Technical note: CO<sub>2</sub> is not like CH<sub>4</sub> – limits of and corrections to the headspace method to analyse <i>p</i>CO<sub>2</sub> in fresh water

Abstract. Headspace analysis of CO2 frequently has been used to quantify the concentration in fresh water. According basic chemical theory, not considering equilibration carbonate system sample vials will result a systematic error. By analysing potential error for different types water and experimental conditions, we show that incurred by headspace is less than 5 % typical samples from boreal systems which have low alkalinity (< 900 µmol L−1), with pH < 7.5, high pCO2 (> 1000 µatm). However, simple calculation can lead (up −300 %) or even impossibly negative values highly undersaturated equilibrated ambient air, unless shift equilibrium explicitly considered. The precision method be improved lowering ratio and/or temperature. We provide convenient direct implemented an R script JMP add-in correct results using separately measured alkalinity.