replicates of 90 to 120μg are sufficient to achieve an external precision of ±0.007 ‰ (1SE) or of ±0.013 ‰ at the 95% confidence level. This study demonstrates that by using a Kiel IV-253 Plus system with LIDI it is possible to achieve the same analytical precision as conventional DI measurements with at least a factor of 40 less sample material. With the new 1013 ohm resistor technology there is the potential to reduce the required sample material even more. This opens new avenues of research in paleoceanography, paleoclimatology, low-temperature diagenesis and other currently sample size limited applications. Copyright © 2017 John Wiley & Sons, Ltd.