Technical note: CO2 is not like CH4 - limits of and corrections to the headspace method to analyse pCO2 in fresh water

Headspace analysis of CO2 frequently has been used to quantify the concentration of CO2 in fresh water. According to basic chemical theory, not considering chemical equilibration of the carbonate system in the sample vials will result in a systematic error. By analysing the potential error for different types of water and experimental conditions, we show that the error incurred by headspace analysis of CO2 is less than 5% for typical samples from boreal systems which have low alkalinity (< 900 mu mol L-1), with pH < 7.5, and high pCO(2) (> 1000 mu atm). However, the simple headspace calculation can lead to high error (up to -300 %) or even impossibly negative values in highly undersaturated samples equilibrated with ambient air, unless the shift in carbonate equilibrium is explicitly considered. The precision of the method can be improved by lowering the headspace ratio and/or the equilibration temperature. We provide a convenient and direct method implemented in an R script or a JMP add-in to correct CO2 headspace results using separately measured alkalinity.

Automated summary

Headspace analysis of CO2 is commonly used to quantify CO2 concentration in fresh water, but systematic error may occur if the chemical equilibration of the carbonate system in sample vials is not considered. The error for typical boreal systems with low alkalinity, pH<7.5, and high pCO(2) is usually less than 5%. Improvements in method precision can be achieved by reducing headspace ratio or equilibration temperature.