3 Measurement and Standards

Isotope ratios are, by convention, the ratio of the heavier isotope to the lighter one. The differences in isotope ratios between materials are slight and are not easily measured in an absolute sense. However, if measured as a relative difference between the sample and a standard of known isotope ratio, then the precision increases substantially. Thus measurements in stable, light-isotope mass spectrometers are made by comparing the sample to a standard, using either a dual inlet or continuous flow system.

Stable isotope ratios are reported as deviation from an international standard.

For both hydrogen and oxygen in water, the standard is SMOW, Standard Mean Ocean Water. SMOW was devised by Harmon Craig in 1961 (Craig, 1961) as an average of previous ocean water samples from Epstein and Mayeda (1953) and Horibe and Kobayakawa (1960), but no actual sample existed. Because of this, SMOW was defined relative to NBS1 (National Bureau of Standards, standard 1), a United States administered sample from the Potomac River. Because of the difficulties of not having an actual standard to analyze, the International Atomic Energy Agency in Vienna commissioned the creation of VSMOW in 1966, which was to mimic SMOW. Although VSMOW is not isotopically identical to SMOW (Clark and Fritz, 1997), it is similar enough (within laboratory error) to be treated as the same. Even though VSMOW may have been used in their laboratories to enable calibration of their local laboratory standards (Gonfiantini, 1981; Sharp, 2007), workers should not report the deviation from VSMOW, as this merely enables correction to SMOW. Data corrected to VSMOW should be reported as deviation from SMOW.

Some natural waters are very isotopically different from ocean water, particularly those at very high latitudes, altitudes and low temperatures, in other words, high mountains or polar areas. For this reason, the Standard Light Antarctic Precipitation (SLAP) standard was created. This standard should be used to calibrate isotopically light working standards for use in high altitude or high latitude environments.

The Greek lower-case delta, δ, is used to denote the deviation of a sample from a standard, as shown in Equation 1.

where:

When relatively more of the heavy isotope (e.g., ¹⁸O) is present in the sample than the standard, then the δ value will be greater than zero, whereas samples relatively depleted in the heavy isotope will have negative δ values. The δ¹⁸O and δ²H values of SMOW are equal to 0. As the variations in isotope ratios are generally quite small, these values are reported in permil (parts per thousand), using the ‰ notation. The equation combining these δ definitions is shown as Equation 2.