VSMOWSMOW

VSMOWSMOW is not a distinct standard in itself; it is best understood as a reference to the pair of closely related isotopic standards used for hydrogen and oxygen in water analyses: SMOW (Standard Mean Ocean Water) and VSMOW (Vienna Standard Mean Ocean Water). Both are central to the way scientists report stable isotope ratios in water, but they serve different historical and practical roles within the same calibration framework.

In isotope geochemistry, two isotopes are routinely measured in water: hydrogen-1 (protium) and deuterium (hydrogen-2), and oxygen-16 versus oxygen-18. The results are expressed as delta values, δ18O and δD (or δ2H), which denote per mil deviations of the sample’s isotope ratios from a reference standard. The delta values are calculated as the ratio of the heavy to light isotope in the sample divided by the same ratio in the reference, minus one, then multiplied by 1000.

SMOW, established in mid-20th century practice, was the original standard against which natural waters were compared. VSMOW, Vienna Standard Mean Ocean Water, later became the preferred international reference because it provides a well-characterized, reproducible baseline. In modern practice, VSMOW defines the zero point for the delta scales: δ18O and δD of the VSMOW standard are set to zero by definition. Samples are then measured relative to VSMOW, with SMOW serving as a historic or supplementary reference in some datasets. The SMOW scale remains tied to VSMOW through established calibrations, enabling cross-lab comparability.

Applications of these scales span hydrology, climatology, paleoclimatology, and forensic science, where isotope ratios reveal water sources, evaporation histories, and climatic conditions. If encountered as VSMOWSMOW, it usually signals reference to both standards and the associated on-scale reporting, rather than a separate third standard.