RESUMEN
RATIONALE: 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. METHODS: 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 (600 s integration) followed by a single measurement of the working gas (WG) with the same integration time. RESULTS: The Δ47 measurements of four calcite standards over a period of 5 weeks 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 120 µg are sufficient to achieve an external precision of ±0.007 (1SE) or of ±0.013 at the 95% confidence level. CONCLUSIONS: 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.
RESUMEN
RATIONALE: High-precision stable isotope measurements in gas-source isotope ratio mass spectrometry are generally carried out by repeated comparison of the composition of an unknown sample with that of a working gas (WG) through a dual-inlet (DI). Due to the established DI protocols, however, most of the sample gas is wasted rather than measured, which is a major problem when sample size is limited. Here we propose a new methodology allowing the measurement of a much larger portion of the available sample. METHODS: We tested a new measurement protocol, the long-integration dual-inlet (LIDI) method, which consists of a single measurement of the sample for 200 to 600 seconds followed by a single measurement of the WG. The isotope ratios of the sample are calculated by comparison of the beam ratios of the WG and sample at equivalent intensities of the major ion beam. RESULTS: Three isotopically very different CO2 samples were analyzed. The LIDI measurements of large samples (50 to 100 µmol of CO2) measured at quasi-constant beam sizes, and of small samples (1.5 to 2 µmol of CO2) measured in micro-volume mode, generated results that are indistinguishable from the standard DI measurements for carbon, oxygen and clumped isotope compositions. The external precision of Δ47 using the LIDI protocol (~±0.007) is similar to that of the state of the art DI measurements. CONCLUSIONS: For traditional and clumped isotope measurements of CO2, the LIDI protocol allows the measurement of a much larger portion of the sample gas rather than only ~20% of it. In addition, the sample can be measured at higher signal intensity and for longer time, allowing the measurement of smaller samples while preserving precision. We suggest that other gases commonly used for stable isotope measurements with gas-source mass spectrometry would also benefit from this new protocol.
