Technical Note: Highly precise quantitative measurements of total dissolved inorganic carbon from small amounts of seawater using a common gas chromatographic system: an alternative method compared to established detection systems

Total dissolved inorganic carbon (CT) is one of the most frequently measured parameters in order to calculate the partial pressure of carbon dioxide in seawater. Its measurement has become increasingly important because of the rising interest in the biological effects of acidification. The coulometric- and infrared detection methods are favoured to precisely quantify CT. However, these methods were not validated for CT samples from acidification experiments investigating biological responses to manipulated partial pressure of carbon dioxide (pCO2), which need an extended CT measurement range (~1250–2400 μmol kg 1) compared to natural open ocean seawater samples (~1950–2200 μmol kg 1). Additionally, the requirement of total sample amounts between 0.25–1 L seawater in the coulometric- and infrared detection methods exclude their use for experiments working with smaller volumes. Precise CT analytics also become difficult with high amounts of biomass (e.g. phytoplankton cultures) or even impossible in the presence of planktonic calcifiers without sample pre-filtration. However, filtration can alter CT concentration through gas exchange. Addressing these problems, we present precise quantification of CT using a small, basic and inexpensive gas chromatograph as a highly sensitive CT-analyzer. Our technique is able to provide a measurement precision of ± 3.7 μmol kg 1 and an accuracy of ± 1.2 μmol kg 1 in a CT range typically applied in acidification experiments. It requires sample sizes of only 200 μL taken from 10 mL pre-filtered samples or from a 10 mL sub-sampled seawater reference (Dickson standard). Our method is simple, reliable and with low cumulative analytical costs. Hence, it is potentially attractive for all scientists experimentally manipulating the seawater carbonate system.