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Yup! It’s another one of those “Randy’s found a weird unit of measure rabbit hole” posts again! Also, I think I finished my talk! Finally!… Probably. Might need more pictures.
Water, the stuff is everywhere and so very important to our lives. I’m sure that most of us just think of water, once distilled and filtered of impurities to leave only H2O molecules, as being identical no matter where you collect it. But over the years of just curiously surfing Wikipedia for metrology related topics (as one does), I kept bumping into the “Vienna Standard Mean Ocean Water” (VSMOW) page. Apparently, there’s a standard “average ocean water” that scientists use to reference measurements against. They’ll cite that a sample has 10% more oxygen-18 than VSMOW. I wanted to know what that was all about.
So the history goes, back in the early 1960s, researchers were measuring the proportion of different isotopes of oxygen (O16 vs O17, and O18) and hydrogen (hydrogen vs deuterium and tritium) in seawater. Samples taken from various parts of the world interestingly yielded different proportions. They could tell that the ratio of O18 to O16 was correlated with the salinity of the seawater mixing with glacial freshwater, but also knew that salinity wasn’t the cause because they could find freshwater sources with similar imbalances.
It should be noted that all discussion of the measured isotopes always talk about the ratio between two isotopes of an element like oxygen because mass spectrometers used in this context essentially vaporize the sample, ionize it, then accelerate the ion in a vacuum using magnetic fields to see how the path of the ions deflect before striking a collector/sensor. You keep doing this to build up counts of strikes and eventually calculate the ratio of the different clusters of deflections you see within the sample. Given the huge number of atoms involved to get readings, there’s not really another meaningful way to report data outside of that single sample context.
Anyways, the reason any differences even exist is because the molecular weights of the different isotopes actually has a tiny effect on whether a given water molecule will evaporate or not. Lighter isotopes are slightly preferred to evaporate, leaving oceans with high evaporation rates near the equator richer in the heavier O18 isotopes, while rainwater will have slightly less O18. The reverse, where O18 is slightly more likely to condense into rain than O16 enhances this effect. O16 can then be trapped away from the oceans for extended periods if precipitates onto a polar glacier and can’t return to the ocean.
Since VSMOW represents “typical” ocean water, there’s also exists the “Standard Light Antarctic Precipitation” (SLAP) that uses snow melt from Antarctica as the standard reference for rainwater. SLAP, by definition, contains about 5% less oxygen-18 and 42.8% less deuterium than VSMOW. The two samples essentially cover the range of water isotope concentrations found naturally on Earth.
Is there a reason for us non-specialists to care about VSMOW? Not really. The closest connection most of us have has to do with the temperature scale. Prior to the redefinition of the SI unit Kelvin in 2019, the triple point of water was used to define the temperature scale. A water sample having a different balance of isotopes would have higher or lower triple points that introduced errors into extremely sensitive temperature measurements. Making triple-point cells with water calibrated to VSMOW helps reduce errors in calibrating thermometers. While the new definition doesn’t rely on the tripe-point any more, NIST still considers using triple-point cells to be a good way to realize the Kelvin for calibration purposes.
For researchers interested in the water cycle, the standard is of course useful. Samples taken across the world are referenced against VSMOW and tell researchers about how water is moving around the planet. Water samples taken from ice cores can give details about what climate was like in the deep past.
Another interesting point to me is that the concept of VSMOW is based on an abstract construct.
Epstein and Mayeda in 1953 had published (doi:10.1016/0016-7037(53)90051-9) measurements of sea water sampled from a bunch of places at various depths, and different labs were calculating an “average” sea water from those tables to use as reference points for reporting how their own samples deviated from the mean. It was apparently very confusing for everyone so Craig (1961, doi:10.1126/science.133.3467.1833) comes around and decides to define a new standard, SMOW, by calculating a mean and then measuring how a water sample available for purchase from the National Bureau of Standards at the time for cross-checking mass spectrometers differed from the mean. This “NBS-1” sample happened to have 5% more deuterium than the definition of SMOW, and 8% more O18.
So what we have is a theoretical definition of a standard reference point that comes first, and then a similar-enough physical example is found and set as the reference point for instruments to calibrate against.
Time passes and the IAEA, the same inter-governmental agency charged with promoting the safe use of nuclear technology (of which studies of isotopes apparently falls under), decides that all the competing standards is too confusing and recommends the official creation of the standards we talked about, VSMOW and SLAP. Craig, who had created the earlier SMOW standard was the person who created a large batch of VSMOW by carefully mixing sea water samples until he achieved a isotope ratio extremely close to the original SMOW standard. The samples were stored in small glass ampules and sold to labs and instrument manufacturers for use in instrument calibration and other research.
Then, eventually the supply of VSMOW runs low and the IAEA had to order the creation of a second batch, VSMOW2 and SLAP2. The new batch is identical to the original samples to within measurement error. You can supposedly buy an ampule for 180 euros from IAEA directly. You can also read the reference manual that comes with it, where it goes into great detail as to where the water came from, Lake Bracciano near Rome, Italy, Lake Galilea in Israel, and “a well located near Cairo, Egypt”. There’s also descriptions of how they checked the samples for various properties.
The stuff is also pretty finicky to use. The instructions say the samples should be stored at room temperature and in the dark. It also states that a sample should never be opened, partially used, and then the remnants saved for use on subsequent days. Evaporation or other isotope exchange in the sample has likely occurred by then and the sample won’t match the properties o the reference sheet.
So, end of the day, VSMOW is just a measurement curiosity to us normal folk. I think it’s really cool that the instruments are sensitive enough to figure out such minute differences in isotope proportions.
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I’m Randy Au, Quantitative UX researcher, former data analyst, and general-purpose data and tech nerd. Counting Stuff is a weekly newsletter about the less-than-sexy aspects of data science, UX research and tech. With some excursions into other fun topics.
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Reference standards are an attempt to impose a non-relativistic perspective on a relativistic world. Illustrating once again that the world in its full variability can only be approached through models.