Preparation and quality control of in-house reference materials for marine dissolved inorganic carbon and total alkalinity measurements

Accurate measurements of seawater carbonate chemistry are crucial for marine carbon cycle research. Certified reference materials (CRMs) are typically analyzed alongside samples to correct measurements for calibration drift. However, the COVID-19 pandemic led to a limited access to CRMs. In response to this shortage, we prepared and monitored in-house reference materials (IHRMs) for total alkalinity (TA) and dissolved inorganic carbon (DIC), over 12 and 15 months, respectively. Overall, TA was stable, but a slight increase in DIC of about 2 μmol kg⁻¹ occurred over 15 months. The increase could potentially be attributed to bacterial growth, despite mercuric chloride fixation and repeated UV exposure. It is noted that this small increase was most likely within our instrument and measurements uncertainties. Our repeated measurements also identified a few bottles that had TA or DIC concentrations 4–5 μmol kg⁻¹ higher than the rest, indicating issues during cleaning, fixation, or storage of individual bottles. This study emphasizes the importance of careful and continuous monitoring if self-prepared IHRMs are used. Given that the amount of work required is very high, IHRM preparation is only recommended when CRMs are not available.