Stable Carbon and Oxygen Isotope Analysis of Carbonates and DIC Using the Delta Ray Isotope Ratio Infrared...

The Delta Ray Isotope Ratio Infrared Spectrometer enables to determine δ¹³C and δ¹⁸O values of CO₂. The measurements are calibrated against internal reference gas standards. For discrete carbonate or dissolved inorganic carbon (DIC) samples, the manufacturer recommends a calibration against two isotopically different carbonate standard materials (i.e., a 2-point calibration). Here, we show that this method is not sufficient because the measured values may drift significantly over the duration of a measurement run (i.e., 58 samples in approximately 24 h).

We developed a new measurement routine applying a 3-point calibration and a tight standard-bracketing. In addition, we present an R code to automatically evaluate and calibrate the measured data.

Our standard-bracketing technique can effectively correct drifts of more than 1‰ not accounted for by the internally calibrated data. A 3-point calibration with four samples bracketed by two sets of standards provides the highest precision and accuracy, that is, the highest reproducibility for our carbonate samples. The third standard stabilizes the calibration curve and enables to better identify individual outliers.

Our standard-bracketing method enables accurate and precise (< 0.1‰) measurements of δ¹³C and δ¹⁸O values of calcite and DIC using the Delta Ray IRIS over a large range of values (ranging from approximately -37‰ to +2‰ in δ¹³C and from approximately -25‰ to -2‰ in δ¹⁸O).