Laser-based spectroscopic techniques, such as cavity ring-down spectroscopy (CRDS), provide a new, cost effective and more widely available approach to measure the oxygen isotope ratio in water molecules, H₂¹⁸O/H₂¹⁶O (δ¹⁸O), and are used increasingly to measure δ¹⁸O in the world's oceans. Here, we present results from an interlaboratory comparison designed...