| 08 Aug 2025
Blank variability in coulometric measurements of dissolved inorganic carbon
Abstract. Marine dissolved inorganic carbon (DIC) is by far the largest pool of carbon in the Earth surface system that exchanges with the atmosphere on human-relevant timescales. Measurements of DIC are therefore necessary to study the changing marine carbon cycle. The most accurate routine DIC measurement method is coulometry. In this method, the signal detected by a coulometer for each measurement must be corrected for background noise, which is termed the blank. The current best practice recommendation is to measure the blank once per analysis session and use this constant value to correct all measurements. However, calculating the blank for each measurement separately shows that the blank sometimes changes during analysis sessions. Correcting measurements to a constant blank when the blank is actually changing leads to an apparent drift in DIC results and therefore lower accuracy. Here, we propose an alternative method for coulometer blank corrections in which the blank is calculated on a per-measurement basis. The per-measurement blank values are then fitted to a smoothing function to determine a set of fitted blank values with which the measurements are corrected. We test the three different approaches (constant, per-measurement and fitted) by applying them to 263 measurements of a laboratory internal standard conducted during 89 analysis sessions over ~7 years. Switching from the constant blank to either the per-measurement or fitted blank improves the precision from 1.85 µmol kg‑1 to 1.31 µmol kg‑1. This improvement is statistically significant and important relative to the climate-quality uncertainty target for DIC measurements of ± 2 µmol kg‑1. Using the fitted blank rather than per-measurement blank eliminates a number of outliers, notably reducing the total range and kurtosis of the residuals. A free and open source Python package (koolstof) has been made available to perform fitted blank corrections for some common coulometer data types. We recommend that in future coulometric DIC analyses, per-measurement blanks should be routinely calculated as part of the quality control process and the fitted blank method applied either as standard or when a changing blank is observed.
Competing interests: Matthew Humphreys is an Editor for Ocean Science.
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This technical note addresses an important methodological issue in high-precision DIC measurements by coulometry, focusing on how to handle the background signal. The authors analyze a long-term dataset of 263 internal-standard DIC measurements across 89 analysis sessions (~7 years) and compare three blank-correction approaches: a traditional constant blank, a per-measurement blank, and a fitted blank. They show that using per-measurement or fitted blanks improves precision, and the fitted-blank approach also reduces extreme outliers.
Overall, the manuscript is clear and well written. The problem is motivated by the need for climate-quality DIC uncertainty. The methods are described in sufficient detail, and the statistical analysis is appropriate. This is a useful and practical contribution to the oceanographic inorganic carbon community, especially since small improvements in precision matter when looking at decadal trends. Making the tool open also helps with reproducibility. I have a few comments and minor suggestions below. I recommend publication with minor revisions.
Comments and suggestions:
Minor points:
Line 75: “mean average increments” could be simplified to “mean increments.”
Figure 3 caption: change “with measurements are sorted” to “with measurements sorted.”