When Seawater is as Valuable as Gold

The Alchemy of Certified Reference Materials for Seawater Analysis

by Marta Álvarez, IEOC-CSIC | March 25, 2025

For the past two years in EuroGO-SHIP, I have participated in different activities towards improving services for ship-based hydrographers sampling the open and coastal ocean within the observation system in Europe. I have focused on improving the quality of data, especially when gathering data from different research cruises, and from different sources, such as labs and/or data that comes from different types of sensors measuring the same variable.

When we combine data to build a data product that stakeholders can use to assess essential ocean variables, for example the increase of temperature in the ocean or the decrease of oxygen or ocean acidification, we need to be careful with the uncertainty of the gathered data. Data coherency is absolutely necessary; in metrology this is called reproducibility. Attaining coherency/reproducibility in oceanography is challenging because sometimes, especially in biogeochemical variables, we often lack reference materials that researchers can use to verify their methodology and results, an important step to ensure that the data among laboratories is comparable.

Within EuroGO-SHIP, I have been looking at ways to improve secondary quality control. This is a method to compare and quantify any difference between data from different cruises that overlap in a region of the ocean. Secondary quality control compares data and tries to quantify the uncertainty in the final combination of data sets from different sources.

The secondary quality control activity is related to other activities I’ve been collaborating on within EuroGO-SHIP, such as how to produce working reference materials for inorganic carbon. When working in the lab, we need standard or working reference materials to control and assess the quality of our analyses. A working reference material is “just” liters of seawater that are homogeneous and stable over time so in the lab it can be used to control over time the results of the analysis. In addition, certified reference materials (CRMs) are reference materials provided with a certified value for a specific variable. This certification is done by very few accredited labs, which are either national metrology institutes or specific companies.

For example, for the variables of the inorganic carbon system in seawater, there are three measurable variables: pH, total alkalinity and total inorganic carbon, and there are certified reference materials (CRMs) for total alkalinity and total inorganic carbon. By using these CRMs in the lab, you can check the reliability of your method and the results of the analysis.  

Unfortunately, the ocean biogeochemical community suffers from the limited production of CRM for several variables, especially for CO₂.

Only two labs in the world

Labs producing CRMs for our purposes are few: for salinity there is only one lab in the UK; and for inorganic carbon, we have just one lab in the United States. This is not a sustainable production model for many reasons but mainly because it impacts accessibility and costs. The CRMs are not digital files; the material we need is actually a bottle of sea water and it must be shipped and that is expensive.

Yes, Actual Sea Water

To produce a batch of certified reference material, for example for CO₂, the production lab needs to verify 100 litres of seawater and then parce it out into 500 millilitre bottles, of which are sealed up and sent all over the world. This process has a cost, and we need to pay for it. So this little bottle of 500 milliliters of seawater from the Pacific Ocean and its corresponding piece of paper certifying the concentration of total inorganic carbon and alkalinity is as valuable and maybe (metaphorically) as expensive as GOLD. No drop is wasted.

Box with 20 bottles of CRM for CO₂ provided by Prof. Dickson lab (SIO, US). The inset shows one of the 500 mL bottles.

For our analysis to be accurate, we need to check them against this certified seawater with the values provided in the certificate. This verification brings the provenance; it’s the alchemy that gives the measurement a, sort of, gold star.

Location, Location, Location

Another issue we face with the limited production labs is that the characteristics of the seawater in the Pacific are not the same as the range of values that we have in European seas. For example, the Mediterranean Sea is high in alkalinity because it is high in salinity, whereas the Baltic has low salinity and low alkalinity. So we are using CRMs with characteristics that are out of our working range in in our labs.

This is one of the reasons we are asking European funding agencies to help build a production and certification hub in Europe—to produce certified reference materials adapted to the European seas characteristics and make material more accessible and appropriate for European researchers.

EuroGO-SHIP – is bringing the framework

We are defining a roadmap for the future in terms of first establishing the best practices (the right processes to measure in the lab, but also to sample and conserve the samples), and how to check the measurements with certified reference materials. The former steps will improve the quality of the data that is gathered from different sources, which would then be more comparable in the secondary quality control. The final aim is to reduce the uncertainty of the data products from hydrogaphy data that we want to release to end-users (society, policymakers, etc).

Metrology and Oceanography: Meeting of the Minds

There is a lot of good work going on to improve the hydrography data quality, but an important next step is the direct dialogue between the metrology and the oceanography community so we can discuss our needs in oceanography. For example, one litre here in Spain is one litre everywhere else. Consensus around measurements might be assumed but historically, it has been an evolution. Thanks to the Bureau International des Poids et Mesures (BIPM), we have the International System of Units (SI) that we need for everyday life applications. In oceanography, we need connection with the metrologist because while everybody understands pH, the way we measure pH in the ocean is a very specific technique and there is no link between what we measure in the ocean and the SI. So, it’s a little like translating languages (English to Spanish, etc) and it requires dialogue between oceanography and metrology; it requires commitment and budget. And maybe some alchemy.