Setting standards with old rocks and ocean water

Last month an international committee agreed to a long-planned Système Internationale revision in which the platinum–iridium cylinder in Paris known as Le Grand K will no longer be used to define the kilogram. All SI units will be tied to seven constants of nature .

Yet as metrologists celebrate a more fundamental SI, scientists in other fields continue to rely on standards that were developed decades ago, and they have no plans to change any time soon. Take the field of stable-isotope geochemistry, in which researchers measure nonradioactive isotopes of carbon, oxygen, sulfur, and other elements to reconstruct Earth’s past climate and biological activity. To conduct their studies, geochemists compare the ratio of two isotopes in a sample such as an ice core with the analogous ratio of isotopes in a standard reference material. The problem is that the standard reference materials haven’t always been very standard. Moreover, some of those still used to standardize measurements don’t exist anymore.

Reliable, consistent standards are invaluable in stable-isotope geochemistry research. Consider a researcher studying powdered samples of ancient corals whose age has been determined precisely via radiometric dating. Using a mass spectrometer, the scientist measures the concentrations of oxygen-18 and oxygen-16 and then divides the isotope ratio by that of a standard material, as shown in the equation. With the result, reported in parts per thousand, the temperature and precipitation conditions at the time the coral lived can be estimated. By scaling isotope ratio measurements to a common material, researchers from other laboratories can attempt to reproduce those measurements without bias.

Measuring stable isotopes became feasible in the mid 20th century, when a research team at the University of Chicago led by Harold Urey, recipient of the Nobel Prize in Chemistry in 1934 for the discovery of deuterium, made dramatic improvements to the precision of mass spectrometer instruments. As part of that work, the Chicago team measured the ratios in marine carbonate fossils. The researchers compared the carbon isotope ratio values with that of a rock sample found in South Carolina. They named the reference material PDB, after the Peedee Formation in which they found the fossil, Belemnitella americana, that they used for their calibration.

PDB was given to other stable-isotope research teams so that their work could be compared with that of the Chicago group. After being distributed to other teams, the PDB supply ran low. Researchers responded by measuring a new sample of South Carolina rock against the original material to create a new reference standard, PDB II. After that second material was exhausted, PDB III took its place.

Today there is no more PDB; the original material was completely depleted by the late 1970s. Yet geochemists still use it as a reference material. In the early 1980s, NIST calibrated a new reference standard, NBS-19, against the extinct PDB. The stable carbon isotope value of PDB is defined as zero, while NBS-19 has an accepted value of +1.95 parts per thousand of its stable carbon isotope ratio relative to that of PDB.

Stable-isotope measurements of oxygen were also referenced to PDB for many years. Things got even more complicated in the 1960s when researchers adopted a new standard named Standard Mean Ocean Water (SMOW). The problem was that SMOW was defined by a single sample of fresh water, NBS-1, from the Potomac River near NIST. To make SMOW a true reference standard, geochemists Harmon Craig and Ray Weiss had to carefully distill water samples from different oceans and then combine them in different proportions until they had formulated a concoction that compositionally matched the old SMOW.

The community of researchers measuring stable sulfur isotopes has a similar story. Early on, samples of sulfur were converted to sulfur dioxide or sulfur hexafluoride and then measured for isotopic analysis. The reference material originally chosen was Cañon Diablo troilite (CDT), an iron sulfide meteorite found in Arizona’s Meteor Crater. Later the chemists learned that the CDT reference materials given out to labs were not quite as homogeneous as originally thought. As a result, geochemists now use a silver sulfide reference standard material instead of CDT. But, just as NBS-19 is still expressed in terms of PDB, the isotope composition of silver sulfide remains tied to the now-defunct CDT.

Currently NIST and the International Atomic Energy Agency in Vienna oversee the preparation, maintenance, and distribution of standard reference materials. Stable-isotope measurements reference VPDB or VSMOW—the Vs stand for Vienna. Various geochemistry labs around the world use their own working standards, typically homogenized and readily available materials with a constant stable-isotope composition. Every so often those working standards are calibrated against an international reference standard.

Though the process by which standards are established has proved complicated, it has led to many successful research efforts. For example, using oxygen and hydrogen stable-isotope measurements referenced to VSMOW, researchers have transformed an ice core from Antarctica’s Lake Vostok into a near-uninterrupted view of global climate as far back as 800 000 years. As long as VSMOW and other standards work as intended, researchers probably won’t be complaining about their difficult-to-discern origins.