High-Precision Simultaneous O/O, C/C, and O/O Analyses for Microgram Quantities of CaCO by Tunable Infrared Laser Absorption Spectroscopy.

Stable isotope ratios (O/O, C/C, and O/O) in carbonates have contributed greatly to the understanding of Earth and planetary systems, climates, and history. The current method for measuring isotopologues of CO derived from CaCO is primarily gas-source isotope ratio mass spectroscopy (IRMS). However, IRMS has drawbacks, such as mass overlap by multiple CO isotopologues and contaminants, the requirement of careful sample purification, and the use of major instrumentation needing permanent installation and a high power electrical supply. Here, we report simultaneous O/O, C/C, and O/O analyses for microgram quantities of CaCO using a tunable mid-infrared laser absorption spectroscopy (TILDAS) system, which has no mass overlap problem and yields high sensitivity/precision measurements on small samples, as small as 0.02 μmol of CO (equivalent to 2 μg of CaCO) with standard errors of less than 0.08 ‰ for O/O and C/C (±0.136 ‰ and ±0.387 ‰ repeatability; n = 10). In larger samples of CO, 0.68 μmol (or 68 μg of CaCO), standard error is less than 0.04 ‰ for O/O and C/C (< ±0.1 ‰ repeatability; n = 10) and 0.03 ‰ for O/O (±0.069 ‰ repeatability; n = 10). We also show, for the first time, the relationship between O/O ratios measured using the TILDAS system and published δO values of international standard materials (NBS-18 and -19) measured by IRMS. The benchtop TILDAS system, with cryogen-free sample preparation vacuum lines for microgram quantities of carbonates, is therefore a significant advance in carbonate stable isotope ratio geochemistry and is a new alternative to conventional IRMS.