ANALYSIS OF δ18O AND δ13C VALUES IN SMALL CARBONATE SAMPLES ON A GAS BENCH-DELTA ADVANTAGE ISOTOPE RATIO MASS SPECTROMETER

Stable isotope analysis of carbonates is vital for paleoclimate studies. In serially sampled or micromilled carbonates such as mollusk shells (bivalves, gastropods, etc.), corals, otoliths, speleothems, and small organisms such as foraminifera, sample size limits spatial sampling resolution. This in turn limits temporal resolution of paleoclimate reconstructions and increases time averaging. Thus, smaller samples are desired in many studies. Small samples, down to about 20 μg, are routinely analyzed on more expensive instrumentation such as a ThermoScientific Kiel Device coupled to a MAT253 mass spectrometer, but not on smaller mass spectrometers such as the ThermoScientific Delta series. In this study we test sample size limitations on Union College’s Gas Bench II coupled to a Delta Advantage using the carbonate standard NBS19 in both 4 mL and 12 mL Exetainer vials. Twenty microgram standards had similar precision to 70 ug samples in 12mL vials. The smaller vials produced slightly worse precision than the larger vials when running 20 μg standards (standard deviations: 12 mL vials δ¹³C = 0.06‰, δ¹⁸O = 0.03‰, n=7; 4 mL vials δ¹³C = 0.08‰, δ¹⁸O = 0.11‰, n=10). We have not yet tested 10 μg standards in 12 mL vials, but even 10 μg standards produced acceptable precision in 4 mL vials (δ¹³C = 0.16‰, δ¹⁸O = 0.12‰, n=4). Our study illustrates that the new Delta Advantage ThemoScientific IRMS and Gas Bench II have the sensitivity to analyze very small samples with good precision down to 20 μg, and even 10 μg (with slightly poorer precision).